I am working on a friends 94 ZJ 4.0L. It will not start - no CHECK ENGINE light, so right away I replaced the CPS, still same problem, replaced PCM with another one which we had, which worked fine before when we swapped it (when the jeep was running). So I started tracing wires, starting at the crank sensor, Sensor Ground (Black/Light Blue) is good have good ground. 8 volt supply wire White/Black) - has no power (with the key on) what so ever, so thinking the wire was grounding out somewhere, I cut the wire a few inches from the PCM and checked for power again still no power, well I decided to if it was grounding out, hokked one end on power at battery and other end to wire (leading to PCM) and I have ground - where it should be power. Why is this, thinking something was wrong with the PCM put the other one on and same thing have ground where there should be power.

Unbolted harness from PCM and check continuity on that wire and I have good continuity. Do anyone know why I would have a ground signal where there should be power.

anyone, experience anything like this or have any ideas.

Hi
New to Forum but have read info lots of times.

The 8 volts comes from a regulator circuit on the PCM board, coming out on pin #7. This 8 volts feeds power to the Crank position sensor, camshaft position sensor and the vehicle speed sensor.
You will need to check for a short to ground with the PCM connector disconnected. If shorted to ground, then check wiring too, and all three above, sensors for shorts.
If there is no shorts found on this line then there must be an internal short in the PCM, at the regulator.

Hi
New to Forum but have read info lots of times.

The 8 volts comes from a regulator circuit on the PCM board, coming out on pin #7. This 8 volts feeds power to the Crank position sensor, camshaft position sensor and the vehicle speed sensor.
You will need to check for a short to ground with the PCM connector disconnected. If shorted to ground, then check wiring too, and all three above, sensors for shorts.
If there is no shorts found on this line then there must be an internal short in the PCM, at the regulator.

Well I know all of this...lol and it's not a short in the wiring, I cut the wire from the PCM and checked for power (on the PCM side of the cut) and get NONE. I had a similar problem with my ZJ a couple years ago (didn't check for power with the wire d/c'd from the PCM tho) and found all three wires going into the crank sensor were bare and shorting out on each other.

My thing is tho, when the CPS is bad and the PCM doesn't see a signal from the CPS doesn't it shutdown this power wire or is that the ground.

I had 11.5 volts on the 8volt powerline when my PCM was failing.This voltage would slowly drop to approx. 10 volts before my check engine light & fuelpump would come on. Truck would run with a 1/8 sec miss/stall randomly.
I opened up the PCM and replaced Three electrolitic capacitors on the board and put it back in the truck. Works fine now starts and runs fine.
The 8 volt line now sits at 9.1 volts but the truck does'nt seem to mind.
I believe the power is not cut off because of the crank sensor, otherwise you could never get a new one to work?
The bad crank signal shuts down the ASD & fuelpump relays, and maybe the check engine light output.
This is just my opinion.
Hope this helps.

And does the ASD relay do - SHUTS OFF POWER TO THE INJECTORS AND IGNITION COIL, right or does it shut down the ground to these components.

And does the ASD relay do - SHUTS OFF POWER TO THE INJECTORS AND IGNITION COIL, right or does it shut down the ground to these components.

Nevermind, just found it while searching through the FSM, when the PCM doesn't get a signal from CPS it shutsdown the ASD which in turns shuts down the ground to the IGNITION COIL and INJECTORS and prolly the FUEL PUMP (or relay).

humm, it just doesn't make sensor that I get this same thing from both PCM's and they both worked ebfore this happened.

My only suggestion, is that you recheck the 8volt power line from pin #7 on the PCM harness where you cut it and ground. A dead short here from the wire or one of the sensors could blow the regulator output on the PCM board.
Do this before you try another PCM.
One of the capacitors on the output of the regulator on the PCM board could also be shorted because they go from power to ground. You have to be careful removing the foam covering on the circuit board.
Not quite as easy as I may make it sound.
Sorry to say, but it sounds as though both PCM's have lost there 8 volt regulator.

My only suggestion, is that you recheck the 8volt power line from pin #7 on the PCM harness where you cut it and ground. A dead short here from the wire or one of the sensors could blow the regulator output on the PCM board.
Do this before you try another PCM.
One of the capacitors on the output of the regulator on the PCM board could also be shorted because they go from power to ground. You have to be careful removing the foam covering on the circuit board.
Not quite as easy as I may make it sound.
Sorry to say, but it sounds as though both PCM's have lost there 8 volt regulator.

I will talk to him at work and get one of the PCM's.