Fuel Gauge HELP

[Cory]

First off, I have an '86 C10 short bed with a 350, 700r4 tranny, and a single fuel tank under the driver's seat. When I first bought the truck the previous owner told me the fuel gauge didn't work because the sending unit was bad. My first mistake was believing that. I replaced the sending unit the week after I bought it and of course it didn't fix the issue. I then pulled the engine to detail it and the engine compartment and also to clean up all the unused wires. While the engine was out, I went through the entire harness and checked the integrity of all the wires and fixed/replaced any that needed it. The fuel system wires were in great shape so I didn't need to touch them. I bought that factory looking tachometer from GMSportsalvage.com (I think that's the name) which came with a fuel gauge at the bottom of the tach since the whole assembly took place of the original large face fuel gauge. I also replaced the circuit board for the gauges AND all the clips that each gauge pushes into to make contact with the circuit board because I figured I was already in there so it was a good time to replace them. Got it all back together and obviously, nothing. Fuel gauge STILL does not work. At this point, the sending unit is new, fuel gauge is new, circuit board with clips are new....the only thing it could be is the wiring. I spent all day last weekend going through the fuel system wires, reading them out for shorts and opens, and to my dismay...found nothing wrong with the wires. The pink wire running all the way from the instrument cluster to the sender on top of the fuel tank read fine. I had continuity through the wire and through the firewall plug. I also checked it for a short by putting one lead to ground, and had nothing. I double checked that the sending unit had a good ground and it does. I checked my schematic and from what I'm reading, the only wires in the fuel system go from the sender on the tank to the firewall plug, through the plug and onto the instrument cluster. I know there is some sort of jumper wire on the instrument cluster plug, which read fine. Also the ground wire from the sending unit to the frame. I have no idea where else to go with this issue?? Am I missing something??? I'm sure it may still be a wiring issue but if I have continuity and no shorts, what else could be the issue??? ALSO, I did the sending unit test using the rheostat from the headlamp switch thinking maybe the sending unit may have been a bad one but the gauge made absolutely no movement during the test, so I know its not the sending unit. PLEASE give me some guidance here. I've searched the forum and read the "functional tests of factory gauges" post a thousand times. This is driving me crazy as I manage to run out of gas on a monthly basis. I have the mileage worked out so I know when to fill up, but some weeks I'm clearly a little harder on the pedal than others Thanks in advance!

[bd]

Do all of the other gauges, instrument lights, etc function correctly?  Does the fuel gauge needle move at all when the ignition is switched on?  Do you have a test light?

The fuel gauge has three pin connections to the circuit board.  With ignition on, how many pin connections are hot?  Does the gauge needle move at all as you probe each pin connection?

[Cory]

bd, yes all of the other gauges work correctly, with the exception of the water temp. gauge but that is a seperate issue. The dash/instrument lights all work. Everything on the dash functions correctly except the fuel gauge. The needle doesnt move at all when the ignition is switched on, or off for that matter. I don't have a test light but I have a Fluke multimeter I've been using. I can't check right now if all three connections are hot because I'm not at home, but from what I remember, they were. Looking at the schematic and at the dash plug connector, are you talking about the pink wire coming from the bulkhead connector, going to the un-numbered pin connector of the dash plug, then coming out of that and going to the #30 pin connector?

[bd]

It sounds like you have an open power or ground connection to the gauge.  The fuel gauge needs ignition (12 volts), ground (zero volt) and sender, which is a variable resistor to ground that modulates the current trending through the gauge.  Using your voltmeter to measure a correctly functioning circuit, you should measure 12 volts on one pin (ignition), zero volt on a second pin (ground), and somewhere in between on the third pin (sender).  Take some measurements and post the results.  Is there a ceramic resistor attached across the back of the gauge?

[Cory]

Ok took some readings at the instrument cluster connector and it's left me wondering which pins on the connector are supposed to read what. Not sure if you have the schematic for my truck or not bd, but looking at the fuel gauge circuit starting from the fuel sender, there is a ground to sender wire and then the pink #30 wire going from the sender to the bulkhead connector. Then from the bulkhead connector to the instrument cluster plug at pin #30. I'm assuming at position #30 is where I should be getting a variable amount of voltage since it is coming directly from the sender?? Then from pin #30 on the instrument cluster plug, there is what looks like a jumper wire on the schematic coming back out of the plug and going to a second pin connector on the plug, also labeled #30. Looking closer, I realized it also looks like the first pin connector on the plug, coming directly from the sending unit, has a jumper wire going from that pin to the #11 pin right next to it, connecting that circuit to the light green #11 pin/wire which goes to the dimmer switch and eventually to the headlights. Is this where the fuel gauge circuit would get its constant 12v from?? If you could tell me what positions on the plug are supposed to be the 12v constant, variable (sender), and ground, that would help out a lot because I am getting zero volts at both pin #30 positions on the instrument cluster plug with ignition on.

[bd]

Reference: 1986 Wiring Manual, pages 10 and 11.

The pink fuel sender wire is circuit #30 (Ckt 30).  The numbers printed at the various connector pin locations are also 'circuit' numbers; not 'pin' numbers.

Reviewing the pertinent wire colors:
Ignition power to the cluster is Ckt 39 - the wires are pink with black stripe (pnk/blk).
Ground for the cluster is Ckt 150 - the wires are black (blk).
Fuel sender is Ckt 30 - the wire is pink (pnk).

The only three wire colors in which you are interested are pnk (sender), pnk/blk (ignition), and blk (ground).  None of the other wire colors running to the cluster are in the fuel gauge circuit.  Although, there are more than one ignition and ground connections to the instrument cluster PC board. 

Checking ignition feed to the fuel gauge:
Begin by connecting the black probe of your voltmeter to a good bare cab ground and use the red probe at the fuse block to find a constant battery source that reads ~12.6 volts.  This step verifies that the voltmeter has a good stable connection to ground.  Without moving the meter's ground connection, and with the harness plugged into the cluster and the ignition switched ON, measure the voltage at all the harness connector pin locations at the back of the cluster that have pnk/blk (ignition) wires attached and you should measure ~12.6 volts at every location.  If the pnk/blk wires measure ~12.6 volts, then probe the three PC board connections to the fuel gauge; at least one should read ~12.6 volts.  If it does, then ignition power is being supplied to the gauge.  If not, then the 12-volt ignition feed to the cluster or gauge needs to be corrected.  You must have ignition power to the fuel gauge for the subsequent tests.

Checking the ground and sender connections to the fuel gauge:
With your voltmeter still connected to ground and ignition supplied to one of the fuel gauge pins, the remaining two gauge pins should measure as follows:

• one of the gauge pins should measure zero,
  
• the remaining gauge pin should measure between zero and 12.6 volts.

In summary, you should measure 12.6 volts on one fuel gauge pin, zero on one fuel gauge pin and somewhere in between on one fuel gauge pin.  If all of the gauge pins measure zero, there is an open ignition feed.  If none of the gauge pins measure zero, there is an open ground.  And, so on through various combinations.  Post your results.

[Cory]

Ok I just did my checks and found the following:
All three pink/black wire locations at the plug have 12 volts, so I checked the 3 PC board locations directly behind the fuel gauge. 1 pin has the 12v ignition power. The remaining 2 both read zero volts. I'm assuming there is an issue somewhere with the fuel sender wire?

[bd]

Is the fuel gauge needle pointing to Empty?

[Cory]

Yes it's on E.

[bd]

Does the gauge swing well past Full (to ~3:00 o'clock) if you unplug the tank sender?

[Cory]

No it doesn't move at all when the sender is disconnected.

[bd]

Unplug the harness connector from the back of the instrument cluster and carefully remove the pink Ckt 30 wire from the connector.  You can release the terminal using a stiff straight wire, such as a heavyweight paper clip that is bent straight, by inserting the wire into the end of the connector from the PC board side, along the backside of the terminal.  With the pink wire removed from the connector, plug the harness connector back into the cluster and switch the ignition ON.  Does the fuel gauge needle swing to the 3:00 position?

[Cory]

Bd, disconnected the ckt 30 wire from the plug, plugged it back in and with ignition on, the needle most certainly moved to the 3 o'clock position...

[bd]

The Ckt 30 sender wire is grounded somewhere between the instrument cluster harness plug and the sender plug.  Start tracing the harness.

[Cory]

Looking at the schematic, the ckt 30 wire only goes from the top of the fuel sender to the bulkhead connector, through the connector and then straight to the cluster plug correct? It should not go through the fuse block and is not fused in anyway? Just want to be 100% before I start tracing. Thanks again for all the help bd!!