[F ROCK]

stuck at full. where to look first?

[sleepy]

Wire connections, wire rubbed through and shorting to a ground, sending unit, gauge.
I would start with the wire from the sending unit to the gauge and make sure it's not shorting to a ground.

[F ROCK]

can i see that wire w/ out pulling the tank? i can see the ground. then i have one that goes over the top of the tank near the bumper, cant see where that one goes. oh, and thanks!

[sleepy]

If you can get to the connections on the sender. the easyest way to check is to disconnect the sender and check the gauge. if it still reads full the wire between the gauge and sender is probably grounding out. That doesn't necessarily mean the gauge isn't bad or just stuck. if it reads empty the the sender is probably bad. Either way you probably need to drop the tank to have enough room to work on it.

[F ROCK]

thanks, it's 60+ degrees out there so i'm just gonna drop in some fuel and rip it up a bit. i'll get to dropping the tank next time.
btw the gauge worked fine when i parked it; dont know what happened.

[Schurkey]

If the wire from the gauge to the sending unit were grounded, your gauge would read EMPTY not full.

If the wire from the gauge to the sending unit is OPEN (broken); or if the wire from the sending unit to ground is OPEN, the gauge will read past the full mark.

If there's high resistance between the gauge and sending unit; through the sending unit; or between sending unit and ground, the gauge will read optimistically--that is, it will always read higher than the actual amount in the tank. Common as dirt.

Another possibility is that the float may be stuck/jammed somehow and not responding to the level of fuel in the tank. A bumpy road--or sudden stop--can sometimes provide a temporary fix.

[PLAY400]

I just finished going thru all the BS with the sender. Work sometimes then not. Needle would go to full which is an open circuit. Bought a new assembly as all the wiring and connector tested good all the time. The brand I got was made in Taiwan but considering the price and quality of some of domestic I think I did well. Gas gauge works now!!!!!

BTW I open the sender figuring I could clean it and ????? after. Found the winding assembly had let of the case at 1 end and there was an break in the winding around the 20 ohms mark that would make and break intermittantly.

[Bill Hanlon]

I agree that a full gauge indication is caused by an open, not a short.

To troubleshoot,

1. remove the wire on the back side of the gauge that goes to the tank. Connect a short piece of wire to the gauge terminal that you just took the tank wire off of. Turn on the key. Gauge should read full. Touch the other end of the new wire to ground. Gauge should read empty. If it doesn't, the gauge is shot. If it does read empty, turn the key off, remove the temporary wire, reattach the tank wire and proceed to step 2.

2. remove the wire from the sending unit and connect it to ground. Turn on the key. Gauge should read empty. If it doesn't, the wire between the gauge and the sending unit is shot. If it does read empty, turn the key off, reattach the tank wire to the sending unit and proceed to step 3.

3. Remove one of the screws that holds the sending unit to the tank. Clean to bare metal where the screw attaches. Put the screw back in using it to hold down a short piece of wire. Attach the other end of the wire to a good clean ground. Turn on the key. Gauge should read empty. If it doesn't, the sending unit in the tank is shot. If it does read empty, turn the key off, make your new ground wire permanent and all is well.

[sleepy]

SO Pontiac is just the opposite from Chevy and every semi on the road? OK
I'd still start with the wire between the gauge and sender, then the ground, then the sender, then the gauge.

[Schurkey]

Originally Posted by sleepy

SO Pontiac is just the opposite from Chevy

Nope. Probably the very same parts used except maybe the gauge in the dash (styling concerns.)

Originally Posted by sleepy

and every semi on the road?

No idea.

Originally Posted by sleepy

I'd still start with the wire between the gauge and sender, then the ground, then the sender, then the gauge.

Seems reasonable.

["QUICK-SILVER"]

IMO Easiest Place to start is body harness connector in trunk.
Unplug it, ground tan wire going to guage, key on, guage should go to empty. If test works, move to trunk side of connector. If not, move to front body connector (near kick panel) and do the same ground test on tan wire. No empty reading, move to guage. If you do get an empty reading, check ribbon (flat wire) for breaks, between front and rear connectors. With long leads, it can be checked with an ohm meter.

From the trunk end of connector, check tan wire to ground with an ohm meter. Reading should be between 0 ohms/empty and 90 ohms/full, If not, faulty sending unit or bad tank ground. Bad unit or ground and ohm meter will have high or infinite reading.

Starting in the trunk and doing the front ground test and rear ohm reading test narrows down problem area fast and easy. Testing at that one plug-in cuts trial and error (diagnostic time) in half and points you in the right direction. While verifying functionabilty of other components.

AND one of them FWIW's, at sometime or another, late 70s or early 80ish, GM truck fuel guages worked opposite to cars. Don't know why for sure but probably had something to do with dual tank switches. Learned this the hard way. May have even changed on 80ish model cars, DK TOO NEW for me.

[SS-TA]

As quick silver has mentioned most GM guages/sending units work with a 0-90 ohm range. However a full tank will have a significantly lower resistance than 90 ohms. I went through this process with 72 Olds Cutlass with a new GM sending unit and tank, and confirmed that a full tank (20 gallons) yields a resistance of 54-57 ohms, numbers like 75 ohms will go way past the full mark.