[1qikta]

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!

["QUICK-SILVER"]

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.

[1qikta]

Originally Posted by "QUICK-SILVER"

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.

[Greg Reid]

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.

[1qikta]

Originally Posted by Greg Reid

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...

[Greg Reid]

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.

[lazlo]

Originally Posted by "QUICK-SILVER"

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.

Are the straps what ground the tank to the car? I am on my second sending unit from Summit and neither shows above 3/4 tank when it's full. Replaced the sending unit because the original developed a leak around the outlet tube. The gas gauge worked fine with the original unit. It doesn't seem to like these Taiwanese sending units. This is on my 1966 Lemans. Any thoughts?

Tomorrow I'm gonna take the original to the radiator shop & see if the can fix the leak..

[Greg Reid]

Originally Posted by lazlo

Are the straps what ground the tank to the car? I am on my second sending unit from Summit and neither shows above 3/4 tank when it's full. Replaced the sending unit because the original developed a leak around the outlet tube. The gas gauge worked fine with the original unit. It doesn't seem to like these Taiwanese sending units. This is on my 1966 Lemans. Any thoughts?

Tomorrow I'm gonna take the original to the radiator shop & see if the can fix the leak..

Did you check the resistance of the unit? Should be 0~90 ohms full range. This is what controls how far the gauge will move up and down.
The tank itself doesn't really have to be grounded for the circuit to work. The sending unit ground wire is grounded with a screw to the trunk pan. The rubber gasket isolates the unit from the tank, more or less. The positive lug on the sending unit gets the hot wire and it must be isolated from ground but that's built into the sending unit.

[lazlo]

Originally Posted by Greg Reid

Did you check the resistance of the unit? Should be 0~90 ohms full range. This is what controls how far the gauge will move up and down.
The tank itself doesn't really have to be grounded for the circuit to work. The sending unit ground wire is grounded with a screw to the trunk pan. The rubber gasket isolates the unit from the tank, more or less. The positive lug on the sending unit gets the hot wire and it must be isolated from ground but that's built into the sending unit.

Went thru this a couple of years ago with my 71. It worked with the original sending unit but not with the two Taiwanese replacements. I've had it apart three times now. I didn't think the tank needed to be grounded but wanted to make sure..

[lazlo]

The original sending unit has been repaired and the ohms check at 1.2 at the bottom and about 87 at the top. After the holidays I'll be putting it back in...

[1qikta]

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?

[Greg Reid]

But, if grounded unplugged without the sending unit plugged in it does not go to full...grounded or not it just lays on full.

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.

[1qikta]

Originally Posted by Greg Reid

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.

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

[Greg Reid]

Originally Posted by 1qikta

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

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.

[1qikta]

[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!

[Greg Reid]

Originally Posted by 1qikta

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?

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.

[1qikta]

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

[1qikta]

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.

["QUICK-SILVER"]

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

[1qikta]

Originally Posted by "QUICK-SILVER"

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

Thanks Clay,

I tested some stuff...

an old plug wire gave me .23
touching the probes together gave me .03
a 20 amp fuse gave me .02
my spare sending unit .0L
the sending unit in the car gave me .0L
an 1156 bulb gave .06
an 1157 gave me 3.3 (on bottom) and 2.8 (side to bottom)

I guess both sending units are bad.Any other tests on the sending units?