myraus

Hi my name is Myron and I'm new here. I got myself a 91 Cherokee recently for cheap, it had been sitting for a couple of years, and I'm trying to start it. It has spark, because I can pour a few drops down the throttle body and crank it and it will run for a few seconds then die. I took the fuel line off the rail, after opening the schrader valve, only to find a few drops of fuel.
I started at the back, by pulling the pump out, tested it briefly with 12v and it spun with no problem. Checked the wiring from the connector by the fuel filter to the pump and sending unit, no problems there. Took the vehicle end of the connector and did continuity tests on the harness to the front, and they checked out okay too.
Then I went to the front, to the main relay box, found the fuel pump relay, checked for power in, then jumped it to the outgoing circuit for the pump, and still nothing.
I did notice the resistor on the driver's side fender, and I am gonna try the jumper method when I get home after work, but when I was using the wiring diagrams to sort thru this {one in the Haynes manual and one from Autozone} both of them showed a ballast resistor relay in the system. Anybody know where it is mounted, and why do they have one?

Gee oh Dee

I thought the resistor was near the master cylinder on the firewall. I could be confusing it with something else though.

tornprince

mine was located on the drivers side firewall. mines also a 90 laredo. unlike yours, my pump doesnt even ingage.

Gee oh Dee

Yeah, that's what I thought.

tornprince

and the purpose is to reduse current to the coil. and if you replace it, be careful mounting it back. there cerramic and crack with to much pressure.

myraus

Yeah my resistor is beside the windshield washer fluid reservoir, just before the firewall. I did a search last night on the forum and found that they could be placed in a bunch of different spots.
Does anyone here have any wiring diagrams from an original service manual that I could use to hunt down this problem? When I got this rig the previous owner replaced the steering column and couldn't get it started again. Also, I found a block that holds 3 relays under the dash. Anyone know what they are used for?

cpnwrench

if you have MPI (am sure you do, I have same year Jeep),,,,,replace the fuel pressure regulator.

good luck with it sir!

cpnwrench

captainofiron

this

it could be that the regulator has blown out and is not allowing the system to build pressure

myraus

Okay, well I just spent sometime doing a trace on the wiring from the pump all the way up to the fuel pump relay, and there are no breaks in the wiring.
One thing I did find was, in the wiring diagram I got from Autozone, was that there is a dark blue wire coming from both the fuel pump relay and the automatic shut down relay. That same wire is connected to one of the terminals from the computer, which from there goes to a box on the sheet, but it is so faint that I can't tell where the wire leads to. I thought it was the ignition switch, but there is not continuity up to it. And I figure that the blue wire, once energized, activates both of the above relays.
Does anyone know where that wire goes? And to what circuit?

P.S. And by the way, the thread has to be read properly to see that I was inquiring about a wiring problem. When I get fuel up to the rail and injectors, and they don't want to work, then I will look at that!

myraus

Well I finally had a chance to go check out a few more things on my Cherokee.
First, there is power up to the relays when the ignition switch is off, but barely any voltage when the switch is on. Is this normal?????
Secondly, I traced the small blue/yellow wires from the fuel and auto shutdown relays to the computer, exactly to the pin for the auto shutdown operation. Now has anyone here heard of a problem with the auto shutdown circuit? The relay is fine, I took it and the fuel pump relays and swapped them with the starter relay, which I know works.

Can anyone help? I am at a loss. And I'm just about to wire in a toggle to operate the fuel pump, but if something is wrong with the auto shutdown circuit, then it would be of little use because the fuel injectors are in that circuit too!

I need someone more knowledgeable about this kind of thing to help me out!

myraus

Does anyone here have a computer for a 1991 Jeep Cherokee with a 4.0 and 5 speed? Federal emissions? That is what I've figured out. The circuit that energizes both the fuel pump relay and the auto-shutdown relay { which controls the fuel injectors } is dead. Everything else checks out. I was gonna just do a bypass on the circuit, but I would rather keep everything original.

Mmurdock

I just looked at e-bay and found six used 4.0 ECMs which will work with a '91 Cherokee priced at $85-$150 (most offer free shipping).

With those prices as a starting point, start calling the wrecking yards in your area to see who has one and what they want for it. Every wrecking yard where I live has three-five Cherokees setting in them but prices for parts varies.

myraus

Well, I finally got a chance to work on my XJ tonight, and I had a reman ECU to put on to see if it was the problem. NO it wasn't! Still no power getting to the auto shutdown relay, which in turn will trigger the fuel pump relay. I can get power to the pump with a jumper wire, but the point still stands to where those relays aren't getting the juice. What am I missing here!!!!!!!!!!!
I know the fuel pump works, been tested and it runs, out of the tank. But I'm still dealing with the power issue.
Luckily i still have the previous owner's email, and I sent him a message to get back to me to find out exactly what happened to make it quit running.
Do you guys have any idea? I could really use a fresh perspective right about now!

myraus

Well, the previous owner replied back and told me that the Jeep was running with the old steering column in place, but when he put in the replacement, it wouldn't run after that. I checked the ignition switch, and it is okay. Checked all the circuits that control those two relays, the wiring is fine. Like I said before, the CPU was replaced, to no avail. The only thing I haven't done yet is replace those relays with new ones, hoping that they are defective or just shorted out inside. It seems kinda hard to understand why a relay would give me the problems I am experiencing, but I need to get this vehicle running!!!!!!!!

Anyone have any ideas or suggestions on something I may have been missing? My gut instinct is telling me that it could be something simple.

myraus

What, no one has any suggestions??????