The Stovebolt Site Forums The Shop Area The Electrical Bay gas guage

I'm having trouble with a no-brain simple circuit, To make this worse I'm using Randy R's printout to boot! The wiring is correct,the tank is full, 12v at the correct terminal of dash,12v at the sending unit. The sending unit(new in box) had to be adjusted(wiper to Rheostat not making contact) now reads correct ohms. The problem? when ign is turned on, needle pegs to empty. I suspect the tank unit is NG. Whats this about grounding? Is the gauge insulated from the mounting pan on both sides, or just at the rear where the wires attach?

tools94,

A gauge that swings to EMPTY when the key is turned on is an indication the circuit has found a full ground. This could be a wire pinched and broken, or as you said, a bum sending unit.

If you turn the key ON and pull the wire from sender, the gauge should read FULL. Leave the key ON and touch the wire to a known good clean ground. If the gauge now swings to EMPTY, the gauge and wire are fine and the culprit is probably the sending unit.

It's pretty typical to run a small wire from an attaching screw on the sender to a good ground on the cab, this insures that the system works. The gauge should be grounded too, but it usually accomplishes this through a bond to the dash assembly.

It might be a good idea to update your profile with the type of truck you are working on, it makes it easier for folks to answer or help with your questions. Good luck.

Stuart

Ok, thats a path to re-check. I did bypass the harness and used fresh wireing, to include the ground you mentioned, with the same results. If the gauge bonds itself via the dash assembly, then the question of an insulator at the rear of the assy becomes moot. For if no insulator was used, a direct short would occur by attaching the 12v lead to the dash bonded assy,no?
The subject vehicle is a '52 3100 5 window w/a 235 L6

tools94,

Both the terminals on the back of the gauge are insulated just as you describe. One of the two coils internal to the gauge is bonded to the case, it's inside, you can't see it. This means the case of the gauge must be in contact with some portion of the gauge cluster which is usually is.

Stuart

There is a contact. Its on the circuit bd between the terminals in the form of a large drop of solder which penetrates the circuit bd and connects to the rear of the coil case. As the terminals are drawn up from the rear, the gob of solder makes contact and thus a ground.Thanks for the exchange of info, I now have a better grasp of the what n why. and, I'm not sure that I didn't self inflict this grief. Tomorrow will tell. stay tuned

On my gauge, the back of the assembly has a small brass plate which contacts the bezel frame when the unit is bolted up. This is where my ground is made.

I would suggest that you check the resistance from the sending wire to ground, with the sender attached. If you get a low reading, you have to find out whether the lead is shorted or the sender has a problem.

I stepped back and reassesed the situation and the $$$ paid out for these parts and dialed my supplier and laid out the facts. I'm awaiting UPS with a shipment of a new sending unit & gauge. The aftermkt supplier is replacing my grief with 'new in box' parts. Perish the thought, but it was 'new in box' parts that got me into trouble at the git go. Has anyone invented quality control yet?

They are really going the extra mile for you. Only one of the parts will be defective, if at all.

Thank you to ATOMARC and TRUCKERNIX for your assistance.the new parts arrived. While enroute, the downtime was used to test the gauge which was OK. The new sender was also less than prime, but a least this one gave ohm readings. It was found to have a limited travel that read 3/4 full at its end limit. A little tweaking and full now corresponds with a full tank. Thanks again!!