Topic: Partial lighting on instrument panel

[jphillips3333]

It's been out for some time ... so I thought I'd take a look.

Symptoms;
Partial lights at the speedometer - right/pass side seem to be problematic.
No lights at the fuel/volt gauges.
No shifter bezel lighting.

Work performed;
Swapped "good" bulbs from working locations into non-working locations.  Bad bulbs worked in prior "good" locations.  Good bulbs didn't work in bad locations
Checked voltage at #1 gray wire (bottom/passenger side) at back of cluster with lights on - 11+ volts.  Reinserted tight, still no lighting to the right side of the cluster.

Also, is it a best practice to pull the volt/fuel gages to get better access to troubleshoot?  I'm under the dash but it's difficult to get anywhere unless you've short arms AND small hands.

Any quick "look here" suggestions?  If one bulb is out, does it break the circuit downstream of that location?

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[ta78w72]

You've got more than one problem.  Most likely you've got a break in the cluster ground circuit for illumination.  On the other two bad lighting area's, check the socket terminals and see if you can slightly bend them out towards the bulb....I don't really recall what the inside of those look like.  Measure for 12 volts at those gray wires.

On access, pull your cluster.  You've got to do that anyway to check the passenger side of the cluster circuit.  You may have to pull a few A/C ducts too.  I don't like trying to access that area laying on my back and trying to hold my head up.

[jphillips3333]

OK ...
I'll disconnect the tach wire and speedometer cable then pull the tach/speedo cluster out.

I'm looking for breaks in the back of that cheesy plastic filmed circuit board?  Can I power it up with a test wire while it's out - positive to the #1 location, negative to the #8?  I think #8 is the opposite end of the #1 location (top, "passenger" side).

I'll pull the fuel/volt cluster forward and check voltage at those locations as well.  I hope there's some slack to do that.

Thanks ... more to come.

[jphillips3333]

Russ,
Checked voltage at both the fuel and voltage guages - 11+ volts.  Didn't see a bulb to check like the instrument cluster.  Is the bulb accessible from the front - plastic removal, pull the gauge and there it is?

Below is the cluster circuit - it appears ratty on the lower speedo side and there appears to be another bulb location under the tach side that is obscured by the circuitry - not one in it.

Part number on the circuit is 8911865 (speedo side) // PED-1 (tach side)

[ta78w72]

The socket for the fuel/volt gauge is in the back of the gauge.  It sticks into the back of the metal canister.

On the plastic circuit, check the ground circuit with an ohm meter point to various points.  It would be pin number two for ground, pin number one for 12 volt.

If you find a break, you might be able to work a jumper in to fix it.  But we'll go over those options once you isolate the problem

I would also check the ground path with a magnifying glass.  Sometimes the breaks are hard to see.  The break would look discolored.

If you have a 12 volt power supply you could connect it to one of the bulbs that work....12 volt to the socket copper connector on the 12 volt path, and ground to the copper socket connector on the ground path.  Move from bulb to bulb and you'll find where the break begins because once you work your way across the break, the former bad side would light up but the former good side wouldn't.....does that make sense?

[jphillips3333]

Light socket;
That's in the center of the fuel/volt gage - black rubber-like connector with a single grey wire?  I'll tug a little harder, it's only been 33 years.

Gauge cluster;
So, with a twelve volt source, power up the connector @ #1, ground @ #2.  Jumper power over to a bulb while grounded and check for illumination.  If one doesn't light up, do a trace route (tracert) back along the circuit and look for breaks with a magnifying glass?

[RENOVATIONS]

Light socket;
That's in the center of the fuel/volt gage - black rubber-like connector with a single grey wire?  I'll tug a little harder, it's only been 33 years.

Give it a turn before trying to pull it out.

[ta78w72]

Anytime you see a gray wire it's for illumination.  It's just a simple hole in the back of the fuel/volt canister.  The bulb gets ground from the canister so I would expect some metal on the outside of the socket.  Just twist and pull.  It should come out.

On the cluster...lets make this really simple.  Number one pin is 12 volt, two pin is ground.  Locate the first bulb on the circuit...the closest to the cluster connector.  Connect the 12 volt power supply to that first bulb..the red line to the 12 volt circuit side and the black lead to the ground circuit path.  The break will be between the last bulb lit and the first bulb out along that circuit path.  Make sense?

[jphillips3333]

Yep, makes sense.  Gotta crash though ... tomorrow.  You'll be up @ 7 Eastern, 4 Pacific, right?  Thanks bub.

[ta78w72]

Yep, makes sense.  Gotta crash though ... tomorrow.  You'll be up @ 7 Eastern, 4 Pacific, right?  Thanks bub.

Sometimes, but not usually!

[jphillips3333]

On the fuel/volt guage light socket;
I had to get a wrench onto the back of light in the fuel/volt guage.  It was torqued to 100 ft-lbs so it must have been like giving birth to a 33 year old.  (LOL)

On the cluster;
I pulled EVERY bulb.  Ran power to each one.  They're all good.

Ran power to the cluster - positive #1, negative #2
Off the 12V power source, positive to the bulbs.  I could only get one bulb to illuminate - the center one right under the speedomter cable.  To the uneducated eye, that looks like a run from the 2nd and 3rd connector? 

I don't see how all of the bulbs receive power just from the #1 & #2 slots.  I can see four bulbs + the fiber optic cable getting power off of #1.  There are 10 bulbs total and I assume that #3, #5, #7 are also 12V+ locations?  I think I'm doing something wrong ...

[ta78w72]

Not all the bulbs provide cluster illumination.  You've got a high beam bulb, seat belt warning bulb, right and left turn indicator bulbs, and probably one or two that I've missed.  So, you've only got four or five bulbs that provide illumination to the cluster.  You've got to follow the path from pin number one, if you've got power to the first bulb on that path and it illuminates, then the break is between the first and second bulb on the circuit.

OK, the result on the cluster you saw is correct.  There are two grounds for illumination.  The other is pin 9.  I guess they couldn't physically tie in all the grounds with one wire.  Pin 2 only grounds one socket...the one middle speedo.

[jphillips3333]

True, so I won't see the bulbs light with the #1/#2 connection at all locations.  So what I did is bend out the little metal tabs to engage the circuit somewhat better and slapped it back into the dash for now.  All the bulbs are good so I'm going to leave it for now as dropping the column is cake to pull this out again.  I'm a pro at that so the experience has been good.

Classic Industries sells this for $66.  I've got the 8K round tach circuit panel which apparently is something they need to know based on my research.  I need part 8911965 or FM864 from some other sites I was reading up on.  For $66, I can avoid going blind trying to see if there's cracks, trying to solder a fix.

Is Classic the best place to pick one up?  Anywhere cheaper or have a better product? 
I looked at e-bay, musclecarindustries wants $75 + $10 shipping.

Thanks,
John

[ta78w72]

Classic is where I would go.  The last time I bought one I got it from GM but that was over 20 years ago.

[jphillips3333]

Thanks dude - again.  I owe you a beer, maybe a case by now.