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post #1 of (permalink) Old 06-08-2009, 09:05 PM Thread Starter
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Smile Toubleshooting a fuel tank sending unit

This has been biting us for along time. A flakey reading of the fuel gauge level. Even replacing the entire gauge cluster, still getting an inconsistent fuel level reading from the standard size tank in my 85 CJ.

Here are the symptoms of the sending unit as I understand it. The sending unit provides a resistance of 22.7 ohms and an approximate reading of a half-tank of fuel on the fuel gauge. This operation continued for about 15 minutes and then the fuel gauge was reading well below empty. A resistance measurement on the sending unit showed OVLD or basically an open circuit, i.e. infinite resistance. This state of operation lasted for about 40 minutes and then a resistance measurement of 22.4 ohms was obtained. Does this mean the sending unit has a fault or is there something else wrong in the circuit?

What do you think? Thanks!

1985 CJ with a transplanted 94YJ 4.0L motor, T176 tranny, mainly stock otherwise. ><>
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post #2 of (permalink) Old 06-08-2009, 10:21 PM
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well you are not saying how you are taking the readings.. but if it's jumping around on you.. i would suspect a bad ground..

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post #3 of (permalink) Old 06-09-2009, 05:46 AM
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There's a wire that is attached to the frame with a sheet metal screw and to the sending unit base with a spade connector. That could be the problem, or it could be the sending unit itself, or the wiring between the gauge and the sender.

What else was going on during your tests? Were you driving around, or just parked with the ignition on? And how did you take the measurements? Did you get any readings other than 22.7 and open?

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post #4 of (permalink) Old 06-09-2009, 09:24 AM Thread Starter
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Originally Posted by jeepgod View Post
well you are not saying how you are taking the readings.. but if it's jumping around on you.. i would suspect a bad ground..
Thank you for the reply. The sending unit is mounted in the tank, gas tank is mounted inside skid plate in the oem location. I have cut a 6" hole in the tub floor so we have driect access to the top of the sending unit.

For the resistance reading, a DMM was used to measure the resistance across the variable resistor in the sending unit. I measured resistance from the postive connection pin and the negative connection pin on the sending unit.

I made sure the connections were good and did simply sanity checks on my DMM, like measuring the resistance of just the leads which produced a stable reading. The sending unit only did this once for a brief period of time, then a reasonable resistance was displayed. Weird!

Let me know if this does not make sense or you need more information.

1985 CJ with a transplanted 94YJ 4.0L motor, T176 tranny, mainly stock otherwise. ><>
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post #5 of (permalink) Old 06-09-2009, 12:08 PM
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Was the wire to the instrument cluster connected while you took the readings? I think that could do strange things to the resistance readings you get.

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post #6 of (permalink) Old 06-09-2009, 12:27 PM Thread Starter
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Quote:
Originally Posted by Jim_Lou View Post
There's a wire that is attached to the frame with a sheet metal screw and to the sending unit base with a spade connector. That could be the problem, or it could be the sending unit itself, or the wiring between the gauge and the sender.

What else was going on during your tests? Were you driving around, or just parked with the ignition on? And how did you take the measurements? Did you get any readings other than 22.7 and open?
Thank you for the reply.

So, during all of the tests the PCM or ECU (not sure of the terminology for Jeeps, but the computer) was disconnected. The vehicle was parked stationary and some of the tests were with the ignition on and off.

NOTE- while performing one of the the resistance tests on the sending unit, the ignition circuit were disconnected.

The measurements were taken with a Digital Multimeter set to measure resistance. The actual resitance varied from ~23-25 ohms.

Does this help?

1985 CJ with a transplanted 94YJ 4.0L motor, T176 tranny, mainly stock otherwise. ><>
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post #7 of (permalink) Old 06-09-2009, 04:04 PM
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With the sending unit connected to the wire that runs, eventually, to the gauge cluster, there's no telling what the resistance readings you get might mean. You're reading the resistance through the sending unit and the resistance through the instrument cluster to ground, whatever that may be.

When you got an OPEN reading though, the unit itself had gone open, unless one of the meter probes had slipped off its point, or the probe lead had come loose from the meter. Either of those possibilities are more likely than the S.U. going totally open, IMO. If you're absolutely sure of those readings, replace the sending unit.

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post #8 of (permalink) Old 06-09-2009, 09:23 PM Thread Starter
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Quote:
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Was the wire to the instrument cluster connected while you took the readings? I think that could do strange things to the resistance readings you get.
Thanks for following up.

The wire was disconnected from the sending unit to the gauge in the instrument cluster. The wire is still attached to the gauge, but not at the sending unit.

Let me know if this makes sense or you need more info

1985 CJ with a transplanted 94YJ 4.0L motor, T176 tranny, mainly stock otherwise. ><>
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post #9 of (permalink) Old 06-10-2009, 12:11 AM
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I think to rule out the sender, you have to try and totally isolate it from the ground of the Jeep and power side of the circuit. Pull the power and ground wires off and make sure the connection "at the sender" for ground is good. IIRC this is a soldered connection and I've seen them get pretty crappy looking over time. Also make sure the spade connector on the power side is solid and not coming loose.

With the sender not hooked to anything and the Jeep sitting still you should get a steady resistance ready. Of course it should be within the normal range of empty to full, but the main thing is that it should be steady.

Still sounds to me like you have some grounding issues.

Once the sender is ruled out (aka steady reading when isolated from the rest of the system) you can go about fixing the grounds.

For now leave the sender and concentrate at the front of the Jeep. Make sure you have very good, heavy grounds from the battery to body (and dash), battery to frame, and battery to engine. Once you've got this accomplished, go back and make sure the ground to the sender is clean and tight. Now double check the power side of the circuit and check the operation again.

If it's still jumping around, check connections at the gauge and then start tracing the circuit wiring itself. You will find the problem... I'm betting sooner than later.

Is this the only hammer you've got?
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post #10 of (permalink) Old 06-10-2009, 06:10 AM
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He said that he had the meter probes on the two terminals on the sending unit, and that the wire from the sending unit to the gauge was disconnected. That covers everything except possibly a loose terminal at the sending unit. That should mean either a bad sending unit or a faulty reading due to a loose meter probe or something like that. I would suggest doing that test over, just to rule out an erroneous reading. If it again comes up open, it has to be the sending unit.

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