OBD1 4.0 no start - no signal from computer

[Xjcrawler736]

I am currently switching from a remix system to an OBD1 HO system. 1991 to be exact.

I bought an 91 XJ that ran and drove just to pull parts off of for my buggy. Ran and started up fine so I just pulled the motor, trans, computer and harnesses and put them on the buggy. Pretty much plug and play minus some of the body connectors for lights, gauges etc...

I go to start it last night and it will not crank and the fuel pump does not prime. I bypasses the Neutral safety switch, so that is not an issue. I have power to the computer and the the fuse box. All grounds are where they should be. I jumped the relay for the starter and it will crank. Also jumped the fuel pump relay and the pump works. But for some reason when the relays are plugged in, I get no signal for them to fire. I traced the fuel pump trigger wire back to the computer and it is not sending a signal. The starter is not getting a signal either. I have checked all relays and they work. Naturally, I am getting no spark either. It's almost like I am missing something simple that would correct everything.

What am I missing? I have looked at diagrams over and over and still can't figure out where the problem would be. I have a race on Saturday and I need thing thing running ASAP.

[Xjcrawler736]

From looking at the diagrams, the Fuel Relay Control wire is not doing anything and I am not getting battery power at the relay.

The battery power is hooked up to the power distribution center, so I am not sure why it isn't getting to the relays.

[xj9140]

I was having a similar problem intermittently ('91 ho) I fixed a couple vacuum leaks and its been fine ever since. Have you checked the fuel pump ballast resister?

[Xjcrawler736]

Originally Posted by xj9140

I was having a similar problem intermittently ('91 ho) I fixed a couple vacuum leaks and its been fine ever since. Have you checked the fuel pump ballast resister?

Its is ok. I have 3 more to test just to make sure.

The more I read, it looks like the Auto Shut Down or NSS. I have bypassed the NSS so that should be out of the picture.

[moparmaniaccuda]

So possibly a dumb question here but how is your ignition setup? I can't remember too much detail of your rig but do you have a factory column in there? Ignition switch junk?

[Xjcrawler736]

Originally Posted by moparmaniaccuda

So possibly a dumb question here but how is your ignition setup? I can't remember too much detail of your rig but do you have a factory column in there? Ignition switch junk?

I used to have a separate ignition switch and starter button. But since I am switching over and don't have time to screw with all that stuff so I put the OE stuff back in from the parts XJ. The ignition switch triggers the starter solenoid but the relays are not getting battery power. The computer plug #3 is getting power and that line goes directly to the ASD and Fuel pump relay.

I am currently looking though this diagram to figure it out.

[moparmaniaccuda]

What diagram did you follow to ground out the NSS? Kind of sounds like that's what's doing it.

Did you modify the power to the fuse box at all? I know there's a small tab behind the nuts that connects two terminals/posts to feed everything on some years (had it on the '93 harness I used).

Did you add/mod the factory grounds? I hate the ground strap on the rear of the head. I used all 0/1 welding wire to upgrade all the grounds.

Do you have any way of just plugging in a gauge cluster and seeing what pops up? Could easily get some codes off of it that MIGHT help point you in the right direction.

[Xjcrawler736]

Originally Posted by moparmaniaccuda

What diagram did you follow to ground out the NSS? Kind of sounds like that's what's doing it.

Did you modify the power to the fuse box at all? I know there's a small tab behind the nuts that connects two terminals/posts to feed everything on some years (had it on the '93 harness I used).

Did you add/mod the factory grounds? I hate the ground strap on the rear of the head. I used all 0/1 welding wire to upgrade all the grounds.

Do you have any way of just plugging in a gauge cluster and seeing what pops up? Could easily get some codes off of it that MIGHT help point you in the right direction.

I used this.



I did not modify the power to the box. And I added more grounds because of the nightmare that they are.

What are the tab you are speaking of?

[Xjcrawler736]

I just checked and I am getting battery power to the fuel pump and ASD.

The issue is, I am not getting signal from the computer, which would be grounding those relays. I am going to pull the harness and put in another that I have and see if it makes a difference.

[Xjcrawler736]

We now have the fuel pump and have hotwired the starter. No ignition power now.

[fantic238]

Did you ground the steering column frame? The ignition switch takes its ground from the column.

The ignition swithch (in run position) enables the ECU functionalities through a wire that goes to pin 9 on the ECU. If you don't get +12 there when the ignition switch is in run position then the ECU doesn't feed the grounds to the other relays.

[Xjcrawler736]

Originally Posted by fantic238

Did you ground the steering column frame? The ignition switch takes its ground from the column.

The ignition swithch (in run position) enables the ECU functionalities through a wire that goes to pin 9 on the ECU. If you don't get +12 there when the ignition switch is in run position then the ECU doesn't feed the grounds to the other relays.

Yes we did.

Ended up figuring it out late last night. It was the ASD. Swapped relays and it still did it. So we bypasses it and it runs great. As of now it is a band-aid and I still need to figure out what it going on after this race.

[4x4prepper]

Does the Jeep have a fuel pump cut off switch like the Fords do, in case of an accident? I can't tell by the above if that is the automatic shut down relay shown or not.

I know from working on non-starting Jeeps (4.0L - automatics) the transmission switch was found to be bad, after I swapped in good relays on the firewall and nothing had happened.

Is there an oil pressure switch and is it run into ECU #8 ? Seems like that would be a ground wire. I know some vehicles will not start if oil pressure reads 0.

[fantic238]

Originally Posted by Xjcrawler736

Yes we did.

Ended up figuring it out late last night. It was the ASD. Swapped relays and it still did it. So we bypasses it and it runs great. As of now it is a band-aid and I still need to figure out what it going on after this race.

This means a lot of things. It means that A the ECU works, B the ASD switched power (30 and 87 on the relay) is wired correctly, and C the ignition switch is fine too.
Maybe the second relay you swapped in was bad too?

[SkooledMedic]

Originally Posted by fantic238

This means a lot of things. It means that A the ECU works, B the ASD switched power (30 and 87 on the relay) is wired correctly, and C the ignition switch is fine too.
Maybe the second relay you swapped in was bad too?

I've had that happen before.

OP, make sure any relays you use to test with are known good relays.

Is the fuel pump relay working like normal?

I suspect you may have a power supply issue on the positive side of the coil circuit for the FP and ASD relays.

You have two different circuit in any relay. One circuit that the relay switches on or off and a second circuit that activates the relay.