My same fuel gauge issue
 

Hey guys,

I finally got around to looking into my fuel gauge issue. It reads waaay past full and lays over fully to the right. It worked periodically a couple of years before it quit all together..now it just lays at the 2-3 O'clock position.

Using my volt meter I have power at the sending unit in the truck...also, I have power into the gauge and out of the gauge...Is it fair to say I need another gauge?

I took the gauge out of the pod and checked it well for good contact re-assembled and still the same result..when you plug it in it moves to the right from around full until it stops..Have I missed anything here?


Thanks!

Grounding the sending unit wire in the trunk, should make the guage go to empty.
Try it and see what happens, that'll really narrow things down.
Straight to ground want hurt it, same thing the sending unit is suppose to do at empty.

If it does bring the needle back down, sending units in trouble or tanks lost its ground.

Check it out, post back, and we'll go further.

Thanks QS,

My single ground wire from the sending unit is grounded well to the inside of the tail pan. I have triple checked it. It has a 100% clean metal to clean metal contact.

Jay, he means to ground the signal side. In other words, the 12v wire that goes to the sending unit in the tank. If that is grounded, it's the same as the sending unit reading 'empty'. The gauge should go all the way left.
When you take it off ground, that is, leave it disconnected from anything, it should peg full.
If those two things happen, your gauge is good and the sending unit in the tank is probably the culprit.

Greg,

Seems like that would just blow the fuse...

No, the gauge has enough resistance to prevent that. Remember the gauge windings are between the voltage source and the ground you're applying to the tank end of the circuit.
Basically, it's what the sending unit does at it's low fuel postion. It's just a variable resistor and when the tank is empty, the resistance is zero....Basically, a short to ground.
Second opinions welcome though! I may have forgotten something in my old age. :old:

PS- By the way, you can do the same check at the instrument panel but by doing it at the tank, you'll also be checking the wiring from the gauge to the tank.

Thanks Greg,

OK,

I grounded the power wire in the trunk(while plugged to the sending unit). Sure enough the gauge went over to empty
as soon as I touched the body. But, if grounded unplugged without the sending unit plugged in it does not go to full...grounded or not it just lays on full.

Now what?

BTW, Is there another ground besides the tail pan ground? Maybe one under the dash?

Greg,

I also just plugged in my spare fuel sending unit...Same result.. I even tested it down on the end for power(where the float arm connects to its body).It has power at the top and the bottom.The gauge still shows past full.

So if I have power into and out of the gauge....power all the way thru the sending unit..and a good ground at the tank(tail pan)..The gauge appears to work(plugged in)..whats left?

A dash ground maybe?

I'm not following you here Jay.
With the switch on, grounding the line that goes into the sending unit from the dash takes it to Empty as you said...That works. All you need to do after that is take that same wire off of the ground (just let it hang) and it should go to Full.
If it doesn't, I'd say your gauge is faulty.
By the gauge going to empty when you grounded it, that pretty much shows the wiring is good.

Sorry,

I am assuming that I leave the tail pan ground from the sending unit during this test..correct? I may have this wrong..

I am conducting your test at the plug that goes thru the body into the trunk. At that plug (while plugged together) if you ground it the gauge goes empty immediatly. However, if I unplug that same plug and ground the hot side back to the gauge I get the same full pegged result...not the empty movement.

Both with the switch on

My test ground or the tail pan ground? Either way I get the same ..Full

That's about it as I recall without having a print in front of me. If you have all that and the gauge is all over the place, I would suspect the gauge.
It's a single line series of circuit.

Simplified, it's kind of like-
12V-----(+ gauge -)----(^^+sending unit-^^)--->ground.

Varying the resistor adjusts the current flow through the series circuit, moving the gauge.

Only three or four things to go wrong if voltage is present;
Bad connection, Bad gauge, Bad resistor(sending unit), bad ground.

We were posting at the same time Jay.
Look at the "schematic" I drew. With switch on...BEFORE the gauge..Where it goes into the trunk sounds like a good spot...unplug the wire. Should be full. Ground the wire...Should be empty. You're taking the sending unit out of the circuit to test the gauge.

[QUOTE=Greg Reid;3609711...unplug the wire. Should be full. Ground the wire...Should be empty. You're taking the sending unit out of the circuit to test the gauge.[/QUOTE]


Its gotta be the gauge,

If I unplug the wire it shows full...if I ground it (unplugged) it stays full..only plugged in and grounded does it go empty.

I found one on ebay..should get it this week.


Thanks Greg!

Something is wrong here
 

If grounding the sending unit wire makes the guage go to empty, plugged or un-plugged wont make any difference, ground is ground.

Something is not getting checked right. Ground the tan wire going to the guage and what happens? No other ifs, ands, or buts.

The connector has nothing to do with this test, just dead ground on the wire TO the guage.

If the guage goes to empty, sending unit or tank ground is bad.

What you're describing is kind of stramge. You ground the wire while the circuit is complete and it goes empty but you ground it with the wire unhooked from the sending unit and it stays full?
I'm thinking it should have gone to empty either way.

Very strange indeed
 

Unless the guage IS bad, but then the first empty reading wouldn't have happened.
Unless another possibility, a broken wire in the ribbon running from front to back and wiggiling the harness/connector had it making and breaking contact. But it doesn't really sound like it though.

Think of the fuel guage as an ohm meter, thats only capable of reading 0/zero (dead ground) to 90 ohms. And 0 to 90 is all the sending unit can produce. 0=empty and
90=full.
With the connector unplugged, on the back/trunk side, you can test the sending unit with an ohm meter. Sending unit wire to ground should read 0 to 90 ohms if it's good.

You can perform the same test on the spare sending unit and move the float up and down to see the 0 to 90, if it's good. If it is, hook it to the car harness and see if the fuel guage works.

I just double checked the fuel gauge needle..

It was wedged against the gauge body (at 4 oclock).I had to get it free. So,it will go to empty now...plugged or unplugged.It being wedged on the unplugged test messed us up. So now we are looking at the sending unit or ground to it?


QS,

I just bought a basic multimeter..still need to figure out how to check ohms or resistance.Do I use both probes on the lead/plug I am checking in the resistance mode?

BTW, I checked the exact power to the sending unit and it is around 8.7 volts.

You use both probes to read across or through something, one probe on each end.

Get your spare sending unit......
Put one probe on the wire terminal and the other againt the mounting plate. If it's good you'll get a reading. Move the float up and down, down should read 0 ohms resistance, and up should read 90 ohms. Readings should be close anyway.

On the car, tan wire to tank and ground will be your touching points, with connector unplugged. With a bad sending unit or tank ground, meter should give an infinite reading. About the same as just holding both probes up in the air, not touching.

Touch your probe tips together to see what it's zero reading is. Most wont show a true zero, usually a .0? number.

Try checking some plug wires from end to end. You'll find that interesting.

Check the difference from contacts to base on a 1157 bulb. You can see the different resistance for bright/dim-brake/tail.

It'll take some trial and error to learn the quirps of the meter, but it's fun and can be very usefull.

PM or E a number if you have any questions.
Clay