Fuel and Temperature Gauge Problems

Q: Fuel and Temperature Gauge Problems

I have an inoperative fuel gauge ( always full ) and temperature gauge ( always cold ); the oil pressure and alt gauges are both operational. Several “shadetree mechanics” tell me the problem may be in an improperly grounded rally cluster metal case. I recall that the wiring harness plug has a green wire that is attached to the metal case with a sheetmetal screw. I thought that was a ground wire. However one fellow was talking about grounding the unit/case by way on the metal strip that is behind the high-beem indicator light in some way ??? I’m lost !!! What advice can be offered. I’ve removed the gauges several times before and may have not gotten a solid ground.

A: There are actually multiple “grounds” on the rally cluster. When I get home tonight, I pull the wiring diagrams and check out where the fuel gauge and temp gauges derive their grounds. Is it possible that the problem is not in the rally cluster itself, but in the fuel and temperature sending units themselves? Do you have an ohm meter that you can use to check the inputs from these sending units?

I know from experience that disconnecting the wire at the sending unit in the gas tank will cause the fuel guage to read full. There is also a ground wire on the fuel guage sending unit which tends to get rusty and break off after a few years. If this happens, you will see the same problem – fuel guage reading full.

Concerning the temperature guage, one simple test of the temp guage is to hook up an ohm meter to the temp sending unit located on the intake manifold when the car is cold. Take take a reading. Then start the car and allow it to come up to operating temp. The value on the meter, (the measure of resistance in the sending unit) should change as the engine warms up. If it doesn’t move, you have a bad sending unit, or the wrong sending unit. I’ve seen a case where someone installed a temp sending unit that was designed for the “idiot light cluster” instead of the rally gauge cluster. The temp sending unit for the idiot cluster is a switch, the temp sending unit for the rally cluster is esentially a variable resister based on temp. If the readings change, then it is a safe assumption you have a rally guage sending unit and it is operating normally. The next step would be to then check if you get the same reading at the plug which goes into the back of the rally guage cluster. The temp wire is the green wire going into pin 2 of the connector.

Did your car come stock with a rally guage cluster? If not, the problem may lay in the fact the the “pin-out” for the rally guage cluster is different than the pin-out for the idiot light cluster. I can provide the pin-out for the connector if you need it. (Note: See ‘Replace an original Dash cluster’ for more information)

A: I pulled the wiring diagrams on the rally guage cluster and wiring harnesses last night. Before I get into the gory details, I’m not sure how familiar you are with basic electrical circuits so here is what you need to understand about them before you can understand and diagnose your problem.

The electrical circuit(s) in the car are 12 volt, DC. That means, that every circuit must have a complete connection from the positive side of the battery to the negative side of the battery. The negative side of the circuit is usually refered to as “ground” and in most cars, (your firebird included), the negative side of the battery is connected to the frame/chasis of the car esentially making every metal part of the car the negative side of the battery.

Now, in reference to your gauge problem:

The first thing to keep in mind is that your problem is probably NOT being caused by ANY missing “ground”s on the guage cluster itself. Why? Two reasons. 1) The ground connections on the rally guage cluster are ONLY used for the instrument lighting, turn signal indicators, and high beam indicator circuits. 2) The “ground”s for the gauge circuits, (oil preasure, temp, and fuel guage), are at the “sending units”, (the ammeter is a completely different type of circuit and totally independent). However, I say “probably not” because I don’t have the specifics on the internals of the gauges themsleves so I can’t say if the gauges use any “reference ground” source.

The way the cluster and gauge circuits are designed is that the fuel gauge, oil pressure gauge, temperature guage, and brake indicator light, ALL share the same +12V feed, which comes into the cluster on pin 10 of the connector via two PINK wires. Since the oil pressure gauge is working, then you have the +12V into the cluster on these pink wires AND any possible “refernce ground” source for the gauges is probably there as well.

Since you have +12V to the gauges, that means that one of four problems exist for the fuel gauge and the temperature guages. (They will have to be diagnosed seperately since the problem with the fuel gauge circuit might not be the same as the problem with the temp gauge circuit.) The possible causes for the malfunctioning gauges are: 1) a break in the wire from the gauge to the sending unit, 2) a bad sending unit, 3) a bad ground at the sending unit, or 4) the gauge itself is bad.

To diagnose the fuel gauge, here is what I would do. Locate where the rear wiring harness, (the one from the tail lights) meets the main harness. (Starting in the trunk, follow the wiring harness from the tail lights up to the where the rear harness connects to the main harness. There will be a seven wire connector at this point. Disconect the two harnesses. (The TAN wire is fuel gauge circuit). Working with the connector coming from the FRONT of the car, take a piece of wire and make a connection from the TAN wire in that connector to a GOOD ground, (a clean metal part on the body.) Now sit in the driver’s seat and turn the key to the on position. No need to start the car.) IF the gauge and the TAN wire from the gauge are good, the fuel gauge should immediately drop to EMPTY. (Don’t leave the key turned on for more than 5 – 10 seconds). If the gauge doesn’t drop to EMPTY, then the problem is either the gauge itself, or the tan wire from the guage cluster connector to the connetor you currently have your grounding wire attached. If the fuel gauge drops to EMPTY, then you are bummin’ because you will have to drop the tank to get at the fuel sending unit. There is either a break in the TAN wire in the rear harness, a broken ground wire on the sending unit, (the most likely source of the problem), or a bad sending unit itself.

The temperature guage is a little easier to test. The temp sending unit is located on the intake manifold on the top left front right near the thermostat housing. Disconnect the wire from the sending unit and run a length of wire from that connector to a nice clean ground on the engine. The sending unit wire should be dark green by the way. Once again, sit in the driver’s seat and turn the key to the on position. The temp guage should immediately jump to HOT side of the gauge. If it does, you have a bad sending unit. If it doesn’t, then either the dark green wire from the sending unit to the gauge cluster is broken or the temperature guage itself is broken.

Keep this in mind, the idea is to complete a circuit from the positive side of the battery to the negative side of the battery by going through the gauge in cluster AND the sending unit. Any break in the circuit along the way and the gauge quits working. The positive side of the ciruit is at the gauge cluster and the negactive side of the circuit is at the sending units.

I hope this helps. Good luck in your diagnosis

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