Fuel gauge diagnostic

R

rabiddawg

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Lafayette, LA
I had to put a new tank in my 81 and now the gauge is pegged to the right, way past the full mark, which I believe means somthing is grounded. Any ideas on how to go about checking everything so that I don't throw away money replacing the good parts? Everthing worked with the old tank.

I have looked at the contacts on the back of the cluster and all looks in order there. Basically no matter what I try the gauge doesn't respond.
 
On my '72 the gauge wire is on the pickup tube under the tank on pass side bottom, I think yours is up top, but the wire is a brown lead that plugs onto the sending unit stud, or a bolts holds it down...is that at all reacting when you disconnect it and leave it loose, and then ground it to the tank???

there is a black wire that connects to the tank somewhere, that is to ground that sending unit if ti's not connected the tank is floating at 12 volts and the sending unit is just a 0-90 ohm variable resistor on a float....so it is in series with the gauge, I forget which way the gauge goes when open vs shorted out....
 
The ground wire is located at the very top of the sending unit, where it bolts to the tank. I think it just slides on, and is very easy to knock off. There should be only 3 wires to verify. Did you replace the fuel pump with a higher pressure pump?
 
GM senders are 0 Ohm empty, 90 Ohm full. Your sender has an improper ground. usually attaching a ground strap from the sender to the chassis will cure it. Std. the sender is grounded via the tank itself and the tank straps to the frame. The tank wire is the signal wire. Check this first if it's making proper contact.
 
Twin_Turbo said:
GM senders are 0 Ohm empty, 90 Ohm full. Your sender has an improper ground. usually attaching a ground strap from the sender to the chassis will cure it. Std. the sender is grounded via the tank itself and the tank straps to the frame. The tank wire is the signal wire. Check this first if it's making proper contact.
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On my '72 there is a black push on spade lug ground wire going to the frame and to the sending unit/pipe assy itself, not relying on any other path to ground, not even the slip/clamp ring to the tank, which can be isolated via the tar/rubber mounting material, to keep from vibration rubbing the tank thin and leaking, I presume....I remounted the tank using roofing tar paper rolled up several layers thick in the rub area/corners....

:drink:
 
Xanders 77 had that too, a tab part of the whole filler plate assembly. The 78 up tanks don't have that, at least I have never seen it.
 
Twin_Turbo said:
GM senders are 0 Ohm empty, 90 Ohm full. Your sender has an improper ground. usually attaching a ground strap from the sender to the chassis will cure it. Std. the sender is grounded via the tank itself and the tank straps to the frame. The tank wire is the signal wire. Check this first if it's making proper contact.
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This could be the problem because the new tank has a coat of paint on it where the old one didn't? I did clean the coat of paint off the sender (I applied) so that the bolts would connect the sender with the tank, but I never checked to see if the tank itself is grounded.

No matter what I have tried, I can't get the gauge to react. I will try a ground wire after work and I will be unplugging the gauge harness and check for polarity between the two wires that plug into the sender. That should tell me if my wiring is good.
 
Okay, I'm an idiot, I just checked my 82 spare sender again and it doesn't have the spade, upon closer inspection it looks like it's broken off, that's what led me to believe it didn't have it. checked another one and sure enough it's there. This means your problem is in the wiring somewhere
 
Twin_Turbo said:
Okay, I'm an idiot, I just checked my 82 spare sender again and it doesn't have the spade, upon closer inspection it looks like it's broken off, that's what led me to believe it didn't have it. checked another one and sure enough it's there. This means your problem is in the wiring somewhere
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I wanted to say I thought the wiring technique would be the same, but it's only been 6? years since working on one....and yes, the problem is in the wiring somewhere, it's WHERE that is the issue.....:bonkers::rofl:

Nah, to the OP, take a nail or something and connect to the brown wire off the fuel sender watch the gauge as you connect and disconnect it from ground, right there on the tank metal....if the meter swings from one to the other, that means the wiring is good and your sender is most likely bad....PIA....

:rain:
 
I would pull the sender just to check the ground. While it's out you can verify if it's good or bad with a VOM. I'll check to see if I have any old pics that might show the spade commector.
 
I unplugged the wire harness from the rear of the gauges. Then using my ohm meter I checked to see if the two wires were somehow connected and got nothing. I then reconnected the harness and tried to ground the sender/tank to the negative post on the battery via a wire and got no reaction from the gauge.

Nothing I have tried has given me anything and I admit to being totally ignorant about eletrical stuff.
 
Dirtbuster1 said:
I would pull the sender just to check the ground. While it's out you can verify if it's good or bad with a VOM. I'll check to see if I have any old pics that might show the spade commector.
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What is a VOM?

If I use my tester set to react when I touch the two leads together should the needle move if I put a lead on each post on the sender? I think it does happen but will verify at lunch if I remember.

I do know this. If I put a lead on each post on the sender and move the float up and down all my needle on the tester does is peg. Moving the float makes no difference.(this was last weekend with the sender in my hand)
 
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VOM= Volt Ohm Meter.

Set on the OHMS position, the meter should go full scale when you touch the leads together, indicating no resistance.

If you hook one lead of the meter to the post on the sender that hooks to the gauge and the other meter lead to the base of the sender it should read differently as you move the float. If your meter goes to full scale and you can get no meter movement either your sender is shorted internally or the meter is set on too high of a scale. WIthout knowing what meter you have it's hard to tell- If you can pick the scale, use X1. X10 or anything higher will make the meter move so slightly you may not be able to see it.

I read that you had check with your meter at the gauge- there, one wire will be 12VDC (key on) and the other one (tan?) goes to the sender in the tank.
 
TimAT said:
VOM= Volt Ohm Meter.

Set on the OHMS position, the meter should go full scale when you touch the leads together, indicating no resistance.

If you hook one lead of the meter to the post on the sender that hooks to the gauge and the other meter lead to the base of the sender it should read differently as you move the float. If your meter goes to full scale and you can get no meter movement either your sender is shorted internally or the meter is set on too high of a scale. WIthout knowing what meter you have it's hard to tell- If you can pick the scale, use X1. X10 or anything higher will make the meter move so slightly you may not be able to see it.

I read that you had check with your meter at the gauge- there, one wire will be 12VDC (key on) and the other one (tan?) goes to the sender in the tank.
Click to expand...

Ok, thanks. I must have a short in the sender because the meter pegs on x1 &x10 and has no reaction to moving the float. I have been all over this sender but don't see anything wrong with it and if it broke, it happened on the shelf.
 
There are 3 wires that go to the fuel pump. 1 Black, 1 brown, & 1other. Black should be ground, brown to fuel gauge, & other 12 volts. The gauge reads full to empty on 0 to 90 Ohms. I would think that the VOM would have to be on an automatic scale, or a manual meter on 100 ohms to see a movement. How are you moving the float? Is the fuel pump out?

I can't find any past pictures to look at. Do you have any pics of the fuel pump?
 
Dirtbuster, I have an 81. No pump in the tank.

I had the sender out last weekend; a neighbor and I tested the sender as you guys have described but got nothing but a pegged meter reading regardless of where the foat was moved. This evening I checked it and rocked the car. When I did this I did observe the meter needle move back and forth as the fuel sloshed about.

Now I am thinking my gauge may be bad.
 
Sorry,

I thought the 81 & 82 had the same fuel pump. The float arm should still work the same. 0 - 90 Ohms. The gauage can be check by jumpering the wires on the back of the fuel gauge. 1 way full, the other goes empty.
 
I took the car out night before last for a little run around the neighborhood. Then I realized the tail lights aren't on. All this worked up until I did the conversion and removed the interior for paint. I must have a bad wire somewhere. :mad:
 
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