fuel guage not working

[Woodie]

Well I ran out of gas yesterday. I thought I had more....
The guage is not working, well it is showing E. The wires from the sender unit are where, into the trunk and then to the guage, or along the underside of the car. ??

 

[RobSelina]

most likely the ground at the tank. Drop it and clean it up.

 

[jlaird]

As I remember the wire comes down through the body from the top inside. First shot, see above, clean things up good, make sure you have a good circut.

 

[dklawson]

The normal Lucas color codes for fuel sending wire(s) are a black ground connection (when fitted) and a green/black wire between the sender and gauge. If you can reach the back side of your fuel gauge you can disconnect the green/black wire there and temporarily fit a jumper wire between that gauge terminal and chassis ground. Switch on the ignition and watch the gauge. If the needle moves to full you have a problem with one of three things: 1) a break in the green/black wire, 2) a failed ground connection at the sender, or 3) a broken wire inside the sending unit.

Assuming the gauge is working properly, you can move that jumper wire to the connections at the fuel tank to determine where the problem is from the three possibilities listed above.

 

[drooartz]

My gauge jumps to 1/4 full when ignition turned on, and stays there. Ideas? Same sort of things to test?

 

[RobSelina]

if I'm not mistaken, the sending units are basically variable resistors, and the fuel guage is basically an ohm meter with a pre-set range (ie 10ohm to 90ohm). Drew, if the best you get is 1/4 full, that probably means you're getting 3/4 of the resistance in the circuit even with the sending unit putting out it's minimum resistance. The most likely problem is just a ground IMO, since it's easy to add resistance at a bare metal connection. Failing that, as Doug said, the sending wire or the sending unit can also fail, but I find it much less common.

EDIT - just remembered you have a new wiring harness. Did you clean the ground at the sending unit before screwing it in? If not, still check there. After that, do Dougs test since the guage or sending unit are more likely culprits than your new wiring.

 

[drooartz]

I didn't replace the sender wire from the tank directly, since it required dropping the tank (and the harness didn't come with a new wire for that). Dropping the tank for cleaning is on my list, so I'll look at it then. Hasn't been a big priority, but with the Tunebug going off the road for a while, I figure now's the time.

 

[dklawson]

If the car where the gauge is "jumping" to 1/4 full is the Bugeye in your avatar, and you do mean jumping (not slowly sweeping), your gauges are pre-voltage stabilizer and work differently than those in later cars. They aren't a volt meter, per-se, they work by balanced magnetic fields. There's a very good explanation at the MGA-guru site. Look here:
https://www.mgaguru.com/mgtech/electric/fg_01.htm


Your sender should swing from 0 ohms empty to 90 ohms full. If you pull the green/black wire OFF the back of the gauge, the needle should peg to FULL when you switch on the ignition. If with the green/black wire disconnected the gauge still only goes up to 1/4 tank, your gauge is out of calibration OR you have a ground problem. On the early gauges it is extremely important that the case of the gauge has a good, clean ground connection. Clean the mounting bracket up and run a dedicated ground wire and repeat your test.

Don't discount the fact that you could have a leaky float. The float may be half submerged and really measuring 1/4 tank. A quick test for this is to switch on the ignition and use a bent coat hanger through the filler neck to lift and submerge the arm while you watch the fuel gauge.

 

[jlaird]

My money says a bad ground. But I've been wrong before.