[1968junkerjim]

On my 67 firebird the fuel gauge pegs pass full with power off turn key on slowly moves to empty and stay there have about. 5 gal of gas in it. Has new tank and sending unit. Unplug tan wire in trunk and has power to it with key on Any ides what may be wrong ?

[Peter Serio]

With the key off GM car & truck fuel gauges will display any random point on the dial, this means nothing. Only with the key on are they taking a reading from the sender inside the tank.

Did you test the sending unit with an ohmmeter before you installed the new tank?

[1968junkerjim]

Ok did not know that. Thought my old 68 firebird stayed at fuel level with key off. But that was years ago

[Peter Serio]

Since 1965 (A body) and 66-67 (B body) gauges in GM cars are made as small electro-magnets. The electrical current creates a magnetic field which acts upon a tiny disc shaped permanent magnet. The pointer is attached, via a very thin shaft to the disc (bulls-eyed) at the exact center of rotation. So you must have 12 volts "+" and a pathway to ground (thru a variable resistor sender) to get a reading. During shut-down there is a very slight amount of left over magnetic energy inside of the gauge so the pointer will sometimes move bit and then comes to rest. Where it comes to rest on the dial is purely random.

They are very well designed and generally last many years past the "normal" life span of the car. When GM car gauges get old they can have issues.

The other big problem that we have is a miss-match with the sender. Many aftermarket and replacement parts (senders) will not work correctly with a car that is nearly 50 years old. In many cases the parts are made wrong because the maker/seller does not have access to the original car manufacturers data on said gauge.

In the case of a gauge which would be very hard or impossible to find; in most cases I can repair those. I also calibrate to a specific sender.

[1968junkerjim]

Peter, thank you for all the information. I did buy the least expensive sending unit on eBay. I do have a spare cluster. I think i will take the time to switch them out , to see the difference between the two

[JLMounce]

Originally Posted by Peter Serio

Since 1965 (A body) and 66-67 (B body) gauges in GM cars are made as small electro-magnets. The electrical current creates a magnetic field which acts upon a tiny disc shaped permanent magnet. The pointer is attached, via a very thin shaft to the disc (bulls-eyed) at the exact center of rotation. So you must have 12 volts "+" and a pathway to ground (thru a variable resistor sender) to get a reading. During shut-down there is a very slight amount of left over magnetic energy inside of the gauge so the pointer will sometimes move bit and then comes to rest. Where it comes to rest on the dial is purely random.

They are very well designed and generally last many years past the "normal" life span of the car. When GM car gauges get old they can have issues.

The other big problem that we have is a miss-match with the sender. Many aftermarket and replacement parts (senders) will not work correctly with a car that is nearly 50 years old. In many cases the parts are made wrong because the maker/seller does not have access to the original car manufacturers data on said gauge.

In the case of a gauge which would be very hard or impossible to find; in most cases I can repair those. I also calibrate to a specific sender.

Interesting information here. I have a tanks inc tank with their 90 ohm sender in my '69 The gauged worked accurately with the factory tank and sender but has never really worked with the new tank and sender. The sender pad is spliced to the tan wire and also was originally grounded to the same ground as the wiring harness in the trunk.

That resulted in a gauge that stayed at just below 1/4 tank while the tank was in actuality full. Testing, I removed the ground which got the gauge working for a short time, but now the gauge again reads just below 1/4 tank at all times.

Any ideas there?

[Peter Serio]

Hard to say from just words in the Message. Is there any way that you can send me the dash gauge & let me test that for you? When you are dealing with a non-working fuel gauge; there are basically four things it could be: One, a bad dash gauge, 2 a wiring (plug-in) connection harness problem (open or intermittent tan wire), 3 the fuel tank sender or 4 some other wiring issue including a bad or loose ground.

The dash cluster in all of these 1960s GM cars is made from steel. This provides a mounting structure for the gauges and is also used as your ground pathway at the dash. The fuel gauge needs a ground at the dash. The sender also needs a ground.

Many gauge issues on older GM cars can be traced to a bad or missing ground.