[Rich-L79]

When the truck is turned on (doesn't have to be running) the fuel gauge works fine and reads accurately, but when I turn the truck off, the gauge needle drops down below empty. The needle can take a few seconds to "warm up" when I turn on the truck, just yesterday the needle was stuck below empty but moved up to register the tank contents when I rapped on the gauge lens.

What's at fault here? What is the fix? This has to be something fairly simple I'm guessing since the gauge works fine when the ignition switch is on. Previously the needle registerd the appropriate tank contents whether the ignition was on or off.

 

[John_Muha]

It's mechanical Rich. The movement is sticking.

 

[Rich-L79]

Originally posted by John_Muha:
It's mechanical Rich. The movement is sticking.

I don't think so, I can sit in the truck and watch the needle slowly drop after I've turned off the key. It used to stay fixed on whatever position it was in at the time the key was turned off.

 

[John_Muha]

But once it falls down it sticks. Battery isn't changing. The float in the tank ain't moving. Back to pickin'on the meter movement.

 

[Rich-L79]

And of course I had a spare gauge a few years ago and practically gave it away. I guess I don't need to replace the gauge until it stop registering altogether when the ignition is on.

The sticking part is new and it only sticks at way below empty. The falling down part started about last Fall. But should it fall down at all? It never used to.

 

[John_Muha]

Sweep type dash? What years or models does it cover. I'll try and keep my eyes open in my travels.

 

[Rich-L79]

I'm talking about my '69 Chevy pickup. The fuel gauges are pretty much the same from 67-72, but the 69-72 versions look correct for my application. I have a good source for a gauge, that's not a problem and I think they are reproduced as well. I just figured the problem might be related to an "electrical leak" or something I could check into and fix if I just knew where to start.

I guess this whole question begs another question: why don't fuel gauges in these older cars drop the needle when the ignition is turned off? My Civic does this but that is the way Honda gauges work. On these old GM gauges, when the power is cut, what keeps the needle in position? If no ohms are read (no power = no ohms in my mind) why doesn't the gauge drop?

 

[John_Muha]

There’s nothing in the circuit to make the gage move once you remove power from it. Any movement after you turn the key off is just the gage itself. These fuel gages are dampened movements or slow reacting. If they weren’t the gage would jump all over the place as the car hits bumps in the road making the sender move as fuel sloshes.

 

[Coppertop]

In addition to what John stated, there is also some residual magnetism in the movement of the guage. There are two opposing coils which create eletromagnetic fields to operate the movement of the needle. My '70 can literally sit for 6-8 months with no power connections what so ever and the needle will stay put exactly where it was when the car was running. Only slowly will it start to "droop" towards "E" after so long. I call it "guage memory".


**Also Rich, if you could please call me tonite I'd appreciate it as I've got some news for you!

Joe

 

[Rich-L79]

Whoops! I didn't see this until today (Thusday) so I'll call you tonight.