Ground Loop Fuel gauge

Question

I just restored my car (1970 Datsun) and the only thing I can't get to work is the fuel gauge. I have a brand new gauge (just a fancy and expensive ohm meter really) that is correctly programmed. A brand new sending unit (giving the correct readings when not installed in the car) and I recently just replaced the wiring which I previously blamed for running out of gas numerous times.

It's an old car which means metal gas tank that is grounded. The sending unit has two pins (one of which is connected to gnd via tank) and the other one is the sensing pin for the gauge. I am running both pins right up to the gauge in a shielded wire (floating at the sending unit and connected to GND at the gauge).

The car is properly grounded and I also tested it by running another wire from the - terminal of the battery directly to the GND pin at the sending unit.

However, I seem to be running into some kind of ground loop issue as I get wrong readings and electrical load on the car (e.g. turning on headlights) affect the gauge.

I drew simple schematics on how everything is wired.

Measuring the pins directly with an ohmmeter (not powered from the car's battery) gives me the correct readings.

Any idea how to isolate the gauge? I thought of maybe using a diode?

Thank you very much

Kind Regards Michael

Answer 1

Fuel and temperature gauges on old cars using variable resistance senders need a voltage stabiliser in the supply.

Answer 2

If the gauge needs a case ground even though it has a ground wire any paint or corrosion where it sits in the instrument panel can effect it and I know the wiring is new but it's easier than you'd think to break shielding or nick it on an edge and not notice it.

Answer 3

Any idea how to isolate the gauge? You haven't posted an image showing how the unit is fixed in the tank, so I'm assuming it is secured there by studs coming from the tank, and nuts, or perhaps just bolted to the tank. If the unit piece that touches the tank is metallic (so you want to isolate it), try to put a rubber seal between the tank and the unit (which I'm almost certain the unit would come with). Then use pieces of wire insulation that you can cover the bolts/studs in such way you prevent them to touch the unit, then plastic or rubber washers, then metallic washers at the end. The most important part here is to isolate the bolt/studs stem correctly with the wire isolation, just find one piece of wire with proper thickness and strip the plastic off it...even a beverage straw would work...

Answer 4

A thing that will solve once and forever the gauge problem:

Replace the wire between the sender and the gauge with two wires. Disconnect the sender from the tank ground. Use the second wire to ground the sender together with the gauge.

What is wrong, then?

"An old car" and "properly grounded" are somewhat contradictory. The headlights affecting the meter prove it.

The car has at least 3 things that can be improperly grounded to each other:

• the chassis. Few things grounded here, but the tank is a good suspect. When not properly grounded, the chassis gets ground from unexpected points with unexpected resistance. In some cases, it may not be grounded at all.
• the cabin (it may have more than 1 electrically disconnected or poorly connected parts). Point welds of 1970 may not be great at conducting electricity. The voltage regulator and the headlights usually are grounded here. The headlights may shift the ground voltage for the regulator.
• the powertrain assembly (engine + transmission). The alternator is grounded usually, but not always, here. The battery minus may or may not join the party.

An old car of mine used to have a copper pipe from the tank to the pump (i.e. the engine). I happily replaced the copper pipe with a rubber pipe and the tank gauge got crazy. Mounting the winch grounded the chassis for good (25mm2 wire) and the gauge suddenly started working as expected... when not winching.