Problem Jeep SRT

Rudy Technician Montebello, California Posted   Latest   Edited  
Discussion
Driveability
2007 Jeep Grand Cherokee SRT8 6.1L 5-spd
Intermittent Stalling
Intermittent Misfire

This vehicle originally came in for a misfire at idle. During the course of testing the vehicle begun to stall out. Initially the stalling was very sporadic,then became consistent to the point of the vehicle being undrivable. All while in my care, of course. This was one week ago. I have approx 6 hours into this car now.

So now I must figure out the stalling and then move on to the misfire (maybe one and the same problem). First thing I did was fuel pressure. A solid 60 psi and holding. During that testing I noticed the ASD and fuel pump relays were clicking rapidly while the vehicle was idling. A quick scope of the injectors and coils on the power side,confirmed that the ASD relay is going wacko. (ASD relay provides power to those two items)

I scoped power input to the relay on pin 86 and the ground side control on pin 85 (fig. 1) While the vehicle was idling. Hmmm looks like a bad ground control from the PCM. Ive not scoped an ASD relay,but IMO the red line should be solid straight across while the vehicle is running,agree? 

From what I have gathered, 2 things will make the PCM unground the relay. They are the ckp signal and ignition switch voltage. Fig. 2 is a CKP signal and ASD again. Everytime there is a "hump" in the ASD, the vehicle stumbles. (I scoped the ignition voltage at pin 11-C1 of the PCM and it was good)

I scoped all powers,grounds (all were on the C1 connector) and both 5v reference signals at the PCM and everything checked out good. I called it a bad PCM. I was wrong. So here I am. 

Advice? Suggestions? Did I take a wrong turn and go deeper into the rabbit hole?

***I wasnt sure how to resize the captures,but if you open them in another window you can move around and get a clearer picture***

+3
Avery Technician
Kansas City, Missouri
Avery Default
 

Based on what you have described it sounds to me that the ASD relay is your most likely culprit. You could install a piece of jumper wire on the switch side of the relay and see if your issue goes away. If your issue is intermittent, use a heat gun to gently warm the relay itself, this in my experience tends to duplicate a more consistent relay failure. Keep this thread updated, tomorrow when I

0 Default Ð Bounty Awarded
Rudy Technician
Montebello, California
Rudy Default
 

I didnt want to upload a wiring diagram because Im not sure if that is copyrighted materiel or not. Yes I of course immediately suspected the relay and swapped it with another in the relay box. No change.

0 Default Ð Bounty Awarded
Joseph Technician
Lexington, South Carolina
Joseph Default
 

Rudy, The first thing that jumps out to me is the fact that those spikes are around 20 volts much higher than charging voltage. to me this would suggest that some type of coil or solenoid had to produce this voltage and may be spiking the ecm. It looks like your using the dual timebase feature and the events seem to happen consistently at about 160ms apart which is about idle or so (750rpm). My

+8 Default Ð Bounty Awarded
Brin Diagnostician
Melbourne, Florida
Brin Default
 

That's a good eye Joseph! I'm impressed!

0 Default Ð Bounty Awarded
Bob Business Development Manager
Ann Arbor, Michigan
Bob Default
 

ive had several 5.7L Hemis in this year range do this. Most were spark plugs/ignition related. Sounds crazy, but the secondary ignition was backfeeding into the PCM, causing it to do a reset. Stalls (reset) then fires right up again. Check the plugs before you do anything else.

+4 Default Ð Bounty Awarded
Rudy Technician
Montebello, California
Rudy Default
 

Oh man,Ive seen this happen on many, many Ford vehicles. It never occurred to me, because the ASD issue was clouding my judgment. Ill look into this for sure. Thank you.

0 Default Ð Bounty Awarded
Rudy Technician
Montebello, California
Rudy Default
 

Yes!! I ment to ask about that. I thought it was an unusually high jump for a relay. Thank you for clarification.

0 Default Ð Bounty Awarded
Robert Diagnostician
Ballston, New York
Robert Default
   

I agree with Joseph that it likely is a solenoid, ignition or injector causing those repetitive spikes. Could the spikes be the result of the ASD cycling and not the cause? If you manually jump out the relay to keep it powered up are those inductive spikes still on the control side? Have you tried unplugging the ignition coils one at a time to see if one of them causes the relays to stop

+2 Default Ð Bounty Awarded
Kevin Technician
York, South Carolina
Kevin Default
 

Can I ask why the plugs should be from dealer for these? I am struggling with a Jeep with the hemi where it came in skipping on the left bank I changed coils and plugs and the right bank now skips.

0 Default Ð Bounty Awarded
Robert Diagnostician
Ballston, New York
Robert Default
 

The oe plugs have a consistent resistance factor in the plug resistor. Because the Jeep uses a two wire coil that the ecm grounds to control the coil that puts the ecm in a position where any inductance caused in the coil firing event can be fed back in to the ecm . The suppression resistor in the plug helps control the amount of inductive kick that goes back to the ecm. If the kick voltage is…

+1 Default Ð Bounty Awarded
Kevin Technician
York, South Carolina
Kevin Default
 

Great help thanks

0 Default Ð Bounty Awarded
Tyler Technician
Hammonds Plains , Nova Scotia
Tyler Default
 

If the ignition coils and injectors are on the same fuse like the older Chryslers an amp clap there should give you some direction if there is indeed a problem with a coil or injector.

0 Default Ð Bounty Awarded
Rudy Technician
Montebello, California
Rudy Default
 

Well guys the answer was staring me in the face the whole time. I knew the inductive spikes on my relay were unusual,but I put it on the back burner and as I got deeper into this,I developed tunnel vision(focusing on the ASD) and I forgot about it altogether. You guys were correct, the spikes were caused by a faulty ignition coil and the issue was causing the PCM to shut down. Appreciate the

+9 Default Ð Bounty Awarded
Rudy Technician
Montebello, California
Rudy Default
 

Wow has it been a year already!?! Seemed like yesterday.....

+1 Default Ð Bounty Awarded