1992 Electric Short - Troubleshooting

[Honcho]

Quick update-

I've short circuited the entire car! Grrrrr...

Now i have to undo all of my wire changes to isolate where the short is. Here are the symptoms if any of you electric-type guys want to take a guess. It seems to be on the Fuse 33 (7.5A) and Fuse 5 (10A) circuits- like they are mixing.

Connect battery:
Front maker lights solid on
Rear marker lights off
ABS pump spinning (2000+ module)
Combination switch dead (headlights, turn signal, etc.)
Headlight raise switch under gauges dead
Horns on solid

Ignition switch to Run:
Blower on full
Oil pressure and Coolant gauges rise to max
Gauge lights dead
CCU dead
Clock dead

Kaz suspects a floating ground. Any ideas? Re-assembly is stopped dead until I figure this out. Note that everything worked properly before I started cutting and splicing wires. So now, I have to undo everything and then do each change one at a time to figure out what's causing it.
[MENTION=26435]Old Guy[/MENTION] [MENTION=30613]Heineken[/MENTION]

 

[DocL]

Regarding the short circuit, I thought I saw in one of your pictures that the ground points in the front bay were painted over. I also think I saw some other ground wires screwed on to painted surfaces. Those ground points need to be bare metal.

Now I'm going back into my cave.

 

[Heineken]

Thanks for mentioning, not sure if I can offer useful support :wink:

Floating ground issues are notoriously difficult to debug .. if you operate switches (headlights, blinkers, etc.) and unexpected things happen (needles on the dashboard start moving, the wrong lights light up or others go out) than it's very likely a ground problem. Finding the root cause is another matter altogether.

Since everything did work before and assuming the grounding points are fine, it will probably be required to really check each and every new connection that was made - the issue must be with one of those changes after all ..

 

[Valhalla]

Quick update-

I've short circuited the entire car! Grrrrr...

Now i have to undo all of my wire changes to isolate where the short is. Here are the symptoms if any of you electric-type guys want to take a guess. It seems to be on the Fuse 33 (7.5A) and Fuse 5 (10A) circuits- like they are mixing.

Where is fuse #33 and any fuse #5 I find in all three fuse box locations are not 10amp. Fuse 5 under the front hood is a 20 amp right headlight. Number five in the engine compartment is 20amp and is the passenger seat recline. Fuse #5 in the driver footwell is 15 amp and is the back up lights, turn signal and alternator. Fuse 33?

 

[Honcho]

Thanks everyone for your help. Electric issues are always a frustrating problem, but at least I know the car was fine before I cut any wires. Thus, it's definitely my fault, which means it should be fairly straight-forward to isolate, since I kept detailed records of each change, complete with wiring diagrams.

Regarding the short circuit, I thought I saw in one of your pictures that the ground points in the front bay were painted over. I also think I saw some other ground wires screwed on to painted surfaces. Those ground points need to be bare metal.

Now I'm going back into my cave.

Ha! Thanks. The painter taped the front bay ground points, so we're ok there. He forgot the engine bay ones, so I sanded them off and they are now ok. I'm starting to suspect that my method of pulling 12V from unused module connectors on the factory loom (EPS, TCS, AT, Radio) may not be as clever as I thought, since there could be internal circuitry in those modules that prevents a short to ground of the 12V power coming into the module.

Thanks for mentioning, not sure if I can offer useful support :wink:

Floating ground issues are notoriously difficult to debug .. if you operate switches (headlights, blinkers, etc.) and unexpected things happen (needles on the dashboard start moving, the wrong lights light up or others go out) than it's very likely a ground problem. Finding the root cause is another matter altogether.

Since everything did work before and assuming the grounding points are fine, it will probably be required to really check each and every new connection that was made - the issue must be with one of those changes after all ..

Yes, I'm disconnecting everything and will connect each change one at a time to try to isolate the problematic circuit. I'm beginning to suspect the dash cam wiring more and more. There, as shown below, I draw constant 12V from Fuse 33 (the clock fuse) and switched 12V from Fuse 5 and combine them into a single power wire to the camera. Then, I run the ground from the camera back to the ground wire on the EPS connector (G403). I used diodes to try to isolate those two circuits, but perhaps it's not enough? Fuse 33 and Fuse 5 would explain nearly all of the symptoms I'm seeing. We'll find out tonight.

Where is fuse #33 and any fuse #5 I find in all three fuse box locations are not 10amp. Fuse 5 under the front hood is a 20 amp right headlight. Number five in the engine compartment is 20amp and is the passenger seat recline. Fuse #5 in the driver footwell is 15 amp and is the back up lights, turn signal and alternator. Fuse 33?

I'm using the fuse labelling from the ETM. It's different than the factory service manual. Fuse 33 (7.5A) is the clock fuse in the front bay fuse box. Fuse 5 (10A) is the middle fuse in the under-dash fuse box.

 

[RYU]

if you unplug the camera harness (leaving an open circuit there) then shouldn't that isolate the short if that's the origin to begin with?

 

[Honcho]

Ok, now it's getting weird. I've disconnected every change I made to the car and it's still shorting out. I pulled every fuse and discovered that connecting the battery trips the horn relay. If I pull the horn fuse, no more relay trip. I can also hear a relay tripping inside the car when I connect the battery (it was being drowned out by the horn relay). I still can't figure out why the front side marker lights are coming on (but not the rears), even when I pull the lights fuse. It's almost like there is a short somewhere that is energizing the horn relay, energizing another relay inside the car, and somehow turning on the front side marker lights even when the fuse is pulled. Also, the blower still comes on full when I turn the ignition to RUN and the oil/temp gauges still shoot up to max. Must also be tripping the blower relay?

Now, I'm poring through the ETM trying to see what these circuits have in common. It's beginning to look like maybe my wiring changes are NOT the cause... Maybe I nicked a wire during re-assembly?

 

[Valhalla]

Is it the fuel pump relay that is clicking on with the ignition in position 2? It should be but if wired correctly it will only energize for a couple of seconds. And by the location of all the interacting circuits, I would be looking at the harnesses and fuse block up front first. My $.02.

 

[RYU]

Ok, now it's getting weird. I've disconnected every change I made to the car and it's still shorting out. I pulled every fuse and discovered that connecting the battery trips the horn relay. If I pull the horn fuse, no more relay trip. I can also hear a relay tripping inside the car when I connect the battery (it was being drowned out by the horn relay). I still can't figure out why the front side marker lights are coming on (but not the rears), even when I pull the lights fuse. It's almost like there is a short somewhere that is energizing the horn relay, energizing another relay inside the car, and somehow turning on the front side marker lights even when the fuse is pulled. Also, the blower still comes on full when I turn the ignition to RUN and the oil/temp gauges still shoot up to max. Must also be tripping the blower relay?

Now, I'm poring through the ETM trying to see what these circuits have in common. It's beginning to look like maybe my wiring changes are NOT the cause... Maybe I nicked a wire during re-assembly?

When it was painted did you remove every single wiring harness? I assume so.

Lights are a bit weird because the current causing that section of the wire harness to energize could be from a feedback loop from somewhere else. You might want to check the voltage and polarity turning on that circuit to the bulb. It's probably reversed. When this happened to me the light(s) that are on are dimmer than normal. I'm not an EE obviously but I recall having to sort through the ETM when I did my 02 headlight conversion to sort this out.

 

[Honcho]

Is it the fuel pump relay that is clicking on with the ignition in position 2? It should be but if wired correctly it will only energize for a couple of seconds. And by the location of all the interacting circuits, I would be looking at the harnesses and fuse block up front first. My $.02.

I'm almost certain it's the starter cut relay. The click is coming from behind the blower on the other side of the front firewall. The starter relay is the only one in that area.

When it was painted did you remove every single wiring harness? I assume so.

Lights are a bit weird because the current causing that section of the wire harness to energize could be from a feedback loop from somewhere else. You might want to check the voltage and polarity turning on that circuit to the bulb. It's probably reversed. When this happened to me the light(s) that are on are dimmer than normal. I'm not an EE obviously but I recall having to sort through the ETM when I did my 02 headlight conversion to sort this out.

The good news is that the wire system worked as normal after paint. About a year ago, I temporarily connected the battery to do three things: (1) raise headlights to install the headlight covers and test the LED bulbs, (2) test the LED side markers, and (3) operate the windows to align and leak-test the rubber seal. At this time, the dash was already installed, as was the steering wheel, HVAC, clutch switch and horn.

Having spent an unhealthy amount of time with the ETM last night, it came to me that the culprit might actually be the Security Control Unit. I remember now that it was unplugged when I first got the car. And, it is connected to every circuit that appears to be shorting. I kept trying to figure out how the horn relay could energize without the horn button. Turns out, the SCU can ground the relay coil as well. That sent me on a hunt through the ETM and both Fuse 5 and 33 are connected to it, along with the side marker light circuits.

I'm going to unplug the SCU tonight and see if that returns the car to normal. If so, I need to find out what's wrong- is it a short in the SCU ground wire or an internal short (leaky capacitor?) I also ordered a ECT3000 circuit probe to start tracing the shorts if it is in fact a wire touching the body somewhere.

The mystery continues...

 

[RYU]

Having installed various alarms to pay my way through college, that seems like a great place to start! Good luck!

 

[Old Guy]

I am a bit late to the party because I have been out hoofing around the Rockies for the last couple of weeks - should have taken my skis as well as my hiking boots because they just opened up the Sunshine Ski resort at Banff because of the late snow!

First off - way too many symptoms to provide a single diagnosis. Assuming you still have the side marker light mystery, I am going to start there as a diagnosis point. The front side marker lights are separate from the rear side marker lights because the front side markers are integrated with the signal lights - the rear aren't. At this point I am working from my 1997 ETS manual so there may be some discrepancies with your car. The side marker lights get power directly out of the side marker flasher circuit in the integrated control unit. That portion of the ICU gets powered up through fuse 38 via the tail ight relay. The tail light relay has a continuous hot supply, but, is switched on by the combination switch. The obvious question is if the combination switch is off - why are the marker lights on? However, I suggest the following diagnostic steps

- Pull fuse #38 - this should kill power to the side marker lights (and a bunch of other stuff). If the side marker lights do not go out then you have a sneak circuit downstream of the #38 fuse which gives you a more focused area to check
- If the lights went out when you pulled fuse #38 , re insert the fuse and pull the tail light relay. This should do the same thing (lights out). If it doesn't then there is a problem between the tail light relay and the #38 fuse.
- If the two above steps test out OK, then you have to figure out why the tail light relay is getting switched on. The relay is normally switched by a switched ground connection at the combination switch. Pull connector C416 at the combination switch and the relay should turn off. If not you may have an insulation failure to ground on the control wire (may have been pinched somewhere?). However, the security control unit will also switches the tail light relay on so pull connector C461 on the SCU to see if that will switch the tail light relay off. If it does then you have some kind of problem with the SCU which is causing operation of the tail light relay.
- If disconnecting C416 at the combination switch fixes the tail light relay problem, then it s switch problem of some kind - test the switch as set out in the service manual.

The tail light relay powers up a whole bunch of stuff. If that circuit is getting powered up without the tail light relay being powered up that might explains some mysteries.

Sort out what is happening with the tail light relay circuit first. If you can get that circuit to behave (side marker lights and other things work normally) we can move on to the next problem area if problems remain.

 

[Honcho]

I am a bit late to the party because I have been out hoofing around the Rockies for the last couple of weeks - should have taken my skis as well as my hiking boots because they just opened up the Sunshine Ski resort at Banff because of the late snow!

First off - way too many symptoms to provide a single diagnosis. Assuming you still have the side marker light mystery, I am going to start there as a diagnosis point. The front side marker lights are separate from the rear side marker lights because the front side markers are integrated with the signal lights - the rear aren't. At this point I am working from my 1997 ETS manual so there may be some discrepancies with your car. The side marker lights get power directly out of the side marker flasher circuit in the integrated control unit. That portion of the ICU gets powered up through fuse 38 via the tail ight relay. The tail light relay has a continuous hot supply, but, is switched on by the combination switch. The obvious question is if the combination switch is off - why are the marker lights on? However, I suggest the following diagnostic steps

- Pull fuse #38 - this should kill power to the side marker lights (and a bunch of other stuff). If the side marker lights do not go out then you have a sneak circuit downstream of the #38 fuse which gives you a more focused area to check
- If the lights went out when you pulled fuse #38 , re insert the fuse and pull the tail light relay. This should do the same thing (lights out). If it doesn't then there is a problem between the tail light relay and the #38 fuse.
- If the two above steps test out OK, then you have to figure out why the tail light relay is getting switched on. The relay is normally switched by a switched ground connection at the combination switch. Pull connector C416 at the combination switch and the relay should turn off. If not you may have an insulation failure to ground on the control wire (may have been pinched somewhere?). However, the security control unit will also switches the tail light relay on so pull connector C461 on the SCU to see if that will switch the tail light relay off. If it does then you have some kind of problem with the SCU which is causing operation of the tail light relay.
- If disconnecting C416 at the combination switch fixes the tail light relay problem, then it s switch problem of some kind - test the switch as set out in the service manual.

The tail light relay powers up a whole bunch of stuff. If that circuit is getting powered up without the tail light relay being powered up that might explains some mysteries.

Sort out what is happening with the tail light relay circuit first. If you can get that circuit to behave (side marker lights and other things work normally) we can move on to the next problem area if problems remain.

I was REALLY hoping you'd chime in here. Thank you! So, I can confirm that pulling Fuse 38 does not make the front side markers go out. According to the 92 ETM, those lights can receive 12V from either Fuse 38, or via the "Turn Signal and Hazard Lights". It's a rather complicated path, but the power appears to originate at Fuse 46 (10A; Hazard Lights) and can only reach the front side markers if the Turn Signal/Hazard Relay is energized by the Cancel Unit (power coming from Fuse 5 to the relay coils).

A third path exists via the SCU, which appears to be able to energize the L/R control relay coils independent of the hazard switch. This would again send power to the front side markers via Fuse 46.

Thus, I will do two things next if pulling the SCU plug doesn't work: first, check that the hazard switch isn't pressed (could easily have done so during assembly). If that doesn't work, second, pull fuse 46 and see if the lights go out. I'll report back and then we can go from there!

**EDIT** Moved this discussion to a new thread in the Electronics section, since it belongs here more than the build thread and can possibly help future owners.

 

[cmc140]

I certainly never achieved a black belt in internet troubleshooting. However, I learned some hard lessons when putting cars together that have been apart for a long time.
When everything worked before was the engine in the car? If I recall?? there are two positive connections (where you can jump start) and 1 ground in the rear hatch fuse box. Is it possible that something got connected to the wrong spot there? Also, you installed a new battery ground cable. Maybe it’s worth retracing your steps on that too. I’d try to relax and just go over the easy stuff first and make sure you didn’t accidentally power your grounds somehow.

 

[Honcho]

I certainly never achieved a black belt in internet troubleshooting. However, I learned some hard lessons when putting cars together that have been apart for a long time.
When everything worked before was the engine in the car? If I recall?? there are two positive connections (where you can jump start) and 1 ground in the rear hatch fuse box. Is it possible that something got connected to the wrong spot there? Also, you installed a new battery ground cable. Maybe it’s worth retracing your steps on that too. I’d try to relax and just go over the easy stuff first and make sure you didn’t accidentally power your grounds somehow.

I suppose a silver lining is that almost 90% of the time spent on the car between the last successful power-up and this disaster was on the engine and transmission- off of the car. Therefore, not much changed inside the car during that time. It's a short list:

(1) Plug in AP1 ABS harness;
(2) Plug in SCU
(3) Plug in HVAC
(4) Plug in CCU, Clock, Cigar Lighter, Aspirator Fan (when installing center console)
(5) Wire AT-MT changes and other modules.

I've unplugged all of the above except for the SCU and nothing changed.

Also, I was using this new battery ground cable last time...when everything worked! LOL

 

[Honcho]

Quick update:

Confirmed hazard light switch in off position.
Disconnected the SCU --> Side markers still go on when batt neg is connected.

Pulled Fuse 38 --> Side markers still go on when batt neg is connected.
Pulled Fuse 46 --> Side markers still go on when batt neg is connected.

We have a result:

When I unplug the green connector from the Integrated Control Unit, the side marker lights go out. All other symptoms remain.

 

[Old Guy]

Since pulling fuse #38 does not cause the side marker lights to go out you have confirmed that you have some kind of sneak or back feed circuit in operation. There is a whole bunch of 'stuff' connected to the fuse #38 circuit which means that there are lots of potential candidates for the source of the sneak or can also be affected by the sneak. See pages 11-7 and 11-8 for the list of stuff. Of note is that the rear side marker lights are supplied from this circuit, yet you report that the rear side marker lights are not lit up. This may mean that the sneak is not affecting this circuit directly. Pull the tail light relay and then pull fuse #38 and check for voltage on both sockets for the fuse. If both sides are dead then the sneak circuit is not originating from the #38 fuse circuit which points a finger at the ICU.

In the 1992 ETM pdf I got from Lars the front side marker lights are on page 110. If the green plug was C401, all you did was disconnect the side marker lights from the ICU and having the lights go out would be normal - because you have disconnected them. If the green plug was not C401 then you might have landed on the source of the problem. The ICU side marker flasher unit certainly gets an input from the hazard lights; but, I don't know that they actually supply power to that circuit. That part is confusing because the diagram on page 70-1 implies that the side marker flasher does not get power from the constant 12v supply to the ICU. The side marker flasher unit is only shown with connections to fuse #38 and the hazard / turn signal circuit. Since the ICU is an electronic module it could have suffered from capacitor failure and electrolyte leakage which is generating an internal sneak circuit.

Since you mentioned that you also had blower 'issues', note that the Blower controls and the Park lights share a common ground G301 with the side marker lights. When your front side marker lights are lit, are they normal brightness or are they dimmer than normal? If they are dimmer than normal then check the connection of G301 to the chassis. If G301 is not well grounded you can have a reduced voltage back feed path which can cause erratic operation.

 

[Honcho]

Since pulling fuse #38 does not cause the side marker lights to go out you have confirmed that you have some kind of sneak or back feed circuit in operation. There is a whole bunch of 'stuff' connected to the fuse #38 circuit which means that there are lots of potential candidates for the source of the sneak or can also be affected by the sneak. See pages 11-7 and 11-8 for the list of stuff. Of note is that the rear side marker lights are supplied from this circuit, yet you report that the rear side marker lights are not lit up. This may mean that the sneak is not affecting this circuit directly. Pull the tail light relay and then pull fuse #38 and check for voltage on both sockets for the fuse. If both sides are dead then the sneak circuit is not originating from the #38 fuse circuit which points a finger at the ICU.

In the 1992 ETM pdf I got from Lars the front side marker lights are on page 110. If the green plug was C401, all you did was disconnect the side marker lights from the ICU and having the lights go out would be normal - because you have disconnected them. If the green plug was not C401 then you might have landed on the source of the problem. The ICU side marker flasher unit certainly gets an input from the hazard lights; but, I don't know that they actually supply power to that circuit. That part is confusing because the diagram on page 70-1 implies that the side marker flasher does not get power from the constant 12v supply to the ICU. The side marker flasher unit is only shown with connections to fuse #38 and the hazard / turn signal circuit. Since the ICU is an electronic module it could have suffered from capacitor failure and electrolyte leakage which is generating an internal sneak circuit.

Since you mentioned that you also had blower 'issues', note that the Blower controls and the Park lights share a common ground G301 with the side marker lights. When your front side marker lights are lit, are they normal brightness or are they dimmer than normal? If they are dimmer than normal then check the connection of G301 to the chassis. If G301 is not well grounded you can have a reduced voltage back feed path which can cause erratic operation.

Yes, the green connector I removed was C401. I also am confused about how the power routes through the ICU to the lights. It seems like there are two possibilities: (1) an internal relay is energized by the internal ICU circuitry and power coming into the ICU from Fuse 5, which allows power to flow to the lights from Fuse 38; or (2) the ICU itself is sending 12V to the lights directly from Fuse 5. Notably, many of the other "issues" appear to be related to items on the Fuse 5 circuit. Page 23-208 of the FSM describes the ICU as a "Side Marker Relay Circuit", which would seem to suggest option (1) above.

If it is an internal relay in the ICU, then we're kind of back to square one. The best I can figure is that 12V can pass though the ICU to the front side markers from Fuse 38 or 46, but pulling those fuses does not cause the lights to go out. If it was a sneak circuit on either Fuse, wouldn't pulling the fuse break the circuit and cause the lights to go out?

I also reached the same conclusion as you and checked G301- it's good and tight on bare chassis metal at the ground point. However, I note that the side marker lights are full brightness.

It seems like there are two "buckets" of problems- Problems related to non-switched 12V power that are caused by simply connecting the battery negative and problems that manifest when the ignition switch is switched on (blower on high, gauges, etc). The ECT3000 circuit tone tracer arrives today, so I'm going to pull the glovebox and expose all of those wires. This is really the only place on the car where I disturbed wires after the successful test of the headlights, turn signals, etc. At a minimum, I can start tracing any shorts fuse by fuse and hopefully the tool will point me to the area where there is a problem. In the meantime, I'll pull the tail light relay and then pull fuse 38 and check for voltage on both sockets for the fuse. Does anyone know a good way to pull those front bay relays? Every time I do it, I end up breaking one of the tabs on the bottom of the relay.

I'm about to go hoofing through he rocky mountains myself, so won't be able to do much more until Monday.

 

[docjohn]

Get shorty.......wow would you have ever have predicted that wayward electrons would have been such a hurdle....vs actually rebuilding a whole car....

 

[Honcho]

Get shorty.......wow would you have ever have predicted that wayward electrons would have been such a hurdle....vs actually rebuilding a whole car....

On a big project like this, I have learned to expect the unexpected. In one way, it's frustrating. In another, it's a new, interesting challenge.

 

[docjohn]

yep that's the only way to deal with a project like this....how many backyards are littered with unfinished projects because of bad attitudes...

 

[Old Guy]

This plier like thing has little tangs that slide under the base of the relay and make it easier (slightly) to pull the relay straight up.

Universele Automotive Relais Demontage Klem Zekering Puller Auto Remover Tangen Clip Hand Tool Geschikt Auto Reparatie Tool| | - AliExpress

Once the relay is out, if you coat the terminals with Ox Gard conducting grease it will make future removal easier. If you know which side of the fuse is the load side, then all you really need to do is check there and don't need to remove the relay. With the fuse removed there should be no voltage present on that terminal. If you have voltage then you know the general location of at least one problem. If yuo don't know which side is the load then you need to remove the relay which would normally insure both terminals should be dead.

I will think a bit about why your headlights don't work. I didn't see any interconnection to the ICU so it may be a separate problem. I will try to generate a fault test tree. I have helped other people with headlight issues; but, a completely dead headlight system that wasn't related to a battery problem is new and interesting.

Enjoy the Rockies and remember your bear spray.

 

[Honcho]

This plier like thing has little tangs that slide under the base of the relay and make it easier (slightly) to pull the relay straight up.

Universele Automotive Relais Demontage Klem Zekering Puller Auto Remover Tangen Clip Hand Tool Geschikt Auto Reparatie Tool| | - AliExpress

Once the relay is out, if you coat the terminals with Ox Gard conducting grease it will make future removal easier. If you know which side of the fuse is the load side, then all you really need to do is check there and don't need to remove the relay. With the fuse removed there should be no voltage present on that terminal. If you have voltage then you know the general location of at least one problem. If yuo don't know which side is the load then you need to remove the relay which would normally insure both terminals should be dead.

I will think a bit about why your headlights don't work. I didn't see any interconnection to the ICU so it may be a separate problem. I will try to generate a fault test tree. I have helped other people with headlight issues; but, a completely dead headlight system that wasn't related to a battery problem is new and interesting.

Enjoy the Rockies and remember your bear spray.

Thanks! A little more to marinate on:

With the tail light relay connected, I confirmed with a multimeter that Fuse 38 (properly) has no power under the following conditions: (1) battery connected; (2) C401 disconnected at the ICU; and (3) SCU disconnected. Under the same conditions, the ECT3000 reported no shorts or opens on the Fuse 38 circuit. Something is causing the ICU internal relay to energize and send power to the front side markers.

It's more than the headlights not working- the entire combination switch is dead. None of the switches work, nor does the headlight switch under the gauges. All of that stuff should (and did) work with the ignition off.

 

[Old Guy]

Thanks! A little more to marinate on:

With the tail light relay connected, I confirmed with a multimeter that Fuse 38 (properly) has no power under the following conditions: (1) battery connected; (2) C401 disconnected at the ICU; and (3) SCU disconnected. Under the same conditions, the ECT3000 reported no shorts or opens on the Fuse 38 circuit. Something is causing the ICU internal relay to energize and send power to the front side markers.

It's more than the headlights not working- the entire combination switch is dead. None of the switches work, nor does the headlight switch under the gauges. All of that stuff should (and did) work with the ignition off.

That is interesting result on the 38 fuse circuit. Have you tried disconnecting the ICU completely to see if any problems 'went away'. Clearly the things directly connected to the ICU like the front side marker lights will cease to function; but, it would be interesting if some of the other problems like the odd blower operation resolved themselves.

Yes, I noted your comment about a completely non operational combination switch; but, being an old guy my multi thread processing ability is limited. As an old mentor of mine said ' eat the elephant one bite at a time'. If you can figure out why the headlights don't function it may reveal a common problem affecting the other functions in the switch. That is for tomorrow's coffee and whenever you get back!

 

[Old Guy]

For when you return!

I had a look at the headlight circuit - pages 100 & 100-1 in the 1992 ETS. The bulb control circuit is fairly simple (the headlight retractable control not so simple). When you tried your headlights, I assume that the retraction mechanism did not raise; but, did you check to see if the headlights illuminated while it was in the closed position?

Assuming the headlights did not illuminate in the closed position :

The headlight illumination circuit is a little more convoluted than shown on 100 & 100-1. Pages 12 and 12-1 give a more complete path showing all the convolutions; but, it looks functionally the same. The headlight relay should have an unswitched supply.
Test steps
1 - The relay coil ground to switch connection goes to the flash to pass switch on the turn signal switch. Its pin 10 blu/red wire on C416. Take a wire and back probe pin 10 to briefly connect it to ground (use a known good chassis ground because the grounds in the steering column area may be faulty). The headlight relay should 'click' and the low beams bulbs should illuminate. The doors may or may not open.

If the low beams do not illuminate then
1a- check the + 12 v supply to the headlight relay (page 10-1). This is in the under hood fuse / relay box and does not have a primary fuse. This supplies a bunch of other important stuff.
1b - If you have 12 v at this point confirm that the relay is not faulty.
1c- if the relay works and you have 12 v then failure of the pin 10 ground test indicates a bad connection between the relay box and the turn signal switch.

2 - If you pass step 1 (ground pin 10 and the lights illuminate), try the flash to pass function on the turn signal switch. If this does not work then I think it is likely that you have a ground connection problem - someplace. Both the turn signal switch and the combination switch share a common ground. This ground exits the combination switch on a black wire and goes to the cruise control switch and then exits the cruise control switch on a blue wire going back to the turn signal switch. The diagram shows no connectors so I assume this is all hard wired. The ground then exits the turn signal switch on a purple wire which goes into C600 on the cancel unit and exits the cancel unit on C600 on a black wire. The black wire eventually gets to G401 which is shared by a bunch of other stuff. A bad connection to G401 could explain a lot of your problems including failure of the retractable headlight control.

If the low beam illumination circuit works correctly but the headlight doors don't open that is a different problem.