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cannot switch off high beam

15 February 2009
Hi dear friends,

anyone ever had this problem? When i turn on the lights, high beam automatically comes on together with the blue warning light in the dash.
When i pull the high beam switch, it stays on, cannot switch off.
I just installed another switch, but the problem remains. Switch was used, so not 100% sure that it doesn't have the same problem..

Is there anything esle i should check or replace or is it certainly the switch which is the problem?

Many thanks,

had this same problem in my old 93 Honda accord it was a relay.

never had this problem in an NSX yet but makes me wonder if an aftermarket alarm may be the culprit.
High beam turns off by breaking ground at the switch. I had an HID set up that grounded itsself out causing the same issue you are having. High beams all the time. Somthing to check.
I would also normally go with a faulty switch (contact not opening) as the most likely cause of the problem. Since you have the original switch out, you should be able to check its high beam switch operation. Get the pin out for the switch from the shop manual. Check between pin 13 and pin 1. You should get continuity when high beam is switched on and open circuit when high beam is switched off (opens the ground connection as noted by DRIFTER). If the connection stays closed all the time then your original switch and perhaps your replacement is faulty. The high beam circuit is connected through an internal diode in the switch, so check the operation with the red (+) lead or your test instrument connected to pin 13 and the black lead connected to pin 1. Otherwise the circuit will test as open all the time.

Since this problem only occurs when the headlights are switched on, I think the problem might be in the headlight flicker / passing circuit. You can check this between pins 6 and 10 on the switch. This should be open circuit except when you do the flick to pass operation. It should momentarily close when you flick the headlights. If it stays closed all the time, then the switch is faulty.

If the original switch is OK, then the problem is elsewhere. The next thing to check after that is the retractable headlight control unit and the security control unit - ouch!
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Thank you for the tips "old guy", i will definetly check when checking the replacement relays i ordered.
Will post if i succeed in resolving..
Hello again "old guy",

What do you mean exactly by "Get the pin out for the switch from the shop manual"?
I took the old switch out of the car completely (so disconnected the green 10-p and grey 16-p connectors) and did the tests as described in the shop manual.
I must be doing something wrong, cause nothing seems to test right although everything was working in the car, except for the high beam staying on all the time the lights were on.

I am not familiar with testing diode circuits; i know they only let current pass in 1 direction, but anyway i put my testing cables, everything (concerning diode circuits) tests open,
only 16 to 7 and 10 to 6 test as they should, but there are no diodes there.
My testing device is a normal 9v multi meter with which i test the circuits sonorically.

What am i doing wrong?

The pinout is the diagram which shows what wire is in what position in the connecting plug for the switch. Its on page 23-177 of the 1991 version of the shop manual. The same page has the description of which pins are connected together when the switch is in its different positions (I call it the switch function diagram).

Multi testers (digital or analog) have an internal battery which polarizes the test leads when you are doing continuity or resistance tests. The red lead is polarized + and the black lead will be the corresponding - (assuming you have them plugged into the tester correctly) . As an example, if you look at the switch function diagram, you will see that pin 13 is connected to pin 1 when the dimmer switch is in the high beam position. You can see that it is connected internally through a diode. Because of the effect of the diode, the red test lead has to be connected to the 'pointy' side of the diode and the black test lead has to be connected to 'flat' side of the diode. If the test leads are reversed, the diode blocks the flow of test current causing the switch to show an open condition even though it may in fact be closed. Just about every contact function in the switch has a polarizing diode. Get the multimeter connected backwards and they will all show open all the time!

Some multimeters have a continuity check function which is sort of a go / no go test. Diodes have something called a forward bias voltage. This can screw up some continuity type testers so that they may always show an open circuit even if it is connected correctly. If your tester has a diode checking function, better to use that or just use the resistance measuring function. 16-7 and 10-6 test out for you OK because they don't have the internal polarizing diode.
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Ok, back again, but no good news yet,

I replaced the lighting and dimmer relays, but still no luck in switching off the high beam..
Testing of the light switch had the following results: 13 to 6 is ok (gives a voltage drop of .657) but stays closed no matter how i put or turn the light switch.
All the other tests 4 to 1, 10 to 1, 13 to 1 and 13 to 6 test open..
The only thing that was wrong are the high beams, so parking lights, low beams all function, even with those test results on the switch???
Testing of the light switch had the following results: 13 to 6 is ok (gives a voltage drop of .657) but stays closed no matter how i put or turn the light switch.
All the other tests 4 to 1, 10 to 1, 13 to 1 and 13 to 6 test open..

13 to 6 is just an internal connection through a diode and should always measure closed regardless of any of the switch positions (leaves me wondering why they wired this up to a switch when it doesn't 'switch). So that test result is correct. I assume your second reference to 13 to 6 being open is a typo?

In order for your low beams and parking lights to function, 10 and 4 have to connect to 1, that is how the lighting relay and taillight relay get their ground path. Since the low beams and parking lights are coming on, we know that the 10 to 1 and 4 to 1 connections have to be working. There must be something odd happening with the test procedure. Is there any chance that you are not making a good connection with pin 1 when you do the test?

Just to re confirm, the unplanned high beam operation only occurs when you turn the headlights on, it does not occur when the headlight switch is off or in the park position and goes away when your turn the switch back to the off position?
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Yes, the high beam just comes on whenever i turn on the headlights and does not go away when i switch on or off.
I remember about a year ago i had the problem that i sometimes could not switch ON the high beam, but that 'went way', maybe a sign that something was already starting to break or go wrong.
Recently i renewed the front fog lights (just replaced the old ones by new Honda ones) and had to replace a connector in the plug, but i don't think this could have something to do with my recent problem.

Connection with pin 1 is certain, i checked a zillion times, but all test open as mentionned above.
But, as you said, 10/1 and 10/4 have to be connecting since parking and low beams work.

Should i try to replace the retractable headlight unti? Does it in any way affect the function of the high beams?

I read about HID sets with ballasts causing the same problem, but my car has just the Original lights and wiring seems ok on first sight..
Yes, the high beam just comes on whenever i turn on the headlights and does not go away when i switch on or off.

That is a little different than what I had thought was happening. I thought that when you switched the headlight switch to off, that the problem went away. According to the wiring diagram, the only thing that can keep the lighting relay energized (the lighting relay needs to be energized for either the high or low beams to operate) when the headlight switch is in off position is the security control unit.

The blue/red wire from the lighting relay and the light switch goes to the 22 pin connector on the security control unit. Switch the headlight switch to on (lights go on) and then switch if off. If the high beams are still on, go to the security control unit and pull the 22 pin connector out. If this causes the lights to go out, there is a problem with the security control unit. The SCU could be faulty or there could be something which is causing the SCU to continue applying a ground which is energizing the lighting relay. You would have to go through the SCU input test to confirm its operation.

It is possible that the headlight retractor unit could be causing the problem; but, in order for that to happen I think you need multiple failures. The retractor control circuit is more complicated and it is not so obvious how it works.

Just to confirm, when you talk about the headlights going on, you mean the bulbs are going on and staying lit, not just that the headlight units are going into the up position?

Finally, I am looking at a North American wiring diagram. The European wiring might be different than the North American wiring. If you have the Euro wiring diagram, is there any way that you can compare the wiring diagram for the lighting switch to the North American diagram on the NSX Wiki to see if its the same?
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Looking for a bit more information: what works and what doesn't: with the lights off, when you flash the lights, do the headlights raise and light up?
have you removed the head lamp fuses one at a time? 49 and 52? Same result for each side independently?
Are the lamps full bright? Have you removed the bulbs one at a time to see different results?
because the ground is switched to turn the lamps on, you probably have a ground issue. check you wires near the back of the head lamp. You could have an open wire or something grounded that shouldn't be.
unplug those fog lamps. (just to see what happens) they should only operate if the headlamps are on and the high beams are off. are they using factory wiring? or after market. how are they connected, and how are they switched?
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Ok, might be that i expressed myself incorrectly;

No lights are burning when all the switches are turned off and retractable headlights are down.
When i switch to parking lights, they light up correctly like they should, headlights stay down.
When i switch to normal headlights, lights come up and light up, but also do the high beams and the blue light in the dash. When i try to switch off the high beams by pulling the lever, i can hear the relay 'clicking' and i can see that the brightness or intensity of the high beams decreases by just a little (to put in percentages; the intensity decreases by let's say 5%) bit, but they stay lit. Same for the blue dash light; when i try to switch off the high beams, the blue light decreases in intensity, but stays lit (so the blue light and high beams ONLY switch OFF completely when i turn the switch to parking lights or off.
I am guessing since i hear the relay and something happens with the intensity, that the switch (my original one and the replacement one) is ok??

When i flash the lights, they come up, flash and go down again as they should.

I still have the factory alarm unit in the car..

I checked the wiring behind the bulbs, but seems to be ok; i agree that i could be having a grounding problem somewhere, but i don't think it is located at the bulbs.
Of course, if you guys think it would be usefull to disassemble the headlights to check the wiring more thoroughly, i could do so..

Will unplug foglights tomorrow, see what happens and unplug fuses 49 and 52 one at a time and see what happens..

Checked the diagrams euro/usa, but they look the same at first sight.
If all the lights go out when you turn the headlight switch to off, then I don't think the problem is the SCU applying a ground to the red / blue wire that I talked about in post 22. Hold off on disconnecting the SCU 22 pin connector.

It seems like the passing function switch is working and it also seems like the 'switch' part of the high / low beam switch maybe working, although your description of the relay clicking is confusing.

I have attached the schematic for the headlight switch.

If when you switch from high to low beam you are hearing the dimmer relay click to the open position, the high beams should be going out. (if the dimmer relay is staying open). The dimmer relay is the normal ground connection for the high beams. The fact that when you switch to low beam, the headlights and the blue indicator light go a little bit dim suggests to me that the dimmer relay is opening; but you have an insulation failure or connector failure that is providing another ground path for the high beams independent of the dimmer relay. On the diagram, there is a thick green line tracing the red/blu wire from the headlight bulbs to the dimmer relay (ignore all my scribbles between the dimmer relay and the switch!). If there is an insulation failure causing a connection to ground anywhere between the headlight bulbs and the dimmer relay, the high beams will light up whenever you turn the headlights on.

Simple test: remove the dimmer relay and then turn your headlights on. If the high beams go on at the same time, then you have a ground problem on the connection between the high beam bulbs and the dimmer relay. If the high beams don't go on, then we move on to a ground some place else.
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FINALLY getting somewhere; what a good tip to take out the relay!
When i take it out and switch on the headlight, what happens????

High beams come on!!

So problem should be between bulbs and relay; i checked behind the headlights, but all ok, so i also took Warrens advice and took out the fog lights.
When i disconnect the passengers side fog light, high beams go out!!!!
The fog light itself is brand new Honda part, so should be ok i guess..
The wiring and plugs are still the Original ones, but i will check the wiring that goes from the fog light into the front bumper later on, the problem or short cut will have to be there...
Good that you have narrowed it down.

With the plugs at the back of the high beam bulbs disconnected, connect your electrical tester on the resistance setting between a chassis ground and the red/blu wire. It should be infinite resistance normally, but, will probably be showing low resistance on your car. You can then wiggle wires and disconnect things to see what causes the resistance to change (other than the now obvious fog lamps).

The fog lamps are a mystery. As Warren notes, there should be no connection of any kind between the high beams and the fog lamps so why the fog lamps would provide a ground path for the high beam circuit is odd, unless it is a common chaffed wire someplace. Previously, did the fog lamps only operate on low beam as they should? If not, there might be a wiring error in the car.
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Will do the resistance checking later on, but can't really say if the fog lights were working correctly before. I suppose so, since i never experienced any problems.
I hadn't made the correlation between changing the fog lights and my high beam problem bacause i don't drive the car daily and it's been a while since i've changed them..
I only changed them because the previous owner (i 've owned the car now for 10 y) put non-Honda fog lights on the car and when i changed back to the Euro Original Honda fog lights
i remember at the passengers side wires were cut so i also had to purchase an Original plug because that was missing.
I will check later on if i connected correctly or if any other problem is visible, but i don't know why the previous owner ever removed the Original plug.. but was suspicious enough
to make me remove the fog light and discover the problem had to be there somewhere..
Ok, reinstalled the fog lights after wiggling the wires in the back of the bulb and now everything works perfectly (parking lights, headlights, high beam + flash) apart from the fact that the fog lights automatically light up with the headlights and stay lit when i switch high beam on or back off again. But maybe this is the way it is supposed to be? With all these tests and trials, i can't remember how it was before...
Nope! Fog lights should definitely not light up with the high beams. Perhaps somebody got creative with the way the fog lights were wired in. Since the North American service manual has no details covering the fog light installation, you might have to try and find somebody with the original installation instructions. Perhaps try contacting Kaz from the NSX club in GB for details.
We had freezing rain here yesterday for the first time. Probably not going to unfreeze until next year. The NSX is in deep storage now so I have to get my NSX 'fix' somehow. Glad it worked out for you!