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Old 12-06-2017, 10:08 PM   #1
Doug R'dson
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Identifying bulb purpose on instrument housing

85 GMC C1500 Suburban
I removed the instrument cluster, and haven't been able to figure out what all the bulbs are for. Anyone have a picture or illustration to help me?
Do I seem dumb.? There are a few that don't light up, and I know the bulbs are good.
Without the bulbs lighting, I'm not sure how to diagnose the problem (s).
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Old 12-06-2017, 10:21 PM   #2
Dead Parrot
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Re: Identifying bulb purpose on instrument housing

Match the bulbs to the little plastic labels or holes in the cluster face. Possible that you have a few extra bulbs not needed for the equipment your truck shipped with. Or a previous owner swapped in a cluster from a truck with a different equipment setup.

It is possible the contacts are corroded. Pull the bulb holders and lightly sand the metal contacts where they touch the circuit board.

Also lightly sand the contacts on the cluster connector.

Pull and insert the bulbs into the holders a few times to clean those contacts.

Always suspect corrosion on a 30+ year old vehicle with electrical issues.

The clusters vary a lot depending on equipment level.
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Old 12-06-2017, 10:42 PM   #3
Doug R'dson
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Re: Identifying bulb purpose on instrument housing

Thanks!! I'll try your suggestion! Why didn't I think of that?
I also have a couple that don't make a tight enough connection. How do I fix them?
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Old 12-07-2017, 03:23 AM   #4
Pontiac1976
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Re: Identifying bulb purpose on instrument housing

You can slightly open ( spread the brass flap ) re fit. Make them snug but do not not over tight.
because you might tear the plastic circuit board or copper contacts.
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Old 12-07-2017, 06:06 AM   #5
Killer Bee
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Re: Identifying bulb purpose on instrument housing

for troubleshooting, you can sort out some known good assemblies and rotate them around.. a test lamp probe is also handy for incandescent lamp circuit testing..

for a sequence of testing, start with KO/EO and headlight switch in either on position to identify all of the dash illumination lamps.. dimming them will confirm their function.. small piece of colored tape or paint marker like a junkyard pen or sharpie can be used to label or mark them.. that will narrow down the field a bit for checking the warning lamps..

I disagree with sanding anything in an instrument cluster without extreme caution.. small particles of copper and sanding grit may be detrimental in there.. and a course contact surface is not as conductive as a smooth one.. if the lamp holders are removed, cleaned, and polished with very fine abrasives elsewhere then cleared of all foreign debris, maybe productive.. but at that point, might be better off replacing them for more reliable operation..

in my profession we encounter oxidation build up on slip rings and commutators frequently.. contacts are fragile so chemical cleaning is usually the safest solution.. small plastic bristle brushes like the free toothbrushes you get from your dentist can be used to gently agitate the cleaners on the contact surfaces as well without damaging them..

I posted some suggestions in a similar thread;
http://67-72chevytrucks.com/vboard/s....php?p=8033814

here;
http://67-72chevytrucks.com/vboard/s...14&postcount=3

and here;
http://67-72chevytrucks.com/vboard/s...7&postcount=10

good luck!
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Old 12-08-2017, 08:40 PM   #6
Doug R'dson
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Re: Identifying bulb purpose on instrument housing

Thanks all for the tips! I think I have this now. If not, I'll be back for another go round.
Hi
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