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c4 headlights issue- how to test between module and switch?

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Old 06-02-2018, 02:50 AM
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Default c4 headlights issue- how to test between module and switch?

Hi all,

My headlights aren't working and I've tested per the manual most of the stuff. I've tested power to both wires. I've hotwired and they've turned on.

Is there an easy way to test whether the signal is getting to the module? I'm 50/50 on ordering a new module. Car is a 92
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Old 06-02-2018, 09:25 AM
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If yellow wire has 12v when headlights are turned on the headlights should come on. The motors have fused red wires. The black wire is gound. When switch is off white wire is applies 12v module to close headlight doors.
if these are present i think you need module.
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Old 06-02-2018, 02:42 PM
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Originally Posted by qwiketz View Post
My headlights aren't working and I've tested per the manual most of the stuff. I've tested power to both wires. I've hotwired and they've turned on.

Is there an easy way to test whether the signal is getting to the module? I'm 50/50 on ordering a new module. Car is a 92
Hi Russ! Sorry, but your question is confusing. Headlights not illuminating, or headlight doors not opening?? The "module" is only for the doors, and has nothing to do with the headlights not illuminating.

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Old 06-05-2018, 12:50 PM
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Originally Posted by Hot Rod Roy View Post
Hi Russ! Sorry, but your question is confusing. Headlights not illuminating, or headlight doors not opening?? The "module" is only for the doors, and has nothing to do with the headlights not illuminating.

Hi Roy,

I don't believe either is happening. I had one of the 12v lines that was bad which I fixed. Now it's time to see if I have a break between the switch and the module.

I was all set to do this Sunday and then the battery was dead.... I guess I let this project sit too long. Hopefully I'll get back to it tonight or tomorrow.
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Old 06-07-2018, 01:24 AM
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Originally Posted by Kevova View Post
If yellow wire has 12v when headlights are turned on the headlights should come on. The motors have fused red wires. The black wire is gound. When switch is off white wire is applies 12v module to close headlight doors.
if these are present i think you need module.
At a quick glance tonight, I'm nt seeing 12v on the yellow position on the plug going into the module. I did see 12v on the red next to it. I'm going to try to test and see where the break may be.
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Old 06-07-2018, 10:53 PM
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as a side note, anyone have any idea if the wire goes directly from the switch on the dash to the module (yellow wire)?
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Old 06-08-2018, 07:24 AM
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Yellow wire goes from headlight switch to dimmer switch. Then through C100 (bulk head connector) to headlight door module. If headlights work and doors don't bulk head connector is a good place to look.
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Old 06-08-2018, 09:14 AM
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No headlights, no doors and no 12v at the yellow. I believe I have a break somewhere between the switch and the module
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Old 06-08-2018, 09:25 AM
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The circuit from headlight switch to dimmer switch is not very long. For nothing to work either no 12v at headlight switch or break between the 2 switches. If you back feed 12v to yellow wire of dimmer switch connector do lights or doors work?
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Old 06-08-2018, 10:07 AM
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Do you have 'PARK' lights? If yes then it's safe to assume you've 'HOT ALWAYS' to light switch.

The dimmer is very accessible under the dash and I believe you just unplug the dimmer connector and check 'cavity D' (I believe that to be correct - check your FSM) and if there's power there (when headlamps switched on) then the H/L switch is maybe OK.

If nothing there do as Kevova mentions and supply a feed and check further.

I think it easier to check at 'cavity D' before going through the required effort to supply a feed.
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Old 06-08-2018, 10:07 AM
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Originally Posted by qwiketz View Post
No headlights, no doors and no 12v at the yellow. I believe I have a break somewhere between the switch and the module
found the dimmer switch and now need to test the line for continuity
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Old 06-08-2018, 04:52 PM
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Originally Posted by WVZR-1 View Post
Do you have 'PARK' lights? If yes then it's safe to assume you've 'HOT ALWAYS' to light switch.

The dimmer is very accessible under the dash and I believe you just unplug the dimmer connector and check 'cavity D' (I believe that to be correct - check your FSM) and if there's power there (when headlamps switched on) then the H/L switch is maybe OK.

If nothing there do as Kevova mentions and supply a feed and check further.

I think it easier to check at 'cavity D' before going through the required effort to supply a feed.
i do have parking lights so the switch appears to be okay so far
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Old 06-15-2018, 11:31 PM
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hi guys. No power at headlamp dimmer switch connector d with the light switch on. Hit connector d with the multi meter to a known ground and nothing. The manual says:
"check circuit 10 or 2 including fusible link m".

The question I have, is or are there any other ways to test to see if this item is getting power? Is there a point on that same harness where the power goes in? How do I test the fusible links being good. I see that they come off the power distribution block, but where do I test them closer to the harness I just pulled off? are they a, c or b by chance?
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Old 06-16-2018, 08:33 PM
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I don't have 92 manual to tell you about fusible link and location. I would check for 12v on red wire at switch, if I could not find fusible link. In sub section 8a-201 there should be an illustration of location of fusible link.
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Old 07-03-2018, 01:43 AM
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got a question. I tested for continuity of the "d" wire to the switch using one of those harbor freight cable tracers. It seemed okay back to the headlight switch. I tested again for 12v at the d position on the headlight dimmer and no power.

took apart the dash to expose the back of the headlight switch. Pulled the connector and tested the red wire and saw 12v. Tested continuity of the yellow cable again and have continuity.

any other ideas?
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Old 07-03-2018, 07:27 AM
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Originally Posted by qwiketz View Post
got a question. I tested for continuity of the "d" wire to the switch using one of those harbor freight cable tracers. It seemed okay back to the headlight switch. I tested again for 12v at the d position on the headlight dimmer and no power.

took apart the dash to expose the back of the headlight switch. Pulled the connector and tested the red wire and saw 12v. Tested continuity of the yellow cable again and have continuity.

any other ideas?
Be sure you check both fusible links coming off the block. I had an odd situation with my headlights, learned a lot, and the culprit was bad fusible links. So far, they have you on the right track and I agree with them (they helped me sort mine!).
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Old 07-03-2018, 12:24 PM
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Originally Posted by JasBass View Post
Be sure you check both fusible links coming off the block. I had an odd situation with my headlights, learned a lot, and the culprit was bad fusible links. So far, they have you on the right track and I agree with them (they helped me sort mine!).
thanks for the input. What type of tests did you have to run to diagnose your situation? I'm wondering if there are additional tests that I can do at the harness to the dimmer or at the switch.
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Old 07-05-2018, 07:23 AM
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Originally Posted by qwiketz View Post
thanks for the input. What type of tests did you have to run to diagnose your situation? I'm wondering if there are additional tests that I can do at the harness to the dimmer or at the switch.
My issue was a little more odd where they would only operate in one direction. Most of what I tested was making sure power was actually making it to the motors. Thankfully the switch itself was functioning. Do you have an FSM for your model year?
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Old 07-05-2018, 10:09 PM
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You have 12v on red wire to headlight switch. Do you have 12v on yellow wire coming out? You could remove terminals from connector inspect crimp for broken wires.
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Old 07-06-2018, 05:52 AM
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If you pull the light sw to On and toggle the Hi/Lo Dimmer sw do you see the Hi indicator on the dash light/extinguish?

On my '88 the Yel wire goes from the HL Sw to the HL Dimmer Sw "C" and then to the HL Control Unit at "C2-A".

Last edited by 65Z01; 07-06-2018 at 05:57 AM.
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