72 GMC CUSTOM 1500 Dashlights Problem
 

I replaced the dash cover on my 1972 GMC P/U.

Since the dash light bulbs needed replacing and I had removed the dash, I replaced all of the bulbs...
plus I went ahead and put in a new headlight switch as well.

After getting the dash cover and headlight switch in, I installed the dash bezel with gauges. Once I had it installed I connected the battery and tried the dash lights out.

I pulled the knob out for the lights and...no dash lights, although all exterior lights are working.

I turned the knob back and forth and the only light that came on was the interior light.

After twisting the knob for the headlight switch back-and-forth several times I finally got the dash lights to come on, but just the upper ones. The bottom row, alt-temp-oil lights at the bottom of the dash didn't come on.

Plus the left turn signal doesn't work, no light, although the right turn signal does.

Anybody willing to help me figure this out please?

I'm going to take my test light and check the connectors for power at the headlight switch and the connector on the wiring harness for the gauges.

What I'd like to know is which wires should show power with the ignition switch off and which should show power with the ignition switch on.

As always, much love and respect to you all and all help is deeply appreciated!

Keith
(slowboy)

Re: 72 GMC CUSTOM 1500 Dashlights Problem
 

Hi Keith did you install a new aftermarket headlight switch ? I bought one and it was defective right out of the box. I would put your old one back in for a simple test…checkyour grounds too.

Re: 72 GMC CUSTOM 1500 Dashlights Problem
 

Hi 54blackhornet, thanks for quick reply!

It is the -
Standard Motor Products, Inc. - Headlight Switch (DS-155T)

I 'did' purchase the switch off of Amazon.

I'll do as you say and put the old one back in, but the knob on old one was having to be moved back and forth sometimes to get the dash lights to work as well. That was one reason why I bought a new one.
I thought 'that' switch was bad.

Re: 72 GMC CUSTOM 1500 Dashlights Problem
 

x2 with the aftermarket switch.

I recently replaced one after refurbishing a instrument cluster. Wasted all kinds of time troubleshooting light issues that developed after installing switch and cluster. Decided to put my old switch back in. Everything worked fine. Just my experience but worth a try.

Re: 72 GMC CUSTOM 1500 Dashlights Problem
 

Thanks for reply CKfan!

Re: 72 GMC CUSTOM 1500 Dashlights Problem
 

Re: 72 GMC CUSTOM 1500 Dashlights Problem
 

I would try some aerosol electrical cleaner on the original switch and then blow it out well. Just might restore the switch back to functioning properly.

Re: 72 GMC CUSTOM 1500 Dashlights Problem
 

Thanks 54blackhornet...I'll do this before re-installing switch.

Re: 72 GMC CUSTOM 1500 Dashlights Problem
 

Can you please tell me where the grounds are located for the headlight switch and the dash, please?

I put old switch back in and I'm having the same problem except now my headlights aren't working [parking lights are] and my dome light doesn't work either now.

Thanks,
Keith

Re: 72 GMC CUSTOM 1500 Dashlights Problem
 

-
If memory serves me correctly there are 2 different switches. One has 7 terminals and one has 8 terminals. Make sure you have the correct one for your wiring plug.

(EDIT) I found a post by GMCPauls:

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Re: 72 GMC CUSTOM 1500 Dashlights Problem
 

The fact you are getting some light and some odd function of the turn signals points to poor ground or connection of power to the cluster. The ground is really a bugger on the cluster. The attaching nuts for the circuit board (plastic film) are the ground points for negative side of the lamp sockets that you rotate into the cluster. The contact of the circuit board to the grounding nuts of the cluster is often compromised when moving things around in that area. I had this problem when I added a Tach. I found the problem by accident after messing with the front turn signal circuit half the day. Adding a jumper lead, looping to both circuit board mounting post, and then to a good ground on the dash fixed all the problems.
This picture is non Tach circuit board as viewed from the front of the cluster. The four lamp sockets across center of the board are the illumination lamps. All the others are indicator lights. (Turn, Brake, Hi Beam and I can't recall the last one).
The two 1/4" holes on the upper left and right are the grounding points. You can also see that one of the grounds only grounds one bulb(Right side) and also runs into the harness connector. The other ground point is common for 3 of the 4 illumination bulbs, the bright light indicator, and the 2 turn signal indicators. Because of the common grounding of these indicator lights, it is why some seemingly unrelated items act strangely when one of the cluster grounds goes bad.

Re: 72 GMC CUSTOM 1500 Dashlights Problem
 

There is only one row of illumination lightbulbs. There are four. Those four are responsible for lighting the entire instrument cluster.

Re: 72 GMC CUSTOM 1500 Dashlights Problem
 

I have 8 pin....have had 8 pin since I bought truck in '89.
Although it is 8 pin there's only 6 terminals being used on the headlight switch.
(1) Dome ground-white wire, (2) 12 V input-red wire, (3) dash lights-dark green, (4) High/Low beams-light blue, (5) 2 brown wires together on terminal 5, and (6) an orange wire to the Dome/Courtesy terminal.
Are the 2 brown wires on terminal 5 - 1 parking lights and the other headlights? If so, why would parking lights work but headlights not work?
Another thing that puzzles me is does the Dome ground terminal count as one of the pins when determining if 8 or 7 pin switch?

Thank you,

Keith
slowboy