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).

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.

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?

Re: Identifying bulb purpose on instrument housing
 

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!

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

Re: Identifying bulb purpose on instrument housing
 

Finally got back to work on the dash. Got all but 1 bulb sorted out and working.
There is a bulb on the upper right hand end of the cluster, behind the fuel guage. It has a white lead attached to the bulb socket, and the white lead has a "bulb" at the socket end, with the bulb protruding through the plastic toward the bulb. That buld doesn't light, and I can't figure what it's purpose is. Should the bulb light constantly? Does the "bulb" heat, then turn on something else?
Any help/ideas?

Re: Identifying bulb purpose on instrument housing
 

That is a fiber optic light. It pipes light from that bulb over to the ashtray.

http://67-72chevytrucks.com/vboard/s...ht=fiber+optic

Re: Identifying bulb purpose on instrument housing
 

^ fiber optic cable or fiber wire with lamp on the end?

low fuel level warning?
lights up right or left side of fuel gauge to indicate which side fuel fill is on?
lights up right or left side of fuel gauge to indicate which fuel tank is aligned?

just a couple of inebriated guesstimates :smoke:

Re: Identifying bulb purpose on instrument housing
 

OK, thanks for that quick response. As it is not lighting, I need to try replacing the bulb, then .... start sorting out where the circuit is broken, if still no light.