Mechanics

FUEL PUMP

1993 Jeep Cherokee

Electrical problem
1993 Jeep Cherokee 6 cyl Four Wheel Drive Automatic 104000 miles

I have a 1993 Jeep Cherokee. Was driving it and it quit running. Had it towed. Key on sends 12V to the fuel pump for just a brief second, but does not energize fuel pump. Of course, I replaced pump and filter. Still no fuel pump engagement. The fuel pump will run if I put 12 V to it, but I am not able to get it to prime and send fuel to the rail, tank is about 1/4 full. Back to the real problem. I do not have 12V to the fuel pump except for a few milliseconds. I have checked ALL the fuses (3 times, because sometimes I am stupid), I have replaced the fuel pump relay. The fuel pump relay has always made an audible click indicating mechanical function. I pulled the PCM, ECM or whet ever it is defined as on this particular vehicle. There was some pretty visible corrosion on, maybe 2 or 3 pins, cleaned this up w/dental picks, elect parts cleaner and compressed air. Still no fuel pump. I would literally have to tear this car apart to trace the wire from the fuel pump to the power center. I have a Haynes manual which is for all intents and purposes useless in this instance. If I had a good wiring diagram or just the info to pin it out I might be able to figure it out. I found a Jeep 1993-94 2.5 and 4.0L EFI powertrain Diagnostics factory service manual. Would this manual provide the info. I need to further diagnose this vehicle? Any help would be gratly appreciated. Thanks, John V.
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Jvansant
June 26, 2008.




Try swapping the ASD (auto shutdown relay) with another one (a/c) and see if that will work. If the PCM doesnt power up, the rest of the system wont work properly

Tiny
Jack42
Jun 27, 2008.
Just to keep everybody posted. The brand new Airtex fuel pump assembly was bad and that's why I couldn't get it to pump/prime. Installed a new pump assy, but of course still no power to the pump, except for just a brief second when the ignition key is first turned on. Vehicle runs perfectly when I hot wire the fuel pump. Anyone have any suggestions? John V.

Tiny
Jvansant
Jun 28, 2008.
Oh, replacing both the ASD and fuel pump relays was the very first thing that had I tried. John V.

Tiny
Jvansant
Jun 28, 2008.
John, I need to know exactly which Cherokkee you have. There are 7 different versions

Tiny
Jack42
Jun 29, 2008.
I finally resolved the fuel pump problem on this Jeep Cherokee. It is a 1993 Cherokee Sport 2 door. It has a 4.0 litre 6cyl, auto, air, 4wd. I purchased the factory service manual, I traced the fuel pump circuit and discovered that I was losing power at the " fuel pump ballast resistor" It is my understanding that this is put in the circuit to reduce fuel pump voltage and thereby noise, under normal driving conditions but is bypassed by the ECM and a " bypass relay" under WOT conditions. The service manual did not give a spec for this resistor but it ohmed out at 78ohms. Seemed kind of high to me. Tried to find the part, but nobody (not even the dealer)knew what I was talking about. I bypassed it, everything worked great. Finally found the part and, new, it had exactly 1.1ohms of resistance. Replaced the bad resistor now everything is peachy. John V.

Tiny
Jvansant
Jul 8, 2008.
John,

I too have the 1993 Jeep Cherokee Sport, 4dr, 4.0L and 6cy. Our problem is very similar I repalced the fuel pump and the fuel filter. Just can't get the fuel going into the line, the fuel is running and measured at the connector to the fuel pump, I am getting only 6.832 VDC. Tried the bypass on the resistor, still won't work and the ballast resistor measured at 1.5 ohms.

With all the experiences that you had on this problem, may be you can shine some light to this.

Dennis

Tiny
Fuzga2
Jul 17, 2008.
Well Dennis, you need to figure out where you have 12v. In the fuel pump circuit, then where you're losing it. 12v. Should be going to the fuel pump for 1 sec. When the ignition is turned on or constantly when the starter is turning the engine over or when the engine is running. I would confirm that the fuel pump is working by VERY CAREFULLY jumping 12v. Directly to the fuel pump to see if it pumps fuel and runs (make sure that you have correct polarity). If that works get a 12v.(Dc) buzzer or light then see if or when you actually get 12v back to the fuel pump. If you don't want to trace the circuit and just punt, I would clean every ground and plug connector in the F.P. Circuit, I would check and clean the big ECM connector, I would replace the fuel pump relay and I would eliminate the ballast resistor. If you truly have a 1993 I could probably help you out w/a factory wiring diagram (if your patient). Good Luck. JohnV
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Tiny
Jvansant
Jul 25, 2008.
I have been working on my 93 Cherokee because it has the exact same problem. I've done the same things you have so far. Where is this ballast resistor? It sounds like that is the problem in my vehicle too. Where did you find this part? Or did you just find a 1.1ohm resistor and replace the piece? Currently I just have the fuel pump hot wired with an in-line fuse holder and I'd really like to get this solved.

Tiny
DEJeepGuy
Sep 10, 2008.
The ballast resistor is located on the drivers side fender. It is a piece of white ceramic material mounted to the fender with a sheet metal screw, has 2 spade terminals, is essentially rectangular. I don't really believe it is needed in the circuit for the fuel pump to operate properly. You can just jump it out and leave it like that. The resitor reduces the voltage to the fuel pump under NOC to extend fuel pump life and reduce fuel pump noise. Any knowledgable Jeep dealer should be able to provide this part. John V.

Tiny
Jvansant
Sep 13, 2008.
I had the same problem. It was hard to find as the resistor worked fine while cold, but freaked out when hot causing the fuel pump to shut down. It was hard to find due to the temperature breakdown. Bypassed the resistor and it runs fine.

Tiny
93Cherokee
Aug 5, 2012.

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