Cranks, No start Part II

Check the threads the last few days looking for more info. Here's where the adventure stands:

Jeep 3 : Tom 0
1987 4.0L/AW4/231 215K on the clock.

New Parts:
Exhaust manifold, Plugs (Champion RC9YC), Crank Position Sensor, L/F motor mount & Trans Mount
Problem:
Cranks, no start, no signal to fuel injectors measured at ECU connector.

Previous attempt:http://www.naxja.org/forum/showthread.php?t=942835 four pages back.

Today’s adventure.
Removed the "new" CPS from the no start 1987 and installed on the running 88 XJ. Tested the 88 XJ and it starts. Determine New CPS is OK. Reinstalled new CPS on the no start 1987, tested cranks won’t start.

1. Removed the ECU from the running 1988 XJ and installed in the No Start 87. Result is it still cranks but won’t start
2. Installed the ECU from the No Start 87 into the 88 and tested. The 88 cranks fires starts and immediately dies.
3. Put ECUs back into respective vehicles, 88 starts and runs, 87 still dead.

Maybe closer to solving. The ECUs had different part numbers.
1987 w/AC EF 8953005026 and ‘S’ 101 140 201 and 8648
1988 w/o AC EF 8953005406 and ‘S’ 101 140 202 B and 8825
I’m figuring that the 8648 means April 8, 1986 and that 8825 means Feb 5, 1988

Anyway still no joy after three attempts. Haven’t located a NOID light yet to test signal at injector, but measuring for a signal at the ECU connector with DVOM shows no VDC.

Hmmm…O2 sensor? As I R&R the O2 sensor with the manifold replacement and the XJ ran into the garage.

check fusible link wires. i had one go bad and it cranked with no start

I've got a noid light that should work - I'll have to dig it up. I've also got a 1987 ECU (4.0L/AW4 & A/C) if you want to test by substitution. The rig got downchecked for blowing a hole in the side of the block - nothing electronic, so it should still work.

I'll have to write down those numbers and compare them to what I've got. Let me know if you're interested.

Do check the fuse link wires and the power feed to the start relay, as mentioned. I don't recall as you've said you've done that or not. The ECU draws power from that junction, so it can cause you trouble. (That's distribution for everything except the starter motor.)

Meanwhile, I'll see what I can round up to-morrow, and I'll pull the numbers off of it all. Note that generally, in electronics, parts are coded by week - 8648 would be the 48th week of 1986, and 8825 would be the 25th week of 1988. It doesn't really nail down the exact day, but you're going to get close.

Thanks again for the info;

I checked three fusible links at the starter relay. (2 green and one red). All three have continuity.

Jon, I’ll make arrangements to get down your way for the ECU. Thank you for the generosity.

I still believe the problem to be an ECU input, why else would the good one from the running XJ not correct the cranks no start? Plus, the ECU from the non running XJ cranked and started the other Jeep, although the engine would immediately die.

I am glad it’s in my garage and no on the trail.

O2 sensor is not used in the start sequence, and not used in the early running seconds, or sometimes minutes depending on its condition. It will start and run with out an O2 sensor.

I did not know Renix made 4.0 Renix ECU in 1986, in fact I didn't know they even built a 4.0 in 86? JON??? Odd date on the 86 ECU, it should be an 87.

Never mind I see Jon's post already answers the question with week 48, like December 1st, 1986, so that would be an early 87 computer model.

Next, didn't they add the ballast resitor in 1988? If so doesn't the 88 ECU shuts off the direct fuel flow and trys to run the fuel pump through the ballast resistor right after ignition? If so that might be a subtle difference in 87 to 88 ECUs. If so, it would prove the old ECU is bad and the new one just could find the ballast resistor in the wiring harness.

From your latest test I would say the ECU fuel injector firing circuit is bad on the 86 ECU, the one part of the ECU that has ever been documented to fail, but only once in xxx 1,000sands, document here by Jon. I would also say you have a shorted Fuel injector wiring harness that maybe burned out the old ECU, or the fuel pump ballast resistor issue, or both. Hell, it could be the wiring, ECU and ballast reisistor mismatch which is just the kind of failure that makes part swapping a difficult diagnostic method, and favors hard core diagnostic testing with DVM or AMM volt ohm meter.

Check the good jeep injectors connections with the 88 ECU to see if you get a DC signal pulse you can read on the Fuel injector connector with that jeep running. If you do, then its got to be the old ECU that is bad and possibly a missing ballast resistor, ECU mismatch, ....

also check the CAM sensor in the distributor. it controls the injector firing. very well could be your ecu also.

RENIX will start without the CAM/SYNC sensor - I've tried. It will stumble a bit while it's guessing, and take a bit longer than normal, but it will start. The effect would be something akin to you've just built a V8, and swapped two spark plug leads (until it starts - then it smooths out.)

I'll dig up those ECUs in a little bit.