MJ not getting fuel

I was driving around town all day with no problems and at my last stop the Comanche ('90, 4.0, 5 speed) wouldn't fire up. It cranked but wouldn't start. Had to get towed home- first time in its 16 year life!! Now here’s where I am:

I unhooked the fuel line to the filter, cranked and no gas flowed
When I turn the key I don't hear the pump priming
I checked all the relays by swapping into my other running Comanche, all OK
I checked the ballast resistor on the driver's side fender in the same way, all OK
Pulled the fuel pump, applied voltage to it, it ran
Voltage at the harness is only ~0.6 V at turn of key (~12 V in running Comanche)
Occasionally I’ll get 12 V, usually first try of the day
Tried grounding the negative terminal, still low voltage
Truck WILL START and RUN if I spray starter fluid down the TB

Could it be the ECU? Are ECU’s the same between manual and automatic trans? All input is welcome.
Thanks.

In the Renix setup, I think the ECU is the same between manual and auto as the trans computer is completely seperate from the engine computer. No wiring between the two. Assuming the running MJ is also a Renix (non-HO) you should be able to swap the computers.

Do you have power at the relay and is it clicking?

check the fusible link.

lawsoncl said:

In the Renix setup, I think the ECU is the same between manual and auto as the trans computer is completely seperate from the engine computer. No wiring between the two. Assuming the running MJ is also a Renix (non-HO) you should be able to swap the computers.

Do you have power at the relay and is it clicking?

Click to expand...

The other Comanche is also a Renix, I'll pull the ECU and swap them. I do hear a click but how do I check power at the relay? The '90 FSM says the fuel pump relay and balast resistor are bypassed at start-up and the starter relay is used, which does work.

87manche, where is the fusible link you're talking about?

Voltmeter. You should see +12 volts at one of the contacts and across the relay coil.

Hey here is my findings when I had the same issue
"Ok well I finally figured it out.
The resistor was fine.
While looking at the relays I was able to pull them out and flip the entire relay and case over and test the wires. When I did this I foudn ou that everything was working except for the orange lead wire to the fuel pump.
On the fuel pump relay the orange wire goes to the fuel pump and wasnt getting any juice. (side note: my auxillary fan is setup on a switch so I popped the relays one by one into it and checked the relays to make sure they had output. Guess what 3 out of 4 relays were bad. No output."

So bottom line just because they worked unhook the entire relay an mounting flip them over and check the voltage

Manual and Automatic ECU are not the same.
The input to the TPS is different and the Auto has a output for the transmission TCU.

fiatslug87 said:

The other Comanche is also a Renix, I'll pull the ECU and swap them. I do hear a click but how do I check power at the relay? The '90 FSM says the fuel pump relay and balast resistor are bypassed at start-up and the starter relay is used, which does work.

87manche, where is the fusible link you're talking about?

Click to expand...

http://www.naxja.org/forum/showthread.php?t=68504&highlight=renix+WOT+dead
There's my thread from a few months ago.
It's a red/green one if I recall. It's attached to the starter relay and goes into the wiring harness. Just tug on all of the fusible links. If one stretches or seperates then it's probably bad. Don't trust an Ohmmeter, I did and it Ohm'd fine, but it pulled apart when I tugged on it.

A recent post and the same problem turned out to be a relay.
He tried swopping relays, it didn't work so he bought a new fuel pump, still no work.
When he got down to testing he found the 3 of the 4 relays were bad.
The fuel pump needs the Power relay and the Pump relay to work.
Because he had only 1 good relay it would not run.

I think I've found my problem. I put the fuel pump back in and was able to start the truck (remember the intermittent voltage issue), but when I released the parking brake it died and this repeated a couple of times (my wife was present to witness it so I'm not delusional). It also died when I slammed the door shut; apparently the wiring harness to the fuel pump runs inside the cab near the E-brake and the connection must have been loose. A couple of months ago I replaced the weather stripping and had to remove all the interior panels and I must have jiggled the connection loose. I should have looked for the simple solution first- Occam’s razor correct again. It’s been running fine for a couple of days now so that must have been it. Thanks for all your help.

langer1 said:

Manual and Automatic ECU are not the same.
The input to the TPS is different and the Auto has a output for the transmission TCU.

Click to expand...

Yes, the ECUs are different between auto and manual for ODB-I and ODB-II. That's because the ECU is providing the TPS signal to the tranny computer. For ODB-I setups, which don't flag auto tranny errors, I believe you can substitute an auto ecm in a manual, but not vice versa since there's no tps output (don't quote me on that though, since I'm not positive).

The original poster said both Comanches were non-HO Renix setups. In the Renix setup, the TCU gets it's throttle position input signal directly from the TPS which has dual outputs. There are no connections between the Renix ECU and the AW4 TCU. I have an AW4 in my MJ with the original ECU working just fine.