fuel guage has stopped working

[Romain]

I have been presented with another electrical opportunity. While working to get the tachometer working (thanks again), the fuel gauge that was working is now NOT. The Fuel Gauge reads empty. I have verified that the fuel sender is working - at the sender, it reads ~50 OHMs and I have over half a tank, so that seems right. I measured the resistance on C203, pin 10 ground and pin 17 ohm from the fuel sender and got the same ~50 OHMs.

 

When I plug up C203 into the cluster, the Fuel Gauge reads zero. Measuring the pins on the rear of the gauge, between ground and +12, it reads 12 volts. Measuring between ground and the sender screw (connected to pin 17), I get nothing. From what I read this happens when the resistance pin is grounded, but I don't see anywhere that could be happening. All my other gauges are working.

This all started with my speedometer cable broke and I thought, I'd love to upgrade my cluster, but it's been nothing but one challenge after another.

 

The circuits do pass through a little circuit board in the cluster and I have an extra of them and have tried both. I also have a spare rear film with the copper contacts (wiring) and have tried both of them. I have ensured the connectors are clean and making a good contact.

 

Please help Obi-Wan-Comanche-Club, you're my only hope.

[Pete M]

@Romain I split the new issue off into a new topic for ya.  

[eaglescout526]

You haven’t over tightened any of the screws have ya? I’ve done that and ruined two gauges. A temp and a full size gas gauge by doing that. 

[scaleless]

I don't have a Comanche fuel guage in front of me at the moment to verify the internals, but I can speak on guages of this era, at least. First make sure you have a good ground. You said you saw 12V between power and ground so you probably do, but a floating ground will peg the guage below E, too.  Failing that, the guage is basically a small electromagnet wired with very thin magnet wire, and if you shorted Gnd to 12V while fixing your cluster, you could have overheated those windings and fused them or burnt them out (depending on the position of your fuel sender). 

 

You could test it out of the vehicle by hooking up 12v and ground (I don't know if it has an open circuit failsafe or not so this might not be definitive) it may peg full.  Hooking a 0-90 ohm resistor or pot to ground on the sender line should change the position, too, if you have that available.  Depending on how it's wired internally you could also short the sender line to 12v and it'll either work or burn up.

 

Either way, if you've got 12V, GND, and your sender line hooked up and showing a resistance, it sounds like your guage is dead.

[Romain]

I tried a new gauge and the same thing. I put the dummy gauge back in and the fuel level stayed at a quarter tank (no movement at all) while I still have over half a tank. What is this notion of a floating ground and how to I identify that?

 

[scaleless]

A floating ground is one that has a different voltage than another electrically connected ground or isn't really even connected at all.  In this case, it would happen if the ground for the fuel sender or the fuel gauge aren't making good contact with the chassis.  It's probably not your issue because you were able to measure both 50 ohms at pin 17 and 12V at the input, but it still doesn't hurt to check both the grounds, I guess. 

 

Based on what you said earlier, it really does sound like pin 17 is shorted to ground, though.  Perhaps try to connect the fuel gauge directly to the C203 with jumper wires to eliminate the flex board from the equation?

[Romain]

Next round. I tried running a bypass wire on my spare rear wiring sheet (whatever that's called and the connector for pin 17 came off. I'm going to have to do a more professional job with a disconnect and solder to the screw and the pin 17 input. I ran the gauge directly to pins 10 (ground), 17 (ohms), and 18 (+12). OK, given the OHMs reading is 50 and my Renix gauge went to 1/4 tank. I have a gauge that looks identical from a '91-'96 that I likewise wired up and it went to FULL. It's so confusing - I'm starting to wonder much much fuel I actually do have. (I'm sure the 50 OHM/over half a tank is correct). I'll try a more carefully constructed connector for pin 17 in the next couple days and report back. She's going in tomorrow for a rear main seal that I replaced and now leaks more. Time for a professional for that.

[eaglescout526]

Woah woah woah. The later HO gauges read backwards with the Renix sending unit. So if the HO read damn near a 3/4-full, the Renix might very well be right with a 1/4 tank. 

[Romain]

Gotcha. No HO gauge for me. That's going back in that cluster I just can't use at all.

Thanks for helping me understand that.

[eaglescout526]

No problem. They look similar but it’s the way Chrysler changed up how the resistance is read for the HO years. Dumb if you ask me but it is what it is. 

[Romain]

Finally I got the fuel gauge working. No matter what I tried, the gauge in the cluster would just not work. I think I somehow ended up damaging/destroying two Renix fuel gauges and the HO gauge is backwards. Instead, I soled the problem by installing a CJ fuel gauge in the area below and left where I believe the Cherokee rear window wiper and defroster switches would go and putting the AMC logo in the fuel gauge spot. Interesting and it works. I'm done. I'm never opening the cluster again...

[Pete M]

sounds like a win to me. 

[Muncher]

Mine didn't work either and then randomly it did

[zomeizter]

My truck has that AMC logo in the cluster, one time I ran the fuel tank pretty low and a low fuel light came on, had no clue there was an indicator there!