Help. Cluster issue

[triophan]

I got this old 79 C-10 earlier this month, apparently the gauge cluster was screwed because all that worked in it is an aftermarket tach in the place of the fuel gauge.

I scored a really nice '79 cluster today at the salvage yard for $20! And I figured it would be plug/play, but NOTHING works. Any ideas? Anyone got any opinions? I sure would like to have this thing sewn up tonight so I can get to work in the morning...

Or if anyone would like to shoot me a price on going through all the wiring on this dang thing...

http://sphotos.ak.fbcdn.net/hphotos-..._5320509_n.jpg

[Mr. 250r]

have you tried checking the copper circuit on back

[triophan]

its burnt on the old one. perfect shape on the new one

[triophan]

[Mr. 250r]

if you notice on your old one the big plugin see how the gold pieces are coming up these things wear and tear so easily i pulled mine out and in the top left corner which i think the the ashtray light? but it was torn look closely to see if there's any tears even the smallest will mess up the whole circuit, if those are all good maybe the actual gauges themselves are bad

[triophan]

via my research, I've found that is a "fiber optic" ashtray illuminator. Could ALL my sending units be bad? Is that possible?

Would it be more practical to re-wire the truck with a kit or resolve the issues with a schematic?

[csb#356]

Those gauges don't look like there out of a 79. And yes it is possible that all sending units are bad, but more than likely they just don't have any wires going to them at all. for example somebody put in a new engine and didn't hook anything up. You could replace it with a kit, but I think I paid $550 for my kit, I know theres cheaper ones out there but I wanted one specifically for my truck. Or you could chase some wires around and see if there hooked up. The easiest thing to check would be to just go look at the sending units and see if theres cut wires or anything.

[Mr. 250r]

its possible they could be bad but i just don't see that try just replacing one if that doesn't work id say its the circuit which are replaceble but rather costly
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[RandyP]

Times like this when a simple tester comes in nice. Just see what has power and what doesn't. Chances are it's in the wiring and/or your sensors.

[triophan]

I've pretty much consigned myself to starting with a LMC re-wire kit and going from there. The further I get into this truck, the more I find that EVERYTHING is going to need replaced... I'm trying to be ok with that...

[customizer76]

Its not likely that all your sensors are bad, I think you have a power problem. Do the lights in it come on with the park lights? If not check for a ground wire loose below where the cluster fits and it should be off to the left side. If you do have a tester you can turn on your ignition and check for power on any of the wires coming in from the back to see if you have anything. Wish I lived closer, I rewired my all over using just straight wires. I think you have a ground problem though.

[triophan]

parking lights, headlights, tails, brakes, reverse lights all work. I think this truck's had this and that swapped, but the wiring hasn't been resolved.

[hatzie]

Originally Posted by triophan

I've pretty much consigned myself to starting with a LMC re-wire kit and going from there. The further I get into this truck, the more I find that EVERYTHING is going to need replaced... I'm trying to be ok with that...

I wouldn't go there yet. Wiring problems are tedious to diagnose but they aren't rocket science to find or repair... It won't cost you $500 just some time and patience.

Wiring diagrams are here 73-87chevytrucks.com Wiring Diagrams

The gauge fonts and colors tell me... The new top cluster is from an 81-87 CK series truck. The burnt bottom cluster is from a 77-80. That being said they should interchange because the connector is pinned out the same...

Your burnt trace is Ground circuit #150 between The TEMP gauge and the connector. You probably have a short... positive voltage where it's not supposed to be.

Instrument Panel Plug Pinout With Contacts Facing You for 1977-1987.
Use the unused center pin to orient the connector.

High Beam	11	30	FUEL
PNL Lamps	8	33	Brake
GND	        150	39	IGN
IGN	        39	931	Choke
OIL	        31		Unused
IGN	        39	237	Seat Belt
FUEL	        30	15	RT Turn
GND	        150	14	LT Turn
TEMP	        35	150	GND

Wire Color Key

Circuit Wire Color
8	GRAY
11	LT BLU
14	LT BLU
15	DK BLU
30	PINK 
31	TAN 
33	TAN/Wht
35	DK GRN
39	PINK/BLK
150	BLACK
237	YELLOW
931	DK BLU

The numbers in both text boxes are GM wiring diagram circuit numbers.


Make sure the Gauge fuse is not blown and is 20 amps.

Get a multimeter and set it for at least 20 Volts DC. A test lamp will work too but a meter is more sensitive. Test the meter or the lamp on the battery to make sure they work and verify the actual battery voltage.

Check from Batt in the fuse panel to each ground contact in the connector to verify that all 3 grounds are good. Fix as needed.

With the ignition on. Ground the common probe. Check for voltage at BATT in the fuse panel with the positive probe. If you get battery voltage then you have a found good meter ground.

Check all 3 IGN contacts with the positive probe to verify ignition voltage to the cluster. Fix as needed.

GM senders provide a variable resistance to ground. There should be 0 volts from the TEMP, OIL, and FUEL contacts to ground. Check all three. If you find volts chase the wire and repair the short.

[triophan]

sweeeeeeet! here I come, 15 hours with a ammeter!!!

[terryj]

With the old flex circuit being burnt like it is, have you checked out the connector itself to see if the problem isn't in the connector.

[hatzie]

Originally Posted by triophan

sweeeeeeet! here I come, 15 hours with a ammeter!!!

Naaah. Doubt it will take that long. A dead short like that will show its face pretty quick. The Gauge fuse is likely blown after your small fire.

terryj has a good point... Check the connector up close and look for chunks of copper from the curling printed circuit contacts bridging the pins.

I work on heavy vehicle electronic systems for a living and I'm pretty patient. I just decide ahead how long I'll chase a really miserable problem before I step back and move on to another. Eventually I have one of those "Geez. Why didn't I think of that?" moments while working on something else.

[triophan]

Fire? Pins? Whaaaaat???

oh wait. had to read it again... I think I got it. Well... 14 hours. I'm a slow learner and a fast forgetter.

Will keep ya posted. I may have some help this weekend. But who knows, beer-paid mechanics are the least reliable.

[terryj]

Originally Posted by triophan

Fire? Pins? Whaaaaat???

oh wait. had to read it again... I think I got it. Well... 14 hours. I'm a slow learner and a fast forgetter.

Will keep ya posted. I may have some help this weekend. But who knows, beer-paid mechanics are the least reliable.

With your flex circuit being burnt like that it could be very likely that the connector will have carbon scoring on the pins, this will cause a high resistance at the connector, just make sure all the connector pins are bright and clean.