Fuel Gauge

I tore into my gauge cluster to figure out why my fuel gauge doesn't work and found an open diode in the circuit. I tried to cross it on the NTE page but it comes up with two different part numbers one is a regular diode and one is a zener diode, so I was just wondering if anybody had any data on the correct diode for the fuel gauge circuit. Oh yea it is an 88 XJ limited. I think the diode is for the low fuel light on the side of the gauge. There is a little circuit board inside the cluster but I could not find anything wrong with any of the components and after taking the diode out of the circuit I was able to make the gauge move back and forth by applying voltage to the gauge. Anybody ever run into this? If so how did you fix it?

Most of what you said went way over my head, but have you check the sending unit in the tank. That is when it usually fails.

and this should be in OE tech unless you fuel pump/sending unit is somehow modified

You do realize that a zener will look like an open, right?

Use the "regular diode" there is no reason they would use a more costly zener.

Sorry my bad I meant it was a shorted diode.

lonecountryboy said:

I tore into my gauge cluster to figure out why my fuel gauge doesn't work and found an open diode in the circuit. I tried to cross it on the NTE page but it comes up with two different part numbers one is a regular diode and one is a zener diode, so I was just wondering if anybody had any data on the correct diode for the fuel gauge circuit. Oh yea it is an 88 XJ limited. I think the diode is for the low fuel light on the side of the gauge. There is a little circuit board inside the cluster but I could not find anything wrong with any of the components and after taking the diode out of the circuit I was able to make the gauge move back and forth by applying voltage to the gauge. Anybody ever run into this? If so how did you fix it?

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You calibrate the fuel gauges by passing 12V through a series resistor which causes full, mid, and empty scale fluctuations on the gauge (simulates the sender in the fuel tank). I only have the cal resistances for the 91+ gauges (the earlier fuel gauges are different), but if you have an FSM for the Renix models the cal resistance values are in there. Try this at the empty end of the scale and see if the low fuel diode works or not w. the diode removed then you'll know for sure.

Well I don't have a FSM been wanting to buy one but can't find one. Does anybody have the calibration resistence's for this particular application? Just to clarify a little bit the guage was waaaaay past empty. When I removed it I took out the diode that I found shorted and applied power to the gauge and was able to move it through its full range but as far as calibrating it I don't have that information. Thats why I assumed it was the diode shorted. I checked the continuity from the cluster all the way to the tank and it was good. The purple wire which is the sending unit has a resistance to ground which means that the float is somewhat working so the only thing I could find wrong was the shorted diode. Any suggestions would be greatly appreciated

For the 91+ models the calibration values are Empty (105 ohms), 1/2 Full (33 ohms), and Full (5 ohms). The earlier senders used opposite polarity to the gauge, so if you reverse these values for empty, 1/2, and full it will probably work. I pretty sure the sender potentiometer in the tank had the same overall resistance in all models.

Thanks for the info. I measured the resistance at the tank and I figured I had just about 1/2 tank and it measured 22ohms so I think the sending unit was fine. But thanks again for the information I appreciate it.

Fuel gauges are just like wifes. They are never right. Just go about your day. Paying them no never mind. Just don't forget to put a little something in them ever now and then so they don't up and leave you stranded.