Okay guys, I only have circuit diagrams for my 2000 WJ and my 2000 XJ, both of which use the coil rail design. They use the "waste spark" system where 2 plugs fire together every revolution, one on compression and one on exhaust. Supposedly, this simplifies the circuitry using only 3 coils to fire 6 plugs thru 3 drivers in the PCM. I have been searching for a long time to find internal circuit diagrams for PCMs, but apparently they are not available to the average person. All OEM circuit diagrams only show a "box" for any electronic module so I'm just going by conjecture. I know enough about digital circuitry to know how solid state drivers function and know they use a 5v signal to switch a 12v load, just like a mechanical relay, only quicker. My coil rails have 4 wires, one is 12v power from the ASD relay and the other 3 are grounds to the PCM drivers. If you put the rail on a bench and power up a coil, it will create a spark when you break the circuit, as in unground it. How long the "dwell" time is on the coils I have no idea. They may only be grounded for nanoseconds, but who knows. I also have enough knowledge about the old "can" coils from point-break systems to know heat is a killer and the point capacitor is there to absorb the primary field winding breakdown current surge to protect the points from burning. I'm assuming that's what the large capacitors in the PCM also do. Again, just conjecture. If there are any electronic gurus out there to either prove or disprove my conjecture, please chime in.

Just an afterthought. I'm thinking the cam sensor has something to do with coil dwell time, energizing it only when it's needed. On your "distributor" systems, you may not even have drivers in your PCM. My FSM shows PCM connector pin-outs that are labeled like "Coil 3 driver."

 

Good discussion, but I think now we are hijacking the OP's thread. Shall we take it to a new thread? I think it would be helpful for all of us to get this sorted out.

Whatcha tink?

 

No, I've already had my say so I don't care. Anybody else want to carry it on, be my guest.

 

Have you tried replacing the newest “dead” PCM with one of the ones that you already replaced to see if it turns on. Cause if it does and then dies again, the PCMs are good and something else is setting it off.

or it could be a short that keeps frying it like bacon. Just getting a quick idea out to be sure the other pcms are for sure “dead”.

youll need the diagrams and a volt meter to start hunting down an electrical problem like this since you may have a shorted wire or part. The diagrams will give you information on how much voltage or amperage each electrical system is running and what voltage is to be expected on the wire, what wires are running to the pcm, is the pcm expecting 5V and you have something shorted to 12V etc etc

 

Also try adding a ground from the PCM case to the body. Sometimes a poor ground there can cause odd issues.

 

Annnnnnd we're back to the grounds. Which is where we should be. Flaky grounds produce weird, mind-bending, 'that can't happen' kind of problems.

Clean them all. Don't assume. A visual inspection will not cut it. Also check the wires themselves. Corrosion can wick up the strands and wreak havoc, while looking just fine.

 

Won't do a thing. The pcm case isn't grounded and has no connection to the pcm circuitry.

 

Well, if you do it right, it might look cool. I''m thinking lime green cable, preferably 00, with gold plated terminals......


But otherwise, agreed!


A bad PCM ground will do weird things. Take a look at this short thread.

 

Well yeah. I confess, I grounded my case at one time. That was before I discovered that the PCM grounds at pins A31 and A32.
As an electrician, I would say you could draw some parallels to the electrical code for services, and just say that every thing in sight should be at the same potential, so create a 'grounding electrode grid' by bonding everything with a #4 copper. Water meter, inside plumbing and gas piping, and now the steel rebar in the concrete form too.

 

The OP has left the building...

 

LOL! A one hit wonder. That's ok we had a party without him.

 

I hate when that happens, but it was fun.

 

 

So i have cleaned all the grounds. And i happened to see that the power wire for my 02 sensor was torn out of the sensor and the wire was touching the metal on the 02 sensor. Could that be why the PCM’s keep getting fried?

 

It's a good possibility, especially if it's power to the heater circuit. I believe that's switched by the PCM.