2.3L ('83-'97) 91 ranger bad pcm?

[00_Ranger4x4]

I bought a 91 ranger with a 2.3/manual from a buddy in a crank/no start condition. After looking it over, I had an intermittent spark condition, it would occasionally get spark once the first time i would crank it over, then be dead. But now it consistantly has no spark. I had taken the icm to the parts store to get bench tested, and it passed. Using a test light, the coils are getting power, but the connector on the bottom of the dis module has a weak ground with key off, and no ground with key in the run position. If i go back and probe pin 16 on the pcm, which is ignition ground, with the bottom connector of the icm unplugged, it has the same symptoms. But if I plug in the connector for the icm, it has a strong ground at the pcm. If im reading the wiring diagram correct, it also ties into the crank position sensor. By plugging the connector into the bottom of the icm, am I pulling the ground off the crank sensor, and thats why my ground improves off the pcm? Im assuming i need a new pcm, because pin 16 looses ground with the key on when unplugged from tge dis module, and the other three main grounds are fine on the pcm.

 

[franklin2]

I do not know much about this particular setup, but looking at the diagram, how do you know the ground originates from the PCM pin 16? Looks like it could be grounded through the DIS module pin 7.

 

[00_Ranger4x4]

I do not know much about this particular setup, but looking at the diagram, how do you know the ground originates from the PCM pin 16? Looks like it could be grounded through the DIS module pin 7.
View attachment 139969

I did some more research, I know the pcm sends a ground to the icm to control timing with the icm. It also pulls a ground from the crank sensor so it knows when to fire the coils. The icm is also grounded though the bolts that bolt it to the intake manifold. My assumption is that when testing pin 16 on the pcm with the icm plugged in, the ground from the icm is backfeeding to the pcm, giving me a good reading. When I isolate the pcm, by unplugging the icm, from my understanding, it should give a good ground through pin 16 when the key is turned on, to go to the icm to tell the icm when to fire. Which it is not. I think that answered my first question about the backfeeding. I'm now thinking i need a new pcm due to the fact that it's not sending a ground to the icm. Unless im missing something. I've never tore into a pcm before.

 

[franklin2]

I do know something about the EEC timing. The PIP goes to pin 56 on the PCM, that is the raw square wave generated by the crankshaft sensor. The PCM takes this raw signal, modifies it making it longer or shorter to vary the timing. This longer or shorter square wave is then output on pin 36 of the PCM. This is called the SPOUT signal, and is what the DIS module uses to fire the coils. This is how the timing is adjusted. These are your signal wires.

 

[00_Ranger4x4]

I do know something about the EEC timing. The PIP goes to pin 56 on the PCM, that is the raw square wave generated by the crankshaft sensor. The PCM takes this raw signal, modifies it making it longer or shorter to vary the timing. This longer or shorter square wave is then output on pin 36 of the PCM. This is called the SPOUT signal, and is what the DIS module uses to fire the coils. This is how the timing is adjusted. These are your signal wires.

Okay, that makes sense. So if I test pin 36 on the pcm, and crank the engine and there is no alternating ground coming from that pin, the pcm is either bad or the crank position sensor is bad? Tonight I need to check and see if the crank position sensor is bad, thats the only thing that I have not checked yet. And if that is good, it would eliminate it causing a no spark other then the pcm, or faulty wiring.