AC only runs for 7 seconds?

[boca rat]

I got an exchange unit from BrianK a few months back on the CC HU for my 91. Worked great, fixed the max or nothing fan issue.


A few weeks ago I started getting AC dropouts, wouldn't work for a bit, then would come back. Finally it seemed to have died completely Sunday,


Dropped it at the dealer this morning, they diagnose and say that the AC kicks in for 7 seconds every time, then quits, so they think it is a computer issue as system is full of Freon, compressor all works OK. I am running a series 1 AEM that hasn't been touched in a few months.


So the question is, do you have any suggestions on how to debug this so I can eliminate either the CC unit or the AEM? Tomorrow morning primer BUGS2K who works at the dealership is going to swap in his factory EMS and see if that makes a difference.


I am assuming that the CC HU sends a signal to the EMS saying "AC on please", then the EMS sends a signal to the compressor, right?


Thanks!

 

[Jinks]

I got an exchange unit from BrianK a few months back on the CC HU for my 91. Worked great, fixed the max or nothing fan issue.


A few weeks ago I started getting AC dropouts, wouldn't work for a bit, then would come back. Finally it seemed to have died completely Sunday,


Dropped it at the dealer this morning, they diagnose and say that the AC kicks in for 7 seconds every time, then quits, so they think it is a computer issue as system is full of Freon, compressor all works OK. I am running a series 1 AEM that hasn't been touched in a few months.


So the question is, do you have any suggestions on how to debug this so I can eliminate either the CC unit or the AEM? Tomorrow morning primer BUGS2K who works at the dealership is going to swap in his factory EMS and see if that makes a difference.


I am assuming that the CC HU sends a signal to the EMS saying "AC on please", then the EMS sends a signal to the compressor, right?


Thanks!

That sucks. Another reason I am staying Oem ecu from Japan and forfeiting the 50 extra ponies.

 

[NSXGB]

Have you run the CCU diagnostics? Run that if not and report back.

 

[boca rat]

Not clear from the description if the CCU diags give general info or if it's all for the CCU unit?

I found this old quote from BrianK: "Any time the ccu is calling for cooling the condenser fans should run. The compressor should also be running unless the Freon pressure is to high or to low or the engine coolant is to hot."

He also gave me this:

"The ac request signal goes to the radiator fan control unit and then to the ECU and then to the compressor relay. When the compressor shutdowns do the condenser fans also go off?"

The condenser fans are staying on apparently so the fault does not seem to be in the CCU.

So that implies that either the ECU/AEM is telling it to shutdown after 7 seconds, or there is a high/low pressure issue in the system. I think!!!

So I guess next step is to check signal at the compressor to see if it is still there after 7 seconds? If so, there is a pressure issue, if not, then the signal is getting cut off by the ECU or between the ECU and compressor.

Did I get that right?

Another prime member locally offered his factory ECU for testing this morning.

If it's the AEM I guess I need to find another one, dump out the tune and load it into the new one.

More great info from BrianK:

"If the fans are running then the signal is getting to the fan control unit so the ccu is outputting as it should. The fan control will stop the signal from going further if the freon pressure is to high or low or the engine temp is too high. You need to check the signal coming out of the fan control unit."

 

[Briank]

I would be checking the signal coming out of fan control unit.

 

[boca rat]

OK, seems that the fan control signal is good, and swapping in a factory ECU showed everything running fine, so seems the AEM is the culprit. Swapping in a spare AEM next

 

[boca rat]

And...this sucks.

Got a spare AEM from BUGS2K, copied over the tune (luckily I went to do this at Stage 6 in Jacksonville as there was a lot at stake if I screwed this up as a first timer, where we found my laptop wouldn't talk to the AEM anymore so used Dwight's), plugged it in, the same issue, no AC. Looking in AEMPro, switch 6 is permanently on. When overridden to run continually, compressor runs.

AEM support says that the AC switch 6 going bad is not uncommon.

So at this point I either a) have two bad AEMs at my disposal, or b) something in the electrical system is frying switch 6, or c) there is something that the factory ECU handles but the AEM doesn't in some way or shape or form. Maybe the signal is too weak for the AEM to recognize?

Further suggestions? I am a little loath to buy another AEM....and an AEM series 2 is cost prohibitive right now.

How can I check that the signal from the harness coming in to the AEM on the pin that connects to switch 6 is being triggered correctly? It needs to be plugged in to test this, right?

 

[jwmelvin]

Can't you verify that signal with a voltmeter?

 

[Briank]

The ac signal from the ccu is active off, the signal from the fan control is active low also so if you are seeing a on signal at the AEM it's not "on".

 

[boca rat]

Can I test any of this without the motor running and ECU harness disconnected from the AEM though?

If so, Brian, once I figure out which pin on the harness si for the AC, can you explain that in simple laymens voltage terms? )

 

[Briank]

Yes you can test the operation with the key on but without anything connected you will not see any switching. The AC run request from the ccu and then from the fan control unit are both active low. When the voltage is low that means its ON and off then the voltage is high the meaning for the system is its OFF. I would assume the aem input being on would mean high voltage.

 

[boca rat]

r.e. voltage what is high and low? 12v and 3v? Or....? Maybe the factory ECU works of a slightly different range than the AEM?

 

[Briank]

I don't know what the exact spec is but you should see near 0 or near 12, I think in your case you have near 12v so that means you are not getting the ac run request from the Fan control unit. Either its broken or one of the signals the fan control unit uses to turn off the ac request signal is not correct. Those signals are coolant temp or freon pressure to high or to low, most likely its the freon pressure.

 

[boca rat]

Update: yesterday the plan was to take it back to the dealer to get Freon level checked again, then over to a tuner to diagnose further with the AEM. On the way there, the AC started working again....so after all this I still have an intermittent issue rather than an undiagnosable failure. So possibly the AC working OK with the factory ECU was also a case of the system working well at the time and I do have a failing component somewhere. Now waiting for it to fail again.....(

 

[GraemeD]

check the wiring at the ECU, specifically the contact/socket in the connector. check it's grip on the pin in the ecu. you will need a spare pin of the same size and insert it in and there should be some drag. they can get damaged by people testing things by forcing wire or other things into it and bending the little spring in the contact. I had the same issue with the EGR circuit on my car.

 

[boca rat]

So the AC is still working fine, but now I have another issue. Seems the blower has started working intermittently now...

Then, this morning, started car up after a week off the road, and heard a kind of squeal from under the dash and then silence. Drove car with windows open (first time this year cool enough to do that, luckily) and at some point turned it back on and all working fine, so not a dead blower motor after all. Ideas? The AC issues are starting to p*ss me off a little...

 

[ftuhy]

If the squeal was from your blower motor, I've had that happen on the SLK320. While I couldn't disassemble the motor without major weld cutting, I was able to drip some Mobile 1 down the shaft into the bearing. I let it sit for a few hours, then cleaned off the excess oil. The squeal has been gone for 2 years now.

Of course your squeal could easily have been from something else (I think you may have a poltergeist in your AC system).