Re:1991 Jeep cherokee with the 4.0 Crank no start.

On Sunday I'm going to go to a friends house to try and help him get this vehicle running. It ran when he got it and then I believe the motor was either removed or partially tore down to do some repairs to it. Now it's a crank but no start.

He says he has no power to the fuel pump or to the plugs. I'm not sure about power to the injectors. He has replaced the fuel pump and I don't know what else. I don't know if there are any common failures on this vehicle that would cause these symptoms. I'm sure there are other things wrong with it but the main thing is to get it running. He has been messing with it for over a year and can't figure it out.

Does anyone know a source to get some wiring diagrams for this truck or is familiar with the wiring schematic for it? I'm thinking that if he has no power to both of these things, they must share a common relay or something else. With no wiring diagrams, I feel like I'm going into this kind of blind.

Any thoughts or experience with this vehicle would be appreciated. Thanks.

The ASD relay powers all of them... man I hate chryslers ASD system.... I've had just as many bad pcms cause this as bad wiring and relay issues. It's a dice roll everytime! I'll get a diagram for it tomorrow if I can otherwise I just posted 1 earlier in the week for a 96 jeep the color codes may differ but the function is the same

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Thanks for the replies and the wiring diagram. It is greatly appreciated. Is the ASD relay located in a central fuse box or is it mounted separately somewhere else?

Well, I went and worked on the jeep today. We didn't really get much accomplished other than verify that the fuel pump would work when jumping the fuel pump relay and that the starter would spin the engine when jumping the starter relay.

Checked power and grounds at the computer and had no power at the battery wire in pin 3 of the ECU. We had grounds at pins 11 and 12, a ground at pin 4 which is the sensor ground but no ground at pin 5 which is the signal ground. No power at pins 8 and 9 which are start and ign start/run.

I had power at pins 30 and 87a at the starter relay and ASD relay. We had no power at the IGN fuse in the power distribution box which also had a solid blue wire that looked to be the same one that went to pin 3 on the ECU and the blue wire that went to the ignition switch on the column. I ran a fused wire from battery positive to pin 3 of the ECU and then I had power to both sides of the IGN fuse in the PDB and power to the blue wire going to the ignition switch.

We had no spark out of the coil or going to the plugs. All relays were new. All plugs, wires, and coil was new.

On the bypass relay, I had grounds on pins 30 and 87a instead of power and I'm not sure if that was supposed to be that way. I'm assuming not.

It's a wiring nightmare under the hood with many connectors not plugged in and no where to plug them in.

I will have to do some more research and get additional wiring diagrams to see where some of this wiring goes and figure out why we have no power at the pin 3 position on the ECU. Any other ideas would be great.

The wiring diagram I posted does not match another set of wiring diagrams in mitchell, some labels were different, oh well. Discard that. Hope this helps.

Thanks for those additional wiring diagrams. I did notice that the one you first provided me did not match the wiring color that was on the jeep. Looking at this new one, you can tell there are some differences with where the wiring goes as well.

I guess the main thing we have to figure out is why we have no battery power at the ECU at pin #3. I got to thinking, we never checked power at this pin with the key off, only with it on. Would that affect our results at pin #3?

While there yesterday, I had no idea where the DLC connector was so I could hook my scanner up to see if there were any codes in the computer. At least now I know where it's at. I have two different scanners that read OBD1 info. Thanks again for the new diagrams.

Another quick question when looking at these wiring diagrams. I'm having difficulty sometimes determining from what direction the wiring is going and what are the inputs going into the computer and what are the outputs. Some are obvious to me but others are not.

For instance on one of the diagrams posted for this jeep. I'm not sure what the power source is from to the "gauge" fuse in the under dash fuse panel. This fuse is totally dead and is in the circuit of stuff that is not working and has to do with the ign start/run pin at the ECU. Fuse #6 in the under hood fuse/relay box is tied to the gauge fuse but I can't determine if #6 is providing power to the gauge fuse or vice versa. #6 has no power either.

I agree, that part of the diagram doesn't make sense. With the key in start or run, pin 9 at ECU should get power. The power should flow from the ignition switch to the ECU pin 9 and also power up the gages fuse.

I don't see why it's tied into fuse #6. Fuse 6 is 60 amps. Just looking at fuses I would assume the big 60 amp one is feeding the lower ones.

Do you have power coming out of the ignition switch on the Dark Blue wire with key position start/run?

Pin 3 should be hot all the time.

Check to see if fuse 6 is really tied into that Dark Blue wire with a continuity check with your meter. Maybe diagram is wrong?

I forgot to provide theory and operation pdf, oops.

 

I do not have power on the dark blue wire at the ignition switch. I also have no power at #3 pin at the computer. When I put a jumper wire from the battery to pin #3, I did get power on fuse #6 and the blue wire at the ignition switch.

I'm pretty sure that fuse #6 is tied to the dark blue wire because we pulled the PDC up to check for corrosion underneath and fuse 6 had blue wires coming in and out of it that looked the same as the blue wire at the ignition switch. I didn't check the gauge fuse after adding the jumper wire so I don't know if it powered up or not.

I'm trying to get a better understanding of the wiring routing before I go back over there. I'm also trying to figure out if there is a fusable link between fuse #15 in the PDC which feeds power to pin 3 at the computer that could be burnt out. I have power at fuse 15

wrote:

I do not have power on the dark blue wire at the ignition switch. I also have no power at #3 pin at the computer. When I put a jumper wire from the battery to pin #3, I did get power on fuse #6 and the blue wire at the ignition switch.
 

Is dark blue the largest wire at the ignition switch? If it ain't check for power on the heaviest one. I don't trust these wiring diagrams but I'm guessing that 60A fuse is the ignition switch feed.

Yes, I believe the dark blue big wire was the one at the ignition switch. I will have to confirm when I go back over there.

Hi everyone. We are still trying to get this jeep to run. There has been some progress made. He ended up taking it to a shop where they found one of the ground wires was bolted to a bolt that had a rubber grommet so the wire was not getting grounded.

Truck would start and run but would die after about 15-20 seconds. Fuel pressure was only getting to about 15 psi and it's supposed to be running at 31 psi. Pressure would also drop to 0 right after the truck would die. The fuel pump and regulator were new. He figured the new pump had an issue since it had been installed over a year ago and had been sitting ever since.

He installed another new pump and it still only gets to 15 psi but now the pressure holds after the truck dies. He said it will now run for a couple of minutes before dying.

Obviously the fuel pressure is not high enough for the truck to run so what else should we be looking at here? What other tests can we do to figure out why we are only getting 15 psi instead of the 31? Thanks for any suggestions.

Voltage drop the power and ground at the fuel pump. Take black lead of multimeter on battery negative red lead Pierce or back probe pump ground wire crank engine or start voltage should be below 200mv. Then do the same for positive side of the pump.

Thanks for the suggestion Cheryl. That was going to be my next step the next time I get a chance to look at it. This vehicle is a wiring nightmare.

I'd try pinching off the fuel return hose to see if it brings up pressure before going after the fuel pump power and ground.

What will pinching off the return line show? Leaks? He has inspected all of the lines from the tank to the engine and cannot find any leaks at all.

I will try this and see what happens. Thanks.

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What will pinching off the return line show? Leaks? He has inspected all of the lines from the tank to the engine and cannot find any leaks at all.
 

It's a quick way to test the fuel pressure regulator. If pressure doesn't increase when you pinch off the line you've got a fuel supply side problem. If it increases above spec your supply is good and the problem is the regulator.

Ok, thanks. The FP regulator is new but I guess that doesn't mean anything these days with junk aftermarket parts. We will check that.