Sending Unit Bad ??? - Off-Road Forums & Discussion Groups
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post #1 of (permalink) Old 01-20-2001, 07:36 AM
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Sending Unit Bad ???

Hey Guys,

I am having a problem with my gauges in the YJ and was wondering if anyone had advice. When i got the rig, the gas gauge kinda worked but the needle was erratic and far from accurrate. I just replaced the whole cluster (used) and the gas gauge now works nice for about 20-30 mins until the truck is warm and then the gas gauge go all the way to F.

Any ideas what this might be or whats causing it? The sending unit for my rig is $150 so i wanna make sure thats what it is before i rip it apart.

[b]Boston Mangler
89 YJ With A Bunch Of Goodies
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post #2 of (permalink) Old 01-20-2001, 08:10 AM
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Re: Sending Unit Bad ???

I think I heard that yj and cj fuel sending units and gauge are calibrated backwards. The CJ fuel gauge reads empty with a restance around 70ohm, and full around 8-10 ohm. SO a yj would be fairly close to those, just reversed. Put a multi-meter on it when the tank is empty (at the gauge) and then fill it. Do this cold. then drive it around until its warm, and take another reading. This should tell you if its something in the wireing. A wire close to exhaust or some other heat source would generate a higher resistance, and cause the fuel gauge to read higher because of the resistance in the wire, not the gauge.

All of your gauges are ohm meters, just instead of calibrated in ohm, they are calibrated in fuel quantity, temprature, pressure, etc. I don't know of anywhere to find resistances or other troubleshooting info for a YJ, but for any CJ pushers out there, there is an AWAESOME gauge diagnostic in the jeep tech section of

'77 CJ-5: 304/T-150/D-20 F-D44 R-9" Power 4 wheel disk, 36X12.5 swampers
post #3 of (permalink) Old 01-20-2001, 08:37 AM
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Re: Sending Unit Bad ???

If you don't have one you definately need to buy a voltmeter and check the ohm's at the gauge like FImud said. Once you find out what the readings should be you can rule out or confirm the sending unit. I'm not sure about the YJ but a CJ's fuel and water temp gauge is more or less together because there is a resistor in the fuel gauge that drops the voltage to both gauges. There is a little arm in the fuel gauge that has points on one end that will only let 6 volts, I think maybe more or less ,but the points were sticking on mine and that was causing all kind of strange readings. If the YJ is the same I could see that happening worse as the points get hot

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post #4 of (permalink) Old 01-20-2001, 09:17 AM
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Re: Suggestion & new gauge info

I like FIMudCJ's idea of testing the sender wire both hot and cold and would do that first.
I have learned that typically if the gauge shows function the culprit is usually the sender wire and most likely the sender it's self.
A good test for the a CJ gauge is to momentarily touch the sender wire to ground.
If you get a good clean sweep on the needle, it is probably OK.
Just don't hold contact long or you will cook the gauge.
The backwards YJ gauge may be different in that one might need to alter this test to include a heavy resistor to the sender and then touch to ground.
I am leery of suggesting you try the CJ test to your gauge.
In the CJ's case, the LESS the resistance the more towards full the needle will read.
In the YJ's case with MORE the resistance the needle will more full.
I don't know if a YJ's fuel gauge needle rests at full or empty when the ignition is off but if you can interpret these facts you should still be able to apply this "shorting" test to see if you can get a good, clean sweep of the needle which should eliminate the gauge as the faulty component.
And yes, the CJ and YJ's sender resistances are backward.
In fact, I was able to sucessfully work with a guy on a YJ body and gauges / CJ frame and sender setup. The resistance paramiters were similar in both - just reversed. He switched the leads on the CJ sender's wiper arm and Bingo! it worked!
The original version of my "CJ Gauge & Sender Diagnostics" article in the ORC Tech section is inaccurate in the jumper strap/regulator section and has been corrected and updated at it's original and primary location at ...
The regulator is actually inside the CJ's fuel gauge and the jumper is just a plain jumper strap. Confusion exhisted among most as to the locaton of the 'bi-metal "strap" or "strip". Only after I later disected a gauge was I able to confirm it's location and discredit my (and many other's) original assumption that the regulator was in the strap!
Getting changes and correctons made at the ORC location is like pulling hen's teeth. Bookmark the URL above for the most current version.

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post #5 of (permalink) Old 01-20-2001, 08:04 PM
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Re: Sending Unit Bad ???

YJ fuel sender resistance readings (from 90 FSM)
empty = 0 ohms
1/2 tank = 44 ohms
full = 89 ohms

At the fuel gauge sender connector, the 16BK/YL wire is ground and the 20TN should be approximately 7.5 volts.


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post #6 of (permalink) Old 01-21-2001, 06:47 AM
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Re: Sending Unit Bad ???

thanks guys

i will do this after a little of the snow goes away and let yall know about the results


[b]Boston Mangler
89 YJ With A Bunch Of Goodies
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post #7 of (permalink) Old 01-21-2001, 09:47 AM
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Re: Sending Unit Bad ???

Make sure you check out the ground to your sending unit. I fought this same type of problem for a couple of weeks and ended up having to change the gauge and then found that the brand new sending unit I put in was bad so I had to get another one of those things plus drop the stinkin tank again to change it.
If you do have to change the sending unit then make real sure you get a GOOD ground and test the unit before you put it in the tank.

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