Do fuel gauges fail in this manner?

[Flatback72.]

Well I noticed the other week that after filling my car up with fuel that the gauge was only showing 1/3 full.

Thinking that the tank sender has gone kaput I dragged out a brand new sender that I bought a while ago for contingency stock and plugged it in and moved the sender arm to it;s extremity to simulate a full tank of fuel.

The gauge pointer with the new sender attached did move a little higher but still sat at around 1/3, so I am convinced now that there is no issue with the current sender unit in the tank.

After a fairly long drive the fuel gauge pointer has moved steadily towards empty (I estimate the tank is still half full), so the gauge does appear to be working, but only in the lower 1/3 range and not accurately.

My guess now is that the fuel gauge itself may be faulty. I do have a spare fuel gauge available to try in an old second hand cluster, but being a fiddly job to remove and replace I'm being a little lazy by posting on the forum first before pulling my dash and cluster apart.

So my question is: do fuel gauges fail in this way, where the gauge still works but the range it works over is diminished to the lower 1/3 of the dial? Or is there some other reason not sender or gauge related that could cause this to happen?

 

[Don C]

When you tried the new sending unit did you ground it to a good ground?

I would start by checking grounds and connectors. The fuel gauge works by the changing electrical resistance in the sending unit. Any bad connection or ground will change that resistance value, causing the gauge to be inaccurate.

 

[Flatback72.]

When you tried the new sending unit did you ground it to a good ground?

I would start by checking grounds and connectors. The fuel gauge works by the changing electrical resistance in the sending unit. Any bad connection or ground will change that resistance value, causing the gauge to be inaccurate.

Mmmmm, I didn't actually earth the body of the sender, just plugged it in to the existing plug. Tomorrow, I'll plug it in again but this time run a test wire from the sender body to a good ground point and see if that makes a difference. Thanks.

 

[Jeff73Mach1]

I would ask Midlife, but I think I would look at the instrument panel voltage regulator. They are known to fail and I am positive that they have an effect upon the gas gauge.

 

[72HCODE]

dash regulator would be the first thing to look at or change.

there is an adjustment on them where you can alter the voltage to the gauge.

the original ones were mechanical like the original Voltage regulators in the engine bay, they make solid state electronic ones now.

 

[red351]

My fuel gauge readings would be way off at times, but so would oil and temp gauge at same time. The old mechanical constant voltage regulator was failing

 

[midlife]

It's probably not the gauge, but the contacts between the dash cluster connector and the circuit card, the connectors to the CVR, and/or possibly the connectors from the circuit card to the fuel gauge. High resistance causes the gauge to read low values. Another place for high resistance to come into play is where the tail-light harness plugs into the underdash connector. I've found that many molded connectors contain high resistance when they shouldn't. To fix that, I sharp tug on the wires going into the back of the molded connectors sometimes causes the internal crimps to catch on the copper wire strands a bit better.

 

[Flatback72.]

Thanks for the replies guys, I had no idea there was an instrument regulator on the back of the cluster. I've just ordered a new solid state regulator off ebay which I will install when it arrives, and also check/move all the connectors on the circuit card while I'm at it. I'm hopeful this will do the trick.

 

[Flatback72.]

In the interest of closing out a conversation thread, thanks to the guys that indicated my fuel gauge issues may be the instrument panel voltage regulator. It only took me 15 months but I finally did the change out the old regulator this week with a new solid state regulator and the fuel tank gauge now seems to be functioning correctly.