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Fuel Gauge broken


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So my fuel gauge doesn't work in the manche. It's a 87 with the 4.0l and the standard gauge cluster (not the dummy gauges). All of my other gauges work just fine. Anything in particular I should look for to get the fuel gauge working again? Thanks in advance for your help.

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Where is the needle? Does it stay in one spot, or just not read correctly?

 

I'd check the wires going to the fuel tank, seen if any of them are broke or worn through. If that's not it, it may be a stuck sender or a dead sender.

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I'd start with the wiring. I don't like blindy changing parts, and dropping the fuel tank sucks. With a jeep (of this vintage), the gauge reads empty with about 100ohms of resistance (it might be 98 actually, but that doesn't really matter), and full with 0ohms. Or at least if my memory serves me. So, if the wire is broken it will see infinitve resistance - and thus peg out on empty. FWIW, a chevy gauge is backwards to that and it kinda sucks because it pegs out on full when it fails, and I think a later jeep gauge is like that too (could be wrong).

 

So, start with a visual, look at the wires to the plug on the tank. Pull the connector apart and make sure there isn't a mess of corrosion in it. Look at the wires going through the floor. If everything down there looked good, that's all the easy stuff to see. From there I'd grab my multimeter and go from the sender pin and ground pin on the connector to the tank, checking resistance. If you've go an amount of or greater than 100ohms, the problem is with the sender or the wires in the tank. If you've got something like 33ohms, and 2/3rds of a tank of fuel, the sender is good. If the sender is good, you then have to pull the gauges out of the dash - kinda a PITA. Check the resistance at the wires going to the gauge with the other connector hooked up - verify it's the same as it was at the tank (if it's infinitive here, the wiring under the carpet to the dash is probably wrecked). If it's the same, I'd say the gauge is trash. You can test them, and I guess you could do that.

 

But umm, I'd probably read through my E manual before I did much. I haven't had to diagnose a jeep fuel qty problem recently. So the old brain is going by memory. Hell, my MJ doesn't even have a fuel gauge other than the mechanical one on the propane tank.

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you don't have to drop the fuel tank on a mj to get the pump out.not on my 88 anyway.

 

Yeah, you're correct... I was thinking chevy when I was typing. That's the problem with owning too many vehicles. You just have to make sure you're suitably low on fuel before you pull the pump/sender out.

 

The only thing that can be a PITA is that the driveshaft might be smack dab in your way depending on the specifics of the truck.

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The only thing that can be a PITA is that the driveshaft might be smack dab in your way depending on the specifics of the truck.

 

I always just chocked the wheels and moved the DS out of the way, then it can also act as a headrest.

 

you don't have to drop the fuel tank on a mj to get the pump out.not on my 88 anyway.

 

Yes that's true, but getting it back in and having it seal properly with it in the truck is a PITA.

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A dirty trick is to either use assembly fluid #1 on the o-ring to stick it in place, or to superglue the o-ring in place and then coat it with vaseline or assembly fluid.

 

Or, sealube, as it works okay for keeping the o-ring place. It's similar to assembly fluid #1, but isn't soluble in fuel (or much else for that matter) so basically once it's on there it will be there forever.

 

If the tank is out, I'd recommend a little vaseline on the o-ring... Or assembly fluid. It'll make it much easier to get the lock ring engaged/locked without potentially damaging the o-ring.

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Start by testing the wiring. On the early XJs and MJs, the sender has an ohm range of 0 - 88 ohms. Zero ohms is empty, 88 ohms is full. So -- gauge reading empty could be a short circuit (zero resistance).

 

The official test procedure from the 1988 FSM is:

 

1. Disconnect C139 -- Needle should go to Full. If the ground to G197 is okay. replace sender. If not ==>

 

2. Connect C139. Test C203 terminal 15. Reading should be between 0 and 88 ohms. If okay, replace gauge. If not okay, repair open to sender.

 

 

The simplified version of that is, unplug the 3-wire connector outside the fuel tank. That gives you an open circuit on the sender (infinite resistance). The gauge should read Full with the connector unplugged. If so, jumper the small wire (violet?) to black on the chassis side of the connector (zero resistance). The gauge should swing to empty. If that doesn't work, try jumping the violet directly to the chassis (making certain you get a good ground).

 

On the tank side of that same connector, put an ohmmeter on the violet and black wires. That's the circuit through the sending unit. The reading should be somewhere between zero and 88 ohms. (I don't think the scale is linear, though, so don't assume that a half tank will read exactly 44 ohms.)

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Start by testing the wiring. On the early XJs and MJs, the sender has an ohm range of 0 - 88 ohms. Zero ohms is empty, 88 ohms is full. So -- gauge reading empty could be a short circuit (zero resistance).

 

Oh, okay, oops. So they are the same as an early GM setup. Well, that changes things.

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  • 2 years later...

The official test procedure from the 1988 FSM is:

 

1. Disconnect C139 -- Needle should go to Full. If the ground to G197 is okay. replace sender. If not ==>

 

2. Connect C139. Test C203 terminal 15. Reading should be between 0 and 88 ohms. If okay, replace gauge. If not okay, repair open to sender.

 

 

The simplified version of that is, unplug the 3-wire connector outside the fuel tank. That gives you an open circuit on the sender (infinite resistance). The gauge should read Full with the connector unplugged. If so, jumper the small wire (violet?) to black on the chassis side of the connector (zero resistance). The gauge should swing to empty. If that doesn't work, try jumping the violet directly to the chassis (making certain you get a good ground).

 

On the tank side of that same connector, put an ohmmeter on the violet and black wires. That's the circuit through the sending unit. The reading should be somewhere between zero and 88 ohms. (I don't think the scale is linear, though, so don't assume that a half tank will read exactly 44 ohms.)

 

So if you unplug the connector and the needle doesn't sweep to Full, then ground to the connector is bad?

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Mine is broken too, but pegged on full. The warning light does come on, but at about half a tank. It was working properly before I did a cluster swap from the dummy gauges to the full cluster, and that's the only gauge that doesn't work right. Was there something else I needed to change?

 

I've been too lazy to fix it, so my poor man's fix is to just use the odometer like on the motorcycle. To be honest I've never missed not having a fuel gauge, you just need to pay attention to the miles and always top it off when you fill.

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I tested my gauge by disco'ing the connector at the pump. The needle swept all the way to F (and past it). I then jumped the ground and violet wires and the needle then swept over to E (and past it).

 

There is no corrosion on the connector and no obvious broken wires.

 

I plugged it all back together and the needle then went back to where it was before disco'ing the connector. It seems that when I fill the tank, the needle moves to the halfway point.

 

I don't recall this happening before I swapped my gauge cluster.

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