635users

1999 4.0 Jeep Grand Cherokee.
I am getting a 0351 code and no spark on my coil pack in plugs #1 and 6. The manual says Ignition Coil #1 Primary Circuit. The coil pack does work correctly as I tried it on my other Jeep and also the camshaft position sensor is good so does anybody know what the problem could be? I'm at the end of my rope!!! Thank you all.

635users

I have checked the wires that come from the coil connector from the PCM and I have 55.0 resistance on pin number 1 and 11.0 on the other two pins, 3&4. I would assume they would all be the same resistance but number one is five times as high so that's where I'm not getting the spark and do you think that is the problem?

dave1123

Welcome to CF!

Okay, the coil rail is shorted between #1 and #6. This is common because there are only 3 coils that fire 2 plugs at the same time, 1&6 being connected together by a long connector that sometimes shorts to the case. The plugs fire at the top of the exhaust stroke as well to help burn what's left of the charge to reduce emissions.

Replace the coil rail and you should be golden.

635users

I'm not sure if I explained it correctly but the rail worked fine on my other car as did the cam shaft sensor so it's got to be coming from the computer to the connection between the rail connector in the computer

dave1123

Okay, here's how the system works. There are 6 drivers in the PCM for the injectors and 3 for the coils. The injector drivers are normally open and the coil drivers are normally closed. The injectors are hot with the key on and the ASD relay pulled in. The coil rail is hot at the same time. The PCM grounds the drivers to signal the injectors to cycle and coil drivers unground to create a spark at the right time. The collapsing magnetic field creates the high voltage.

SO...with the engine stopped and the key on, you can jumper the ASD relay and the coils should all show 12v on both sides showing the drivers are grounded. The injectors will show 12v incoming with an open ground on all. If you have coil #1 not energized, you've got an open circuit to the PCM or the driver is at fault. An open circuit could just be a bent pin in a connector or even a dirty one. If you need it, I think I can find the pin-out diagram for the PCM plugs.

dave1123

Here they are;

635users

Dave, thank you for the wealth of information. I will get on this ASAP and let you know what happens.

635users

I did a continuity test from the PCM plug to the rail connector and it was fine so I must have a problem in my PCM. My rail and crankshaft sensor both work because I tried them on the exact same Jeep as I have as I have two of them. So I'm ruling both of those parts out as the problem. I find it strange that when I check continuity to ground from rail connector coming from the PCM it is 55 on #1 and 11 on #3 and #4. Shouldn't #1 wire RD/TAN be at 11 also and 55? Any more suggestions for me to try? Again much thanks

dave1123

Okay, looking at the schematic for the coil rail, pin2 is supply from ASD. Pin1, 3,and 4 are primary coil grounds. Check the resistance between pin2 and the others. They should all be the same, thus eliminating the coil rail as the culprit.

For several years I've looked for a PCM internal schematic and nobody will divulge it's secrets so you're on your own there. It is possible the #1 coil driver is at fault. I know it's a PITA, but could you swap the PCM from the "good" jeep as a test? That may not work unless they are the same year.

635users

The "bad" RAIL works on my "good" Jeep so I'm assuming it is not a problem with the rail. But what did you think of the measurement from ground to pin number 1 of 55 as opposed to pins 3 and 4 which are 11?
ps... both of my jeeps are 99 and yes the PCM swap is a PITA.
Resistance from pin 2 to 1 on rail is 2.0, pin 3 is 1.2 and pin 4 is 2.0

dave1123

As I've stated already, I've no freaking idea of the internal circuitry of the PCM so I can't tell you if that's normal or not, but I suspect not. Try ohming out the pins on the other jeep. There are capacitors in the PCM to absorb the electrical surge when the coil magnetic field collapses. You could have a bad cap, who knows? On the 4.7 V8 there are large capacitors on each bank for this purpose. That engine has 8 coil-on-plugs.

BTW, all this information I'm giving you is in the Full Service Manual you can download from pacificcoastmanuals.com for $7.99+tax. It's a copy of the Factory Service Manual used by dealer service techs. It's 93 mbs of compressed pdf files about everything on your jeep including system descriptions and functions, diagnostic procedures, component locations, EVERYTHING! It's year specific for your jeep so you don't have to find your application in a sea of stuff about other models. It's the best $8 you'll ever spend on your jeep. DON'T try to print it out all at once! There are over 150 pages on the transmission alone! You'll have to ignore the stuff on the diesels. The last page is an index called a "Group Tab Locator."

I'm a dunce when it comes to transporting stuff like those pages I posted. I had to print them out, scan them into my pictures files, then post the images.

635users

I really appreciate you printing those out and sending me those pictures, that was above and beyond the Call of Duty. I did download something like you said which was a service manual I paid 8 bucks for it and I did go through it a little bit but you helped me out first so that was great. I will let you know what happens when I switch the PCM units. Thanks again

dave1123

WAIT!!!...Why switch the PCMs? Did you ohm out the other jeep's harness? If all 3 drivers show 11 ohms, it proves the one is bad. I have a thing about disturbing something that's working if I don't have to. Sorry, but that's the way my mind works, I didn't think about proving the readings on the good one before telling you to swap them! I sort of think on the fly as I'm typing. Breaking the connections on a 32-pin cannon plug invites problems. Don't do it if you don't have to.

635users

To be continued!

635users

I swapped the PCM units and it turns out that my PCM was bad so I ordered another one and we'll see what happens.