84 GMC 3500- Help with fuel gauge

[2badgn]

I had the sending unit in the tank changed as well as the tank itself.

The fuel gauge is still not working.

What do I do next?

Check the gauge? what else could be wrong?

[INSIDIOUS '86]

Gauges are known to go bad. Change it and be amazed
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[2badgn]

My temp gauge & volts gauge work fine but the oil pressure gauge is off.

Since I will have the dash out to change the fuel gauge, I thought about also changing the oil pressure gauge. Are these also known to fail?

I know that the oil pressure is fine. The truck is regularly maintained by a mechanic and it runs like a champ. When the timing chain was changed at 108,000 miles, the mechanics asked me what kind of oil i used because they said that the engine looked so clean!

[Ville K]

I have also problems with fuel gauge, I have double tanks, and the gauge shows almost same readings with both tanks, so assuming its not the sending units that are faulty? When the tank is full, gauge shows only half tank, and when the tank is empty, it shows 1/4 and both tanks act the same way. Also oil pressure gauge doesnt work properly. Is the solution to buy new gauges? I also have problems with the illumination of the gauges, I changed all the bulbs, but only few days after half of them are not working.. I guess it has to do with that plastic film that has the wires in it, right?

[bk2life]

the plastic "film" (circuit board) could be the problem check the pink wire running from the fuel gauge to the plug on the firwall and all the way back to the fuel sending unit. could be bad, dirty, insulation broken causein the wrong reading. a wire check is simple, costs nothing.
if you replaced the fuel sending unit, do you still have the other one? you could hook the sending unit to the wire, and physically move the float to see the gauge work. or you could shell out the 35 bucks and get a new gauge.

its a very simple circuit, shouldnt take very long to narrow it down to the culprit.

[hatzie]

You didn't tell us whether the gauge is pinned at 3:00, stuck somewhere else, or just not reading the correct level.

Troubleshooting Fuel Gauge.

1. If the gauge reads at the 3:00 position check that the sender is grounded to the frame. Since you just replaced the tank I'd check here first.
   [**]73-86 all & 87-91 Diesel should have a ground wire from the frame to the sender collar on the tank.
   [**]87-91 TBI should have a ground wire that runs from the frame through the sender plate.
2. Unplug the sender wire. The gauge should go to 3:00 if it's not there already.
   [**]If the gauge doesn't go to 3:00 with an open sender circuit check the instrument cluster power and ground with a meter. Sometimes the splices buried in the dash wiring behind the cluster have problems.
   [**]If the power / ground wiring is good check all three fuel gauge traces on the flexible printed circuit with a meter. Repair or replace as necessary.
   [**]If the power / ground wiring and the flexible printed circuit are good continue to next step.
3. Short the sender wire to the frame or other good ground. The Gauge should slowly go to EMPTY.
   [**]If it doesn't move check the wiring from the gauge to the sender you probably have a broken wire somewhere in that circuit.
   [**]If you haven't done it in the prior step... Test the flexible printed circuit fuel sender trace on the instrument cluster with a multimeter.
   [**]If the wiring checks out and the needle moves, but doesn't go to EMPTY, then the gauge is probably bad.
4. If the gauge and wiring pass the above tests you'll need to test the sender. GM 90ohm fuel senders move from 0-3 ohms EMPTY to 87-90 ohms FULL 1/2 tank should be 43-47ohms. The resistance from the sender pin to a frame ground should track the level in the tank. I bench tested some gauges with a calibrated resistance decade box and a lab grade DC power supply to generate the attached excel graph. The PDF is a lot clearer than the JPG.

• Gauge sweeping from E when empty to 1/4, 1/2, 3/4, ... when full is usually the calibration resistor on the gauge can. It's usually easier to replace the gauge than find that white ceramic block.
• Gauges sweeping from above E when EMPTY and below F when FULL are usually but not always sender problems.

[hatzie]

Originally Posted by Ville K

I have also problems with fuel gauge, I have double tanks, and the gauge shows almost same readings with both tanks, so assuming its not the sending units that are faulty? When the tank is full, gauge shows only half tank, and when the tank is empty, it shows 1/4 and both tanks act the same way.

That sounds like a bad gauge.

Originally Posted by Ville K

Also oil pressure gauge doesn't work properly. Is the solution to buy new gauges?

It might or might not be a separate problem. What is it doing?

Originally Posted by Ville K

I also have problems with the illumination of the gauges, I changed all the bulbs, but only few days after half of them are not working.. I guess it has to do with that plastic film that has the wires in it, right?

A flaky printed circuit usually causes that kind of intermittent lighting.

81-90 with gauges without tach is AC Delco part #25044897 $40 US from Amazon. Don't know about international shipping...

http://www.amazon.com/ACDelco-25044897-Printed-Cir...

[Ville K]

Originally Posted by hatzie

That sounds like a bad gauge.

It might or might not be a separate problem. What is it doing?

Thanks for your reply!
I think it shows only a part of the real oil pressure, engine is fine and only 92K miles, and if the pressure would be that poor, it would have broken up allready.
I havent check what the oil pressure really is, but it seems like it works otherwise as it should but shows only a part what it should. It barely moves when the engine is warm, with cold engine it shows the first line.

[hatzie]

Originally Posted by Ville K

Thanks for your reply!
I think it shows only a part of the real oil pressure, engine is fine and only 92K miles, and if the pressure would be that poor, it would have broken up allready.
I havent check what the oil pressure really is, but it seems like it works otherwise as it should but shows only a part what it should. It barely moves when the engine is warm, with cold engine it shows the first line.

Showing low pressure like that may be a bad sender. They're pretty cheap here. I don't know what you'll pay for one across the pond. Crossreference to Standard Motor Products PS154 (60psi) or PS155 (80psi).

You can check the sender with your digital multimeter. Set it to 100 ohms or the closes scale that's >90ohms. Unplug the sender wire and attach one meter lead in its' place. Start the engine. Touch the other lead to a good ground. If the sender is OK and you really have normal oil pressure you should get a reading between 20 & 80 ohms.

[Ville K]

Thanks, will do that and report. We have a great shop here that has almost everything in the storage for these old trucks and other american cars too, they have their own import for the parts, and I think its cheaper to maintain US-made car here than one made in europe..
Thanks again for your time and advice!