1999 Jeep GC P1391

Hi Guys,

I am working on a 1999 Jeep GC with a 4.0. It has a P1391 code after having replaced the crank sensor (Autozone part) for a no-start condition. The vehicle runs great until it gets to operating temp and then it gets funky. (chugs and misfires) I have installed a known good cam sensor and the owner has installed a new coil pack. (NAPA part) Neither have made a difference. I took a screenshot of my scope while checking the cam and crank sensors and what I'm seeing is a downward spike in the crank signal. Am I to look at changing the crank sensor again or am I just chasing a phantom?

Thanks,
Jason

Hey Jason! Thanks for including thr scope picture, love it.

Could be off here, but I think those spikes you're seeing in your scope traces is interference from ignition events. The spike occuring every time a set of four 'windows' goes by on the crank signal backs this up.

Either your test leads/scope is too close to an ignition component, or ignition voltage is interfering with the cam/crank signals somewhere. Maybe try moving your test leads away and recheck? Also, are the spikes there all the time, or only when the symptom occurs?

Thanks for the reply Tyler. I had thought of the ignition thing (Thanks to the scope videos from SD) and moved my leads all the way to the passenger fender. I also used the ground line from the coolant temp sensor instead of battery ground. That lead was brought up to the core support and across to the scope. The spike is there all the time that I've been looking but I can't get it to act up without loading the engine. I will have to get someone to help me create the misfire under load to see what the pattern looks like.

jconiber wrote: Thanks for the reply Tyler. I had thought of the ignition thing (Thanks to the scope videos from SD) and moved my leads all the way to the passenger fender. I also used the ground line from the coolant temp sensor instead of battery ground. That lead was brought up to the core support and across to the scope. The spike is there all the time that I've been looking but I can't get it to act up without loading the engine. I will have to get someone to help me create the misfire under load to see what the pattern looks like.

Understood, you're on top of it! I see that the cam/crank sync looks good, too, so it's probably not a jumped distributor or flywheel.

Also noticed the fourth pulse in the first group of crank signals seems to be a little shorter than normal, not sure if that's scope aliasing or not. Aftermarket crank sensors make me nervous in this application

I'd be very interested to see the scope pattern when it's loaded. I know it's a pain to take pictures, but we love 'em! Let us know what you find.

Well, I wasn't able to catch the poor running on the scope because I was working alone. The scan tool data showed that I was losing the cam/crank sync. I decided to go ahead and change the crank sensor again. This time I used a NAPA sensor. ( which I've never had problems with). Ran the Jeep for almost an hour and a half with no issues. It took less than 20 minutes to mess up before.

Thanks for all the input guys. I will try to get another scope picture now that it's running like it should be.

Nice! To be honest, I had considered suggesting replacing the crank sensor again just because of the brand, but didn't want to suggest 'parts changing'.

Curious to see if the ignition noise is still there...

99 was a weird year on the Grand Cherokees. First year new body style, lots of one year only interchange parts...
I've got a regular customer who drives one and it usually likes to test me...