FIXED-2004 Jeep Grand Cherokee Limited 4.7 AWD.

Problem= Fuse #6 in the under hood fuse box keeps opening (not really blowing) its getting hot and after some time it finally gives up. DTC P1389-No power from ASD relay. Which is expected. Traced through wiring diagrams, Fuse ^ feeds the work side of ASD relay and upon relay closing power then goes 3 places, 1) to the ignition coils, 2) feeds power to fuses 16 & 26, those 2 fuses never blow, So leads me to believe an issue with the coils, capacitors or wiring. Visual inspection of wiring appears to be ok, disconnected capacitors and fuse still blew. Did some reading on several other tech pages, 1) suggested repair was that the alternator may be shorting out, I though ok fine, the one i have is squeaking and seems a little hot, replaced it, using a amp clamp and a lab scope I measured the current draw before ASD relay, ran engine for about 45 minutes, you can see a static draw of around 7 amps, seems normal due to loads, also you can see each coil fire, about 16 amps per coil, again seems about right, test drove it, no problems. The next day drove it again and after about 5 minutes it blew the fuse again. Did some more research and the topic that keeps coming up is Dwell? meaning that under certain rpm the pcm may be firing more than 1 coil at a time, which makes sense as to why the fuse is getting hot and going open, 1 coil draws 16 amps, 2 coils draw 32 amps, add the static loads to that 32 amps and you have an issue where that fuse is not going to hold together very long. So Why is the pcm firing more than 1 coil at a time? If that indeed is whats happening?!?!?!

amp draw on Fuse 6 before ASD relay

wiring diagram for fuse 6

Hey Eagan11! It may be that the PCM isn't necessarily firing more than one coil at a time, but that the dwell is so far off that it's charging the coils for far too long. If this is true, then we may end up with overheated coils, too.

If we wanted to prove the dwell theory, then we'd want to look at the coil current by itself (without the O2 heaters and other stuff that goes through fuse #6), or possibly a primary voltage waveform. Doing some digging, and it looks like the coils get power through a dark green/orange wire that goes through C102. Borrowing a picture from JWR Automotive Diagnostics blog :



Not sure how easy it is to get at this connector, but it'd be great to get a current reading there. A shorter timebase would be needed, too, probably around 10ms. The guideline Paul uses is around 8 - 10 amps per coil at the peak.

If not, no big deal, we can still work with a current ramp and primary ignition waveform from one coil. I just checked some known good waveforms of mine, and it looks like a dwell of around 1.5ms is typical. That's not a specification, just something to compare to.

If we do find a dwell problem, then we can look at the cam/crank relationship to see if there truly is some kind of timing problem. I can find a known good waveform for comparison.

For reference, this video from Paul was a great example of a dwell issue, due to a crank sensor air gap problem. Distributor system, but the principle still applies:

Nice Tyler! Thank you.
To the OP, are there any other driveability complaints with this? These are known for the cam sensors to wig out and cause multiple random coil firings with just the key on! Coils will melt, fuse will blow etc.

ScannerDanner wrote: Nice Tyler! Thank you.
To the OP, are there any other driveability complaints with this? These are known for the cam sensors to wig out and cause multiple random coil firings with just the key on! Coils will melt, fuse will blow etc.

Good point, sir!

Great help and advice was given to me related to this repair and also future repairs.

Ill add now, not that it matters but, This is my wife's truck, so I was allowed a tiny bit of experimental freedom and testing.

The vehicle is now fixed and over 160 miles have been driven. Although I want everyone to know that I did not create this post in hopes of fixing my wife's truck, If you go back and read the OP it kind of hints towards getting the information that Tyler posted and of coarse Paul's help as well. What I mean is "what is actually happening and how does one go about proving it". This is a failure that would challenge quite a few guys in the field. The use of a lab scope would be key in a proper diagnosis. Also having the familiarity of what the wave forms should look like and also how to identify a bad wave form.

I learned something new on this one,

Thank you for all the help.
'

so what is the repair??

cheryl hartkorn wrote: so what is the repair??

^^^^^^^^

Oh Yeah! that would've been helpful.

I scoped the cam sensor, easy to get to and may have been the issue I was chasing but the patterns looked ok, nothing to unusual. The crank shaft sensor is very hard to get at, behind the starter, you can access the pcm connector C2 and back prob it there. I chose to not look at that signal due to not having any drivability issues. It would've been a good thing to do but even to access the PCM I wouldve had to remove a few components and brakets.

I pulled the passenger side valve cover to inspect and it also was leaking and I had already intended on replacing them. Upon inspection of the cam gear with reluctor wheel for cam sensor, which by the way this engine only uses one cam sensor to control both banks I found no issues, all looked fine. continuing to inspect I took a close look at the timing chain and followed it down into the motor as far as I could see, no issues. I looked at the guides I could see and all looked intact. I could also see the chain tensioner and noticed the the lower part of the plunger was very clean and shiny where as the upper part of the plunger seemed stained. Looked to me like that plunger had recently been allowed to protrude further from its cavity.

Since I had already planned on doing the valve cover gaskets due to leakage I pulled the drivers side valve cover. The timing chain tensioner for that side was also sticky out pretty far, harder to see it the drivers side.

These findings lead me to believe I had some stretched timing chains, Order a complete timing chain kit and oil pump (for safe measure)
Tore the front cover off and did not find any other broken guides and all looked good. I would guess the chains just wore out, 176k on the engine.

Replaced all the timing components, 3 chains, both secondary tensioners, main chain and guides along with center gear and crank gear, Used the original cam gear. replaced the Oil pump and water pump too.

I installed a 30 amp circuit breaker in #6 fuse location to avoid burning up fuse's if my repairs did not fix the issue. This way if the breaker opened and the vehicle stalled it would at least restart after the breaker reset.

Its been over a few weeks now, still no issues. Breaker has never tripped.

I do not normally replace parts, specially an entire timing chain job just to see if it fixes something but all my testing was inconclusive, Nothing was unusual and all sensors seemed to be ok.

I believe I could have proven my theory of the chains being stretched if I had scoped the Crank and Cam sensors on a Dual graph, Then I probably could have identified a chain Issue. But because its my own vehicle I had no issues replacing the chains after seeing how far the tensioners were sticking out.

Sweet! Sorry, didn't mean to nag about the fix, just interested to know what you found.

How'd you like keeping those secondary chains timed to the center gear? :silly: Not the worst chain setup ever, but certainly not the best.

My issue is why the PCM figure out there was a timing problem? Clearly the cam/crank relationship was off, causing the dwell issue, but why didn't it flag codes? :blink: I don't get you sometimes, Jeep!

Hi from New Zealand.
I am trying to cure the exact same fuse 6 blowing due to excessive coil charge time (dwell). Above 1200rpm all is good and cam crank correlation dose not seem any different at just above idle when its is the same but the current draw for any of the coils caused fuse 6 to blow.
The heads have been skimmed as purchased car cheap with blown head gadgets and I wonder if skimming has had affected the basic timing.
Even tried a reconditioned ECU but still the same. Something is triggering strange coil charge time.
I am looking for a good known cam crank wave form to compare to.
This is a friends jeep he is setting up for a hunnting vehicle so he wanting it fixed. It's the high output overland version.
Any ideas any one