Fuel sending unit... different from 88 to 92?

[NotMatt]

Alright, got an 88 comanche with what appears to be an gauge cluster from a 92 cherokee in it, but the fuel gauge doesn't work right. It reads backwards, and in a smaller "window". Example, if I fill the tank up all the way, the needle goes all the way past E and the low fuel light comes on. As I drive, eventually the low fuel light goes out and the gauge moves up to somewhere just under 1/4. I usually fill it back up once the fuel light goes out, since I'll have gone about 215 miles by then.

Does this sound like a faulty sending unit, or is it a sending unit mismatch due to the later model cluster? I've never read anything about the sending units not being compatible, but I'm thinking maybe it's because it's a cherokee cluster in an MJ? All the other gauges work perfectly BTW.

Any ideas?

Ron (original owner), if you're reading this... can you verify the cluster info, and if you know whether the original problem was due to the mismatch or if it was just a bad sending unit?

 

[rsalemi]

87 - 90 clusters have mechanical spedos, 91 up have electronic so I think its unlikely you have a 92 cluster.

There are 3 87 - 90 clusters, idiot lite with fine white needles, guages with fine white needles, and Limited - Laredo clusters with flat needles with red tips. (Also with and without tachs)

 

[tchase]

Can't say for sure if the sending units are compatible. Here's what I can say though:

The guy who fixed up my MJ after I wrote it off replaced the idiot light cluster with a full gauge cluster from an XJ of undetermined vintage. It worked fine and was a nice upgrade but I'm pretty sure he only did it for the 100k decrease in the mileage reading.

The original fuel gauge in my XJ showed the same symptoms, however I never did figure out why. The problem spread to include the speedometer and tach. It was an intermitant prblem with the tach functioning normally about 90% of the time, fuel gauge worked maybe 75% and speedo 25% if I was lucky. Voltage gauge always worked, and while the oil pressure gauge did also die, a new sending unit immediately fixed that. The rest of of them acted that way for close to two years then it just went away. Worked fine for another two years with only an occasional speedo flutter. Now the last time I had it running the speedo was dead again. Looks like its time to track it down before she hits the road again. Hmmm... if it doesn't work I won't have to change the speedo drive gear so it will read properly will I? Who needs it anyway.

Sorry that was kind of long winded and not incredibly helpful, just my own experience

Tony

Ps: my XJ also has several other "self healing" electrical problems so it's gauge sickness may not be representative of anyone else's

 

[NotMatt]

87 - 90 clusters have mechanical spedos, 91 up have electronic so I think its unlikely you have a 92 cluster.

There are 3 87 - 90 clusters, idiot lite with fine white needles, guages with fine white needles, and Limited - Laredo clusters with flat needles with red tips. (Also with and without tachs)

Keep in mind he also rewired the whole thing with the 92 wiring harness and converted it from 2wd to 4wd with the cherokee's transfer case and tranny, so if the wiring's all there... maybe? The cluster itself looks EXACTLY like this:

Which is out of a 94 country, which has the electronic speedo.

So, since it's not really that the gauge doesn't read or reads off, it actually reads BACKWARDS, I'm guessing the sending units must be different... maybe I'll try plugging the cluster in from my 87 and see if the fuel gauge works right then...

 

[myjeepsbigger]

Diag flow from "Motor's Alldata":

1. Turn ignition switch to Run position.
2. Disconnect fuel gauge sender connector. Gauge needle should move to E.
3. Connect a jumper between terminal A on two halves of fuel gauge sender connector. Gauge should move to F. If gauge is operating properly, replace sender.
4. Measure resistance of sender. If resistance is not between 5-105 ohms, replace sender.
5. Check for an open between sender connector and gauge

 

[NotMatt]

myjeepsbigger, excellent info... I'm gonna try that tomorrow morning when I get a chance to work on the truck and see what the results are. Thanks.

 

[Eagle]

Keep in mind he also rewired the whole thing with the 92 wiring harness and converted it from 2wd to 4wd with the cherokee's transfer case and tranny, so if the wiring's all there... maybe? The cluster itself looks EXACTLY like this:

Which is out of a 94 country, which has the electronic speedo.

So, since it's not really that the gauge doesn't read or reads off, it actually reads BACKWARDS, I'm guessing the sending units must be different... maybe I'll try plugging the cluster in from my 87 and see if the fuel gauge works right then...

The photo looks exactly like the instrument cluster in my '88 Pioneer -- which came that way stock. The later mechanical clusters and the 91 thru 96 electronic cluster LOOK the same. Have you verified that he did, in fact, rewire the vehicle to use an electronic speedo, or are you assuming that because of the dial faces?

If he did convert, you're screwed. When Chrysler changed to the all electronic cluster in 1991, for some inexplicable reason the changed the polarity on the instruments. I can never remember which way is which, but for one group of years the low end of the scale is 0 ohms and the high end is 88 ohms. In the other side of the change, the high end is 0 ohms and the low end of the scale is 105 ohms.

You might be able to replace the gauge in the cluster with one that matches your fuel sender, or you might have to cut the trace on the printed circuit and hard wire it to the opposite polarity.

 

[NotMatt]

Well, if the polarity was changed, wouldn't the other gauges read wrong as well? If that's the case, then I must have the older cluster because everything else works perfectly.

I'm going to test the sender unit tomorrow when my brother gets home, and go from there.