71D20 gas gauge non-operational - help!

[MSN Member]

From: 3x2x4sp389
Sent: 3/15/2007

I bought this otherwise very functional D20rd a year ago and the gas gauge never worked. Taking down the gas tank is a big pain but did that, got all the rust out, sealed it, new sending unit and sock filter on the pickup tube. Made damn sure the wires were connected and grounded properly, then put the tank back. Still no movement on the gauge. Temp gauge works fine as do all the others so I don't think it's the limiter. How do I test for a bad sending unit without taking out the tank again? It is so hard to get to the wires, I'm tempted to cut an access panel in the floor . . . Thanks in advance.

[75Travco]

Sent: 3/18/2007

A few suggestions for troubleshooting the fuel gauge without dropping the tank:

With the ignition switch on, grounding the sender wire should make the gauge read "FULL".  This will show if the "front half" of the system is working properly.

The sender needs a good ground.  A continuity check between the tank and the frame would confirm this.  The literature with the new sender I installed recommended running a bonding strap from one of the flange screws to ground.

If the sender is new it should be OK, but it wouldn't hurt to check it.  The resistance measured between the sender terminal and ground (when disconnected from the gauge) will vary with fuel level, something like 20 to 80 ohms.  An open circuit, or no resistance indicates trouble with the sender or wiring.

[denisondc]

Sent: 3/18/2007 10:07 AM

If you don't find the problem with the advice so far, see if the case of the fuel gauge itself is grounded.
If all else fails, it could be a bad resistive winding on the bimetallic arm inside the gauge.
If that is the case, you might find a replacement on ebay. I just did a search for " fuel gauge mopar " and got several listings. I would suspect some of them could be made to fit your instrument cluster with minor mods. I believe most of them would use the same 'input voltage' from the voltage limiter, and the same 10-80 ohms input from the sender unit.
I just put in a bid to ebay for a box of 8 'mopar fuel gauges'. If I win, and if they work with the mopar sender unit values, I would be willing to sell you one for what it cost me.

[Slantsixness]

Sent: 3/18/2007

To go al little further...

The gauge wire should read about 10K ohms to ground with the tank mostly empty.

There is a connector at the floor, just behind the accelerator pedal where the wire comes up, then it goes directly (but in a harness) to the fuel gauge.

The dash regulator is a common problem, but you've indicated here that it isn't. But here's a simple test. on one side of both the temp and fuel gauges, there will be a voltage that pulses (no, it's not 5v, rather a thermoelectric 5v regulator, and its common to read from 4v to 9v from the output, depending on how good your multimeter response is). if you dont have at least 4v... something is wrong. Again if the gauge is shorted, or the gauge input is shorted, the pulsing of the voltage will be quicker, and the voltage higher. You will also notice, on the 68-72 models only, that the alternator gauge might get hot to the touch, there's a reason for this.... the regulator is inside the Amp gauge! if it's getting noticeably hot, and the fuel gauge appears inoperative, the gauge itself could be shorted, or even the oil pressure or temp gauge. the Amp gauge is the only gauge that is unaffected by any other gauges, however the regulator is built in to it.

So, unfortunately... here's what to do:

Be sure that the float actually operates correctly in the tank, and that a low (10-50) ohm reading is obtained at the "full" float position, and that in the empty position, the float reads high (10K-100K) ohms. Sometimes, after market fuel floats can be wired more than 1 way, so the tank would read empty when full (that sucks!)..
if you get nothing at all in step 1, you have a grounding problem to the sending unit. the tank is at ground, however there should still be a grounding wire from the sending unit screw to the frame. this wire does not have to be insulated, but the original strap was. bare braid near a gas tank, could cause a spark (although I doubt it ever would)
Remove the backer board (on the floor,carpeted, with the hi-beam switch in it) and look at the 5 connectors. the largest one is where the fuel wire is, others are lighting, turn signals, dual/mom wires etc.... this wire comes in on the main connector. Ohm it out with the sending unit, as you could just have a broken wire under the chassis (common) or the connector is corroded.
check the gauges. A good indicator that something is totally buggered up is if other gauges don't work, OR... after market gauges have been added. It's unfortunate, but every back yard mechanic knows little or nothing about electronics, and this is a VERY common problem (butchered dash wiring)
Check the regulator. If you find it is the problem, you can replace is with a solid state 5v regulator, or an original 3 pin mopar regulator without sacrificing the alternator gauge.. if that's the case, post that here, and I'll post how to do it...
This is an IMPORTANT safety call while you're under the dash: EVERY DODGE MADE FROM 1966-1990, REGARDLESS OF AGE OR CONDITION, WILL BURN THE IGNITION CONNECTOR AT THE STEERING COLUMN. SHODDY EFFORTS TO FIX THIS (IMPROPERLY) HAVE RESULTED IN ELECTRICAL DASH FIRES. THERE ARE QUITE A FEW MEMBERS HERE THAT HAVE HAD THIS UNFORTUNATE EVENT HAPPEN. IF YOU SEE THE BURNED CONNECTION ON THE BLUE WIRE IN THE COLUMN CONNECTOR, DON'T IGNORE IT! REMOVE THE WIRE FROM BOTH SIDES OF THE CONNECTOR AND REPAIR IT BY BYPASSING THE CONNECTOR. If it has been repaired before, check it thoroughly again.   

So.. I hope this helped you. If you have to get under the dash of a 71, remove the driver's seat, but either way, I hope you are a really small guy or a contortionist... cause it's a pain in the butt to get under there and work! As you might have already suspected, yes... you can take the left grille panel and black under panel (covered plywood) off the front of the winnie, but you'd have to have some long arms, the dash is 2 and a half feet back on one side.. whadda pain!

Been there, still got the backache...

Tom

[MSN Member]

From: pinballlarry1
Sent: 6/16/2007

I just checked mine out, easy to do once you cut the floor open. 78 Dodge Itasca.After corrosion cleanup, it reads 200 ohm when full, 12 ohms when empty. The level sender is easy to remove, be sure to clean the metal really good around the sender screw hole that completes the ground path.

pinballlary in San Diego