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Question: C4 (1990 -1996) Headlamp Switch testing and replacement


Well-known member
Feb 27, 2010
Southern California
1974, 1990, 2008 Coupes
C4 (1990 -1996) Instrument Cluster Dim Lighting Troubleshooting

A few questions related to a 1990 C4's headlight switch...

1. What are the expected symptoms when the dimmer switch on the headlight switch is faulty, but not broken or dead completely?

2. What is the procedure to remove the headlight switch?

3. How would one test the potentiometer or voltage on the switch to know if its OK or damaged?

Case in point:
My LCD backlighting on the speedo is dimmer than it should be. Sometimes its fine, others its barely readable at night. When I put my cell phone up to the sensor at night, the LCD is very bright, which means that something is limiting the current to the bulbs. The first thing I changed out was the DIC because I suspected the light sensor, but after doing that, Im still seeing the problem. It has to be something else. This is not a problem that seems to be related to vibration or intermittent. WHen the car first starts up, the LCD is very bright when the parking or headlights are on. But then very quickly goes dim if at night. During the day, its bright enough to see the LCD and yet not fully bright. I also suspect maybe a bulb replacement might be in order, but I don't know how to pull the dashboard and gauge PCB out.

Anyone out there that has actually seen this very thing?

First picture is as bright as it gets before it fades further and darker,

the second picture is when the ignition switch is first turned on with the light switch on also

For references purposes, I also authored a related post:
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I may have a solution afters hours of researching...
The "Incandescent Power Driver" affects the dash back lighting amongst other things orange in the cockpit. It is also known as the "ACDelco D1509B Instrument Panel Dimmer Switch" which is located under the dash to the left of the steering column.

I should have a new one by tomorrow for about $35. Will post my results later. Hope that's it because a new headlight and dimmer switch is $230 and I've already swapped out the light sensor (DIC panel) for around $50, and have a new set of dash bulbs for about $25.

The ACDelco instrument panel dimmer switch (power transistor on a heat sink)

This IC power amp/transistor (293-86655) made in Mexico and is sourced in China. Why am I not surprised.

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Looks like replacing the instrument panel dimmer switch (power amp IC) has solved most of the problems.
Now the lighting stays constant and doesn't drift in and out from very dark to being visible, plus its now brighter than it was before.
Its not perfect or like factory fresh brightness levels, but to get that I think Id have to replace the bulbs.
Thats next when I figure out how to remove the dash, without breaking anything.

First picture is the dimmer switch under the driver's side and to the left of the steering wheel column.
I had to remove some screws and take out the under dash kick panel first. WIthout breaking courtesy light and test jack.

Second picture is a comparison of the new (left) and old (right) power amp ICs

Third is my test drive at about 5pm PST where the dash is brighter than night levels due to the DIC light sensor

Fourth, is a night drive at 7:30pm PST at full brightness slider position (same with day test).
Not as bright as I wish I could make it, but its now stable and readable at night.


Would someone else with a 1990 take some comparative pictures so I can see how yours looks?
Note mine were taken with an iPhone5 which makes the LCD look a bit brighter because of how its photocell camera take in the light (no flash)

Im glad i did not buy a new headlamp switch with the dimmer slide switch on it, as now i don't think it would have helped much or at all.

Still not satisfied fully with the brightness...

im going to try replacing the LDR on the DIC PCB.

the current 23 year old one measures:
3.5 Ko-hm when placed in front of my PC monitor
2-10 M-ohm when covered or dark ambient

These values don't match any white light LDRs at DigiKey.
I suspect the dark impedance is too high by a factor of 10 - 100 (the eye responds logarithmically)
Will try a best guess replacement and see how it goes.

If someone with tech manuals for the DIC button PCB with the LDR on it
could tell me what part number is called out, or what the test specs are, that would be helpful.

Heres the DIC buttons PCB

Original and modded (on LHS, new light sensor of my choosing) which is more stable
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My modded DIC PCB with a new LDR worked in that it is now always bright. Need to tweak it some more.

Also want to replace the bulbs with a kit I bought from Mid America Motors site.

Found written instrument cluster removal instructions without picture
How to Change the Instrument Cluster in a 1996 Corvette | eHow.com

note to self:
light sensor turns off curteousey lighting if daylight, so if sensor is set for bright LCD, no door and under dash lighting.
Also CCM takes DIC and light sensor data and drives LCD and its lighting.
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I have been following your thread with much interest.

Looks like you are very near to the end.

Where did you find your new light sensor? What caused you to buy that particular one?

Where did you find your new light sensor?

VT90N1 - EXCELITAS TECH - LDR, 200KOHM, 80mW, VT900 SERIES | Newark

What caused you to buy that particular one?
A guestimate based off the the light and dark resistance specs.

The below graph is key in making a selection of the LDR as a function of ambient lighting in your car


The other part is understanding what is the relative ambient lighting in my vette to get a range to look at the above LDR response curves.
Below are lux values taken with my iPhone5 at dusk (left), noon (middle, on a cloudy day), measurement facing toward driver like the LDR sits in the DIC (1/10th of pointing up)

At this point Im still tweaking to get the desired brightness over ambient night and day control of the LCD. The wildcard is that the bulbs might be aged and my tweaks might not be necessary with new ones. This is why Im considering replacing them before I dial in the DIC PCB to work with aged bulbs.Theres a balance between getting the brightness I want, and also leaving enough control to auto dim the LCD and manual headlamp switch dimmer. If it were not so, then they would never have used a LDR in lieu of a passive one in the first place.

Note too... that if the CCM is messed up, or the wiring to the DIC from it; then thats yet another failure path that I hope isnt the case.
If there are SYS errors or mumbo-jumbo on what is displayed on the LCD; then thats a good indication that CCM has failed (there are codes to test this too).
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Ambient Sensor on a DIC I have shows a part #VT6861.

The schematics are a little blurry. So if you can't read them right click on the image, save it then print it out.

The first schematic shows the sensor. Since your replacement is making the display to bright, I would think you could put a variable pot in series with the sensor and dial in the brightness.

Next schematic shows the headlight dimmer switch and the color of the wires going to the dimmer switch.

If you unplug the headlight switch connector you could measure the resistance of the dimmer part between Pin A and Pin G and verify at the lowest setting if there is any resistance.

Last schematic shows the seat belt switch. The switch is normally closed and the Fasten seat belt indicator would light and the chime should sound as a warning to buckle up. When you fasten the belt the switch is open. If you look at where the seat belt buckles follow that piece down along the transmission tunnel there is a two pin electrical connector. Someone may have unplugged the connector.



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Thanks for the schematics and how to try an in-circuit LDR sensor resistance check. The VT6861 part number wasn't identifiable as a LDR part, it may be an internal reference rather than a vender p/n.

As for putting a series resistance to dim the LCD, I want the LCD to be as bright as she will go under sunny California ambient lighting and yet not go so dim at night that I cannot see the speed at night, as is with my case. Its logical to add resistance to reduce brightness, but it would reduce both the day and night time brightest levels. That's not the desired outcome. Which is why choosing a LDR that fits both the real life ambient LUX and what is still within the parameters to keep the dimmer switch function and the courteously triggering in range.

Lessons learned:
I suspect changing out the dash bulbs might give a brighter LCD, which might be closer matched to the original LDR sensor. Over time as the bulbs dim or other driving components or grounds fade, the optimum current flow gets compromised and the net result is a dimming of the display. Just a theory. Maybe just replacing the bulbs might have been the first remedy, assuming one can access them. My tweaking efforts are a workaround that compensated for the dimming for whatever the cause was. If it continues to fade, out go the bulbs next. For now, its working fine.
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The VT6861 part number wasn't identifiable as a LDR part, it may be an internal reference rather than a vender p/n.

From my research that part number was made by a company called Vactec. They were acquired by EG&G and became EG&G Vactec. Subsequent to that, Excelitas bought them. As noted earlier, they are still using the Vactec part numbers (VT followed by a number). I independently identified the VT900 series as a likely replacement for the VT6861, but haven't zeroed in on the exact replacement. It appears that the VT6861 part number is for the LDR with a light shield. From what I could find, Excelitas does not currently make a LDR with a shield. It might be a special part for GM or just old, obsolete and no longer listed.

The resistance of the LDS goes down as more light hits it. That means that the dashboard will be brighter if the LDS has a lower resistance with bright ambient lighting.

It appears that the photocell (another name for LDR) is glued into the light shield. I tried to push the LDR out of the light shield but could not do that.

Just for reference, my '86 Corvette uses the VT6861 LDR. I am currently having a random brightness problem, which is why this thread got my attention.
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