Fuel Injectors off with Key in Run Position

A little long...but need to give all the information.

My son's jeep...'88 Cherokee, 4.0, Renix multi-point injection...has had intermittent running problems, finally totally quit some time ago. Replaced crank position sensor with new part, replaced coil/ignition control module from parts jeep, relays from parts jeep. Even an ECU from the parts jeep. Have verified it has spark and timing appears to be good. Injectors are firing (has fuel) during cranking.

After multiple cranking/no-start sessions in cold weather, changed the oil today after finding low compression. After oil change it started right up, but only ran for a few seconds. Did this 2-3 times, then started and ran fine. We ran it for 10-15 minutes, stopped and started it a couple of times with no problems. Turned it off to button up some stuff, now it won't run. It starts, but only runs for a few seconds.

Have verified running problem is that injectors quit firing (can keep it running using starting fluid, and verified fuel pressure at the rail remains at 31-32 lbs). Interestingly...will keep running if key is held in the START position, but injectors quit firing when key returned to run position.

I'm hoping one of the experts here can help. Is this a problem with the ignition switch? Do the injectors operate on a different circuit during cranking than they do during running?

As always...help is greatly appreciated! Thanks in advance.

This sympton sounds like a bad ballast resistor....Jumper out the resistor to see if it will stay running. Try search on Ballast Resistor.

OK...but are you talking about the one on the drivers fender? The one that runs the fuel pump? Would that cause the injectors to quit firing? I've checked fuel pressure...is good throughout cranking and during the short time it runs.

I'll check it out, and let you know.

Well, jumpered across the balast resister...still no run unless I hold key in start position.

Anyone else? I'm at my wits end here.

From everything thing that I can see on the schematic, the injectors operating circuit is the same for cranking and running...the thing that changes is the ignition switch position...so you need to check for power at the igniton switch pin 11 in both the start and run position...(Yellow Wire). If power disappears on this wire in the run position the switch is bad.

Thanks! I will check tomorrow...wife says I have to leave it for now and tackle it again tomorrow. I'll let you know.

IT RUNS !!!! Went back into it late last night. After several hours searching, decided to clean under the relay bank (it was pretty ugly under there) and replace the fuel pump relay (even though pump appeared to be working). That seems to have done the trick!

Thanks for your help!