I recently replace my original instrument cluster with another one with a tach. Everything worked except the fuel guage. I had gone as far as buttoning everything back up before I found out the "new" fuel gauge didn't register. I removed the tach guage and replaced "new" fuel gauge with my "old" fuel gauge and reinstalled it. This time I didn't reconnect the speedometer cable since its tough to get my fat hand behind the cluster to disconnect it. The "old" fuel gauge worked on both tanks.

I re-screwed the tach cluster back into the dash and finished putting the dash panel back during the middle of the week. Yesterday I started the truck up to push some snow from my driveway, the fuel gauge started coming up to about half tank on the front tank, which I knew was at least 3/4 from the ten gallons I had put in to make sure the gauge worked before buttoning up the dash, then dropped to empty. I switched to the rear tank which had approx. 3/4 of a tank started to rise but stopped and went back to empty almost immediately.

I have previously replaced both tanks but used the original tank sending units. I have replaced the switching valve on the driver side frame when I replaced the both tanks.

How can I test or isolate the fuel selector switch in the dash as being the problem? or am I off base on where the problem may lie? I can't believe that both tank sending units and/or a "second" fuel gauge in the dash could go bad at the same time.

hugger94!

 

While you were in the dash, did you locate the ground(s) for the cluster?
The truck didn't move at all while you did the swap, right?, so how likely is it that something else broke?
AL.

 

Dealford - I believe that the ground for the cluster is built into the imprinted circuitry on the back of the instrument cluster. I know that the only things I had to disconnect when removing the original cluster was the speedometer cable and the main electrical connector that controlled all of the gauges and instrument lighting.

Since the original posting, I have verified that the fuse controlling the Aux. fuel tank is good and that I have power to the brown wire plugging into the fuel selector switch on the driver side frame controlling the tanks.

I did drive it out of the garage with the cluster only connected by the main electrical connection to warm up the engine to make sure the temp gauge worked before I put the additional fuel in the front tank. The fuel gauge was working at that point. After verifying the temp gauge was working, I backed it back into the garage and finished fastening the instrument cluster back into the dash. So yes the vehicle moved but less than 20 feet.

I am leaning toward the switch in the dash to select either the front or rear tanks, unless someone has a better idea.

hugger94

 

This is not a very handy thing to do(requires crawling under the truck) but it will test the system.

Take the guage wire off the sending unit on the front tank, and with the dash switch in the front tank position, and the key in run, ground this fuel tank sending unit wire to a shiny place on the frame.

The fuel guage in the dash will peg in one direction full scale.

Then take the wire off the frame and let it hang in the air.

The dash guage should swing full scale and peg in the opposite direction.

If the guage does this, then the wiring from the tank to the dash and the guage are working correctly. You can also test the rear tank this way with the switch in the rear position.

If you knew which wire feeds the tank selector switch from the dash fuel guage, you could also ground this wire and then disconnect this wire, and it should also make the guage swing back and forth.

 

Okay I think I grounded the front tank wire to a ground, in fact tried both wires in the connection but the fuel gauge didn't register anything. I however did not verify that I had a good ground with a test light but did use a ground point used previously to test the fuel selector valve on the frame.

Should one of the two wires have power? It would seem that it should! I didn't check them for any voltage and will do that tomorrow night. I also didn't check the back tank frankly because I was very discouraged.

Does this test indicate the fuel gauge or printed curcuit as being the problem?

hugger94!

 

Yes, one wire should be live with the ignition "on". Test light both to find the hot lead. Ground the existing ground wire to "shiney" clean spot on the frame. Ground the hot lead & the guage should spike.
If the guage does move, then you're into sender(s). If it does not, then you have to back track the wireing right to the guage.
AL.

 

If you test the "hot wire" that goes to the sending unit with your testlight, you may get a flashing light or the voltmeter will jump up and down. This is normal, it's the voltage regulator for the guages in the instrument panel doing that.

I have a funny feeling it;s as you suspect, and there is something wrong in the insturment panel guage. You can also find the sending unit wire leaving the guage panel(this will be the wire that goes to the fuel selector switch) and also do the ground and non-ground test to make the guage swing back and forth.

That is all the sending unit is doing. It is varying the ground of the guage with a resistor on a the float arm. More or less ground, gives you more or less fuel guage reading. The other side of the guage is fed with the blinking voltage from the instrument regulator.

 

Okay, I have no power to the sending unit connector. I also have no power to the fuel guage wire at pin # 4 on the main electrical connector. I have not tried to remove the fuel switch from the dash yet to test for power coming into the switch, to bypass the dash selector switch.

I know that (fuse # 3) the aux. fuel switch does have power to the fuel valve on the driver side frame when the dash selector switch is in the front tank position and no power when in the rear tank position.

Any other ideas?

Thanks hugger94

 

Where is this at? The large connector going to the dash cluster? If so, I would check for power right at the pins connected to the fuel guage. If your other guages are working, you should at least have power coming in on one side of the guage.

 

Franklin2 - Sorry, I didn't make myself clear. Yes the pin# 4 or yellow/white wire on the main electrical connector plugging into the back of the instrument cluster does not have any power. All other guages work correctly, temp, oil and Amp.

I re-tested the pin# 4 with a test light last night for power and in fact touched all the pins on the connector (both sides) to insure that I was working with a good ground connection. I again did not have any power either at the connector pin #4 or at the sending unit connector.

I will be checking the tank switch on the dash for power.

I guess my ignorance is what path does the power run? From the ignition switch to the voltage stablizer, to the fuel guage, then to the tank switchin the dash?

Since all of the other guages work I have to assume that they are getting power through the voltage stablizer. Therefore the only difference that I can determine is the tank switch on the dash.

I am sure that I am missing something! hugger94

 

If you do not have any voltage at the yellow/white coming out of the cluster connector, I would not check the fuel switch. Your problem is in the cluster. Can you get to the fuel guage terminals? One side should have on/off power from the regulator, and the other side also should have on/off power. If you ground the yellow/white, the guage should move.

 

Franklin2 - I hurriedly check the posts of the gauge this morning before leaving for work. It did not appear that I had any on-off power to either terminal. But I also have both the connector off of the dash switch as well as the sending unit connector at the tank when I checked the gauge this morning.

When I originally said that I didn't have any power to pin# 4 (Yellow/white wire) going to the dash switch, I was using a test light against the bare connector at pin #4. I didn't have the electrical connector plugged into the instrument cluster. I was making an assumption that one side of the gauge was a ground, not understanding that the pulsed current passed through the gauge and out the other terminal to pin# 4 and on to the dash switch then eventually to the sending unit.

I will reverify my testing this evening. Sorry for being so dense on this! Guess it shows that I am really ignorant about electrical circuits on vehicles.

hugger94

 

How the fuel guage circuit works is difficult to grasp , because it works backwards to most people's thinking. That is, the gauge is fed with power all the time, and the ground is varied to make it work.

 

Franklin2 - Fuel guage problem solved! I was able to verify that I did have pulse current to both sides of the fuel guage, then checked the connector that would plug into the aux. tank switch.... had pulse current there.

So I reconnected the sending unit connector and put a short jumper wire from the middle connection (yellow/white) to both connections on the aux tank connector, both tanks registered okay or having gas.

I just need to replace the aux. tank switch and I will be good to go!

Hope this is the last electrical issue with the truck but I am sure that it is wishful thinking. Thanks for your patience!


hugger94