Fuel Gauge Pegged...

A few weeks ago my old fuel gauge decided to start showing full all the time. As soon as the ignition switch is on it pegs. I replaced the gauge: same thing. Replaced the sending unit: same thing. I bypassed the line from the gauge to the sending unit to see if it was ground: same result. Temp gauge is working fine: in case someone thinks of the voltage regulator. Any ideas?

I thought maybe it needs a resistor, but then why was it working before and not now? I don't see a pre-existing resistor anywhere. Thanks.

 

Ok, here's the deal. The PO installed a VW sending unit on the under seat tank. When I bought the Jeep it had an aftermarket fuel gauge on it. The gauge worked, but read full at about the 3/4 mark. One day it starts pegging past full all the time. I replaced the gauge with a Sunpro gauge almost identical to the "original" aftermarket gauge. Same result. I replaced the VW sending unit with the same part, and same result.

I'm getting 12V at hot side of gauge. 6.2V at sending unit with key in ON position. Tank is full and I get 20.2 ohms of resistance. Wire to sending unit from gauge gives me .6 ohms resistance. I'm baffled. Do these numbers send off any red flags for anyone here? Thanks. :?

 

The gauge and the sender have to be "matched." Meaning the resistance the sender sends at full or empty must match what the gauge is designed to respond to at full and empty. If the gauge will read full at 10 ohms and empty at 73 ohms (those are standard values for the original gauge/sender, not aftermarket) the sender has to send those values. With mismatched gauge/sender setups, the sender may be sending things in reverse (73/10 instead of 10/73), and/or might be sending the wrong resitance values entirely. Best way to check what you have in there is with a meter. Easy way to fix it if this is the problem is to get a matched gauge and sender set.

If the sender is grounded well, the next thing to check is the ground at the gauge itself. If the gauge is mounted in the speedo head, then check the gauge to speedo ground contact and also the speedo head ground. In original Jeep setups, if you are getting anything above about 5V at the gauge you will fry the gauge. That is, IF it's an original style gauge. If it's aftermarket it might work on 12V-14V.

(Note that old analog type multimeters will show the 5V reading in the original Jeep gauge setups. A digital meter will show a rapidly fluctuating reading rather than the 5V reading.)

From what you describe I'd look for a bad ground, probably at the gauge or speedo head if the gauge is mounted to that.

Stan

 

6.2v at the sender is a bit on the high side, high enough to peg the gauge. with 12v at the guage, check the post that goes between the fuel and temp gauges, should not be over 5.5 with an analog meter.

 

Yeah I checked it. Seems to be fine.

 

Thanks for all the info. I'm going to double check the grounds today. If everything checks out then I'm going to go buy an adjustable resistor and try that. Like you said the gauge and sending unit are not matched so maybe too much voltage is passing through the gauge.

 

Well I added some resistors to the line going to the sending unit and finally got the gauge to come down a bit. The tank is full right now so I added enough resistance to make it read full. I'll have to wait until it's almost empty and probably readjust the resistance to make empty the accurate end. Thanks for all the input.