I was driving my Jeep the other week and it just shut off while i was driving. I checked the coil and wasn’t getting power to it and so i got a new coil and still nothing so i ended up replacing the coil, cap, distributor, cam sensor, crankshaft position sensor and still nothing. I’m getting fuel, i can hear the pump turn on and I’m getting fuel pressure. All it does is crank and crank and not turn on. So i went and replaced the pcm and it fired right up. And the while driving it the next day it shut off again and was just cranking and wouldn’t turn over and start. So i got another pcm and fired right up again and drove it for a couple hours and turned off again and won’t turn on again. I would like to figure out what is killing my pcm before i go and get another one and put it in. If you have any idea please let me know i would really appreciate it. Also i have check engine codes 21, 22, 23, and 24

It sounds more like a failing crank sensor to me. I would have changed that before paying the big bucks for a PCM. People are way too quick to fault the computer today.

That does have “crank position sensor” written all over it.
Try to get a Mopar part, they are higher quality but will cost a little more. Well worth the price.

Michael

Take the PCM apart and see if something is fried, particularly anything that looks like it's coming from a relay.

I'm having trouble wrapping my brain around that. How does "no power to the coil" mean that the coil is bad?


The coil (like the injectors) gets supplied with power all the time. When the PCM wants to fire the coil, it provides ground. That connector to the coil is sometimes a trouble spot. If it's flaky, it will kill your engine intermittently.

More likely, as the others have said, your Crank Position Sensor is bad (again), because you bought an aftermarket sensor. They are known to be bad out of the box, after a few weeks, or after a few months. For the Crank, Cam sensors, don't cheap out. Buy only Mopar parts.

The PCM does sometimes go bad, but not nearly as often as it gets replaced. Typical failure is the capacitors drying out, which are almost impossible to replace because of the silicone get they use to encapsulate them.


But when all else fails, do the free thing that should be done first but rarely is. See my sig, #1.

See also Cruiser's tips.

Good luck with that. I tried taking mine apart and ended up using a reciprocating saw to cut through the potting material. Needless to say I was buying another one. Fortunately Advance had a Cardone on the shelf ..the 96 and 97 you just plug in and go, no programming.

Just a quick word about the electrical system on these jeeps. The coil and injectors won't get any power until the PCM sees a crank sensor signal, meaning you are actually cranking it. THEN it pulls in the ASD and fuel pump relays. The ASD relay powers the ignition and injector circuits. The moment you turn the key to the on position, the PCM sends a 2 second pulse to the fuel pump so it can charge the fuel rail with pressure, then it shuts off.

ASD means Auto Shut Down. If the PCM looses the crank or cam signal, or the system voltage gets too high or too low, it turns the ASD relay off, thus killing the engine. This is why you can crank it until the battery is dead, but unless that ASD relay is turned on, it won't start.

Mark, actually when the PCM wants to fire a spark, it ungrounds the coil, causing the magnetic field to collapse inducing the spark. The injectors, on the other hand, must be grounded to spray fuel. This is accomplished by digital drivers (solid state relays) in the computer. So what you're saying is correct, power is always present in the ignition and injector circuits as long as the ASD relay is turned on.

Agree with DAve's post.

Or if you're going to throw out those dead PCMs anyway send me one and I'll take care of it. I'll give you the 10 bucks for the Express Mail box.

Or just bring the chopped up one in to Advance and get the $90 core on the spot.

As this presumed short is intermittent, check the wiring harness, particularly in areas where it's near the exhaust manifold or areas of wear.

Thanks for the info, Dave, but I'm struggling with that from a design point of view. A coil secondary doesn't care if the primary field is expanding or collapsing - it will produce voltage either way. All that is required is that the field is moving. (It could even be movement of the field due to mechanical movement of the primary coil. Output is the same.)

Having current flowing constantly through the primary as it's normal state is wasting power (which becomes heat), and that power has to come from somewhere. That somewhere translates to load on the alternator, which translates to load on the engine, which translates to the last thing any vehicle manufacturer wants - wasted fuel.

Not only that, but that same heat problem exists for the PCM. It has to deal with the current flowing through that coil because it flows through the PCM, too. More heat where you least want it.

How much heat? Dunno. I can't tell without knowing the resistance of the coil, but I can do a back-of-the-envelope rough calcuation. We know the nominal voltage is 12v. The circuit is fused at 30 amps. That means it normally draws less than 30 amps. How much less depends on some design considerations, but lets take 20% headroom as a good ballpark. That means the circuit normally flows 25 amps. So by law (Ohm's Law), that circuit is dissipating 300 watts (12 x 25 = 300). Now, that circuit is shared by the injectors, so some of that 300 watts goes to them. how much? Dunno. Let's suppose they share equally. They're nice, well-mannered injectors, so they are happy to share with their friend the coil, right? After all, they are all on the same team!

So we are asking the PCM to soak up that kind of heat? True, it's not constant, but it's still more than if we only fire it when we need a spark. It's a duty cycle thing, and I'm sure it varies with RPM.

Attached is a diagram of the coil circuit (from a 98) which is wired as you describe. Constant 12v to the ASD (pink highlighting), which sends it to the coil (yellow highlighting) and ground then comes from the PCM (green highlighting). However, the diagram doesn't give any info about when the PCM grounds the coil.

Can you point me to a circuit function description that lays out the grounding as you describe?

Heh. Had the same problem. That potting material is tough stuff. I could not get the board clean enough to allow a good soldering job when removing and replacing the capacitors. Trashed the PCM trying.

Old points style ignition usually had a ballast resistor and a higher coil resistance and drew a few amps almost full time. In modern fuel injection ignition, the coil primary is only energized long enough to build the field and let it collapse to generate the voltage spike on the secondary side. There is also a CDI (capacitive discharge ignition) design style that dumps a capacitor across the coil. It's not like old points style that always let current flow through the coil. Modern coils are typically very low in resistance, around 1-2 ohms dc resistance.

This is what I believe we have in our Cherokees, but Dave says otherwise. I'm hoping he can point out the proper section of the FSM on this point.