winnbigg

So I have been having trouble occasionally with my fuel pump occasionally turning off for no reason. If I let my car sit long enough for (what I assumed) was my relay to kick back over then it would start run and drive no problem for a while. Then I got rear ended pretty hard and it wouldn't start. I disconnected the battery and it started right up. 3 days later it started, died, and I haven't been able to start it since. Fuel pump is good, I recently replaced it. Haven't found a short anywhere and replaced the relay. Could it b the crankshaft positioning sensor?? The knock sensor?? Does an 89 xj have either or a inertia or roller over switch?? Please help!!

616_95xj

I just recently had a similar problem. With my 95. Mine was alittle different. It kept blowing the 30amp engine control mod. fuse under the hood. Ended up being a short in the harness...where? Idk. I just ended up snipping the wire coming out of the relay box and ran a new one back to right before the harness for the fuel pump. I haven't had any problems since.

I was getting rough starts and idle, then one day it stalled on me in traffic and wouldn't fire. Got it off the road. Looked around tested coil for spark..nothing. Started checking fuses and found the 30amp blown. Replaced it, she fired right up. Drove about 60 feet bam, same thing. Got it to my parents house in the garage to take a better look around. Replaced fuse and it'd idle all day long until I tryed driving it. Within a mile it'd go. Did some test like the unplug; coil, o2, injector, fuel pump didnt have any effect on it. So we went for along shot and tryed the fresh copper to the fuel pump. And it's been great since.

Probably no help...but I figured I'd share since it was similar.

616_95xj

ASD relay? Idk if 89 has one

winnbigg

Well anything helps. I was thinking about using a volt ohm meter to find both ends of the same wire and start replacing whole sections of wire to bypass any possible shorts. Then figuring out exactly where every wire coming to and from the relay is going. I don't really think the 89 has a crank position sensor, or a inertia switch, but could a busted knock sensor act as a trigger to force the fuel pump to not function?? Kinda like a cps or a fuel cutoff switch would when damaged??

Jareduhhh

I would first remove the fuel pump relay and jump the terminals that would connect to 30 and 87 on the relay with a paper clip to verify that the wiring is all good from the battery to the pump, and to also verify that the relay is working properly. This would force the pump on. While I was right there, I would check the terminals that would connect to 85 and 86 on the relay for voltage. If there is no voltage there, then I would start pulling up some wiring diagrams to find the cause.

Jareduhhh

Sorry to double post, but you can also check for a short to voltage on the pump side. The socket that terminal 87 plugs into should be grounded with the relay removed. If there is voltage there, you have a short to voltage.

cruiser54

Knock sensor is definitely an issue. Try jumping the terminals for the relay as suggested. Sure you're working with the correct relay?

cruiser54

Also, the fuel pump ballast resistor can be bad or have loose connections. On the driver's side inner fender. It's okay to bypass it.

winnbigg

I thought about that ballast, will it click when trying to start the car?? As in out loud. Haha. I hear a noise that I usually don't coming from the driver side of the engine bay. Could that b it??

cruiser54

Ballast doesn't click. Put a jumper wire across it and bypass it. See what happens.

winnbigg

Tried that, tried bypassing the relay, still nothing. Next option run a straight wire back to the fuel pump and time it right and crank it. Unless someone has any other ideas? My knock sensor was busted so I disconnected it. Could that b it?? Also, before I did so, my gauges would adjust after one click on. Now, nothing. Not even when I crank it. Oh, and when it would die like this, the gas gauge would peg, and the relay would trip. Ideas??

Jareduhhh

Now we need a wiring diagram to check what controls power and ground to the pump. I would guess its a straight shot to the pump from the relay. If you bypassed it and nothing happened, you either have a break in the wires, a bad fuel pump, or some switch/redundancy is controlling the fuel pump. The last option doesn't make sense to me, I would think all redundancies would be on the low power circuit of the relay, so I'm betting its a fried pump or a loose connector. At my shop we use PowerProbes, they make diagnosis like this take a couple seconds. I could check the relay, check each connection to the relay, and force power to the pump in a minute.

cruiser54

You've got battery/low voltage issues I'm guessing.

That, and/or grounding issues. Do this first.

Renix Ground Refreshing
The Renix era XJs and MJs were built with an under-engineered grounding system for the engine/transmission electronics. One problem in particular involves the multiple ground connection at the engine dipstick tube stud. A poor ground here can cause a multitude of driveabililty issues, wasted time, and wasted money replacing unnecessary components.
The components grounding at the dipstick tube stud are:
Distributor Sync Sensor, TCU main ground, TCU "Shift Point Logic", Ignition control Module, Injectors, ECU main ground which other engine sensors ground through, Oxygen sensor, Knock Sensor, Cruise Control, and Transmission Sync signal. All extremely important stuff.
The factory was aware of the issues with this ground point and addressed it by suggesting the following:
Remove the nut holding the wire terminals to the stud. Verify that the stud is indeed tightened securely into the block. Scrape any and all paint from the stud’s mounting surface where the wires will attach. Must be clean, shiny and free of any oil, grease, or paint.
Inspect the wire terminals. Check to see that none of the terminals are crimped over wire insulation instead of bare wire. Be sure the crimps are tight. It wouldn’t hurt to re-crimp them just as a matter of course. Sand and polish the wire terminals until clean and shiny on both sides. Reinstall all the wires to the stud and tighten the nut down securely.
While you’re in that general area, locate the battery negative cable which is fastened to the engine block just forward of the dipstick stud. Remove the bolt, scrape the block to bare metal, clean and polish the cable terminal, and reattach securely.
Another area where the grounding system on Renix era Jeeps was lacking is the engine to chassis ground. There is a braided cable from the back of the cylinder head that also attaches to the driver’s side of the firewall. This cable is undersized for it’s intended use and subject to corrosion and poor connections at each end.
First off, remove the cable end from the firewall using a 15mm wrench or socket. Scrape the paint off down to bare metal and clean the wire terminal. Reattach securely.
Remove the other end of the cable from the rear of the head using a 3’4" socket. Clean all the oil, paint and crud from the stud. Clean the wire terminal of the cable and reattach securely.
A suggestion regarding the braided cable:
I prefer to add a #4 Gauge cable from the firewall to a bolt on the rear of the intake manifold, either to a heat shield bolt or fuel rail bolt. A cable about 18" long with a 3/8" lug on each end works great and you can get one at any parts store already made up. Napa has them as part number 781116.
A further improvement to the grounding system can be made using a #4 cable, about 10" long with 3/8" terminals at each end. Attach one end of this cable to the negative battery bolt and the other end under the closest 10mm headed bolt on the radiator support just forward of the battery. Napa part number 781115.
For those of us with Comanches, it’s very important to remove the driver’s side taillamp assembly to access the ground for the fuel pump. Remove the screw holding the black ground wire. Scrape the paint from the body and corrosion from the wire terminal. Reattach securely.
　
　
If you want to upgrade your grounds and battery cables in general, contact Jon at www.kelleyswip.com. He makes an incredible cable upgrade for a very reasonable price.
　
Revised 12-04-2012

Busaart

hello,
i refresh this post.
I have a problem with low voltage on fuel pump.
I bypass fuel pump ballast resistor and on this conection i have 12,4V, but on fuel pump is only 6,34V

Where i should search missing voltage from ballast resistor to fuel pump?

Jeep cherokee 4.0i 1988 renix

cruiser54

Behind the driver side kick panel, where the hood release is, is connector C119. gray in color. 10 pins. Pink with tracer color wire
Next C139 behind the panel that's behind the spare tire. 6 pins. Pink with tracer color to orange.