91xj ECU QUESTION,

Okay I got a 91 jeep cherokee that will not start and we went through just about everything so im looking for an ecu for my jeep! Last 3 digits are 707 on the ecu


MY question is, Being that the ecu's all look a like and will plug in and run 4.0 jeeps does the ecu # really matter? My last 3 digits of my ecu model number are 707 and I found some on ebay for out ragous amounts and then I found some from the same year jeep and looks identical for cheap but they have different ecu model numbers!

Any input would be great or if you have a ecu let me know

Never in my life have I heard of, or seen a cherokee of any year model not starting because of a bad ECM. Have you checked/ replaced your crankshaft position sensor?

Yeah its been through a lot! Jeep Starts up and shuts down! I actaully Drove it to get out of the garage to work on the boat and when I went to pull it back in the jeep would just Start right up and then die!

Bypassed resistor
new cam sensor aka complete distributor
new cps
Starts up fine and keeps running IF YOU KEEP SPRAYING STARTING FLUID
fuel pump primes
fuel pressure at the rail is great
SEEMS LIKE ECU IS SHUTTING DOWN INJECTORS
New plugs-rotor-cap-wires - oil - filter - fuel filter -
1991 HO 4.0 L


IF anyone has any idea's Please chime in!:angel:

I would be more inclined to suspect the injector harness wiring/plugs than an ECU given the symptoms you describe. Or, it could be a grounding issue at the dipstick tube or cable from the back of the head. You know how an ECU actuates things, right? By grounding them........

I'd suspect the injector harness or the ASD relay maybe, though it controls the ignition coil as well. As cruiser54 said, most of the ECU outputs are so-called "active low" or "open collector/drain" drive circuits, they connect the negative side of the device being activated to ground.

You need the same exact year and/or part number ECU - not all 4.0s are the same, not all 4.0 sensors are the same, and not all engine wiring harnesses are the same. There are a few exceptions but in general you must use the same part number.

Make sure your MAP is working and connected. I have an adjustable MAP on my 90. I had to remove the console where my switch is installed and forgot to hook the switch back up before starting it. It would start and immediately die just before ti got to idle RPM. Just a thought

I highly doubt it is your ECU as well. That would be the absolutly the last consideration.

If you still suspect the ECU, remove it's harness, spray contact cleaner on the harness plug and the ECU pins. "Tighten" the contacts on the harness plug with a small pick so they grab the ECU pins better. I had a mechanic friend who experienced some weird problems and after troubleshooting it for a long time, tried this, and the problems went away.

Hm. Starts, then won't run without starter fluid?

My vote is the fuel pump relay took a crap.

Fuel pump prime is ASD relay, and it also provides the coil with 12 volts--so it is Ok. Cranking the fuel pump is powered by the starter relay--it cranks and starts so that is probably Ok. Won't run with key in ON position unless you feed it fuel--well, when the key is in the ON position, engine running, the fuel pump power supply is through the fuel pump relay--that is where I would check.

I think Joe is on to something here.