blue01xj

ok guys, trying to help my buddy troubleshoot his 88 xj.

4.0L automatic, 208K on body/harness/etc but 50K on rebuilt motor.

Been driving it for a few months with no issues. Was pulling in his driveway and it shut off. It will start but shuts off in 2-3 seconds.

he has done the following:

1. New fuel pump (he thought it was fuel related)
2. New CPS
3. Cleaned C101 connector
4. Swapped out all relays (they all ohmed out good)
5. Tested TPS -good
6. Tested new CPS - good
7. Cleaned all grounds

he was thinking ballast resistors, ECU, or possible ignition switch.

here is the weird thing...................
If you hold the key in the start position, have someone pull the solenoid wire off stater as soon as it starts it will stay running as long as you hold the key in the start position. you can give it gas and it will rev up and idle fine. soon as you let the key go back it will shut off.

Trying to find a wiring diagram to see what the difference is between those circuits and what signal is it loosing.
dont wanna just throw a ignition switch at it and it not be the issue and dont really wanna buy a new ECU with out knowing.

Any ideas? Anyone in middle tn have a spare 87-90 4.0L ECU we can try?
thanks

blue01xj

im so lost on this older stuff. All my jeeps have been 97 and up.

I would not assume a cam shaft sensor if it was bad would allow the jeep to run normal when holding the key. Gotta figure out how to pull codes if we can on this old girl. and see if the wiring can tell me a way to trouble shoot the ECU

cruiser54

Bypass the ballast resistor shown in the photo and see what happens.

blue01xj

ok, think he already did but will double check. Thanks

XJEEPTRUCKNORRIS

check fuel pump relay. mine blew out on and that's what happened to my xj also the map sensor. and map sensor vaccum line

blue01xj

all relays up by the fender well where swapped around and sockets double checked. will check other stuff

find it strange it will run and stay running if you hold the key forward in the start position (pulling wire off starter so it will disengage after it starts).

XJEEPTRUCKNORRIS

relay.pdf

XJEEPTRUCKNORRIS

when key is turned to on the relay primes pump for 3-5 sec then shuts off when key is turned to start it turns the pump back on. on my 89 that relay went out causing my truck to only run when I held the key in the start position . soon as I let off the key it would die. took for ever to track down even replaced the ignition switch witch wasn't it . ended up being that relay!

blue01xj

ok so maybe its the socket for the relay if we swapped the relays around? will have to unbolt it all and flip them over to check the wiring and sockets

you are talking about the ones on the passengerside behind the battery close to the shock mount.

XJEEPTRUCKNORRIS

Yes ! If u can open that pdf I uploaded on page52 IT shows the schematic pag. Page 53 shows fuel pump relay location

XJEEPTRUCKNORRIS

Fuel pump relay functions same as b latch relay to if that helps

blue01xj

yes opened it and forward it to him. he should be back home soon. thanks

XJEEPTRUCKNORRIS

Also I know the brown ignition wire coming off the ignition switch will burn out

cruiser54

That doesn't affect starting.

blue01xj

he was able to throw 12v power to the reply and start the jeep.
gonna dig into it more later.
thanks for the help but think were on to something.