2001 Jeep Grand Cherokee (WJ) Limited 4.7L - P1698

Hello Danner community,

I posted up for help about an Infiniti Q60 previously and your feedback helped me tremendously so thank you for that.  I am back again with a new issue on a different vehicle and hope you guys/gals can offer up some advice.

I have a 2001 Jeep Grand Cherokee Limited 4.7L V8 that appeared to have the transmission in limp-mode.  It seems to stay in 3rd gear and I have no OD so I did some research and here is what I have so far...

I found that a 30A JCase fuse (Fuse #5 in fusebox next to the battery) appeared to be blown so I replaced it and it blew immediately so i appear to have a short somewhere.  I am pretty handy with electrical troubleshooting and have most of the tools needed so I am hoping just a little advice on how to troublshoot around the modules without letting the smoke out of them will be greatly appreciated.  This particular model fortunately has a known hack where you can gain an extra gear simply by upgrading to a newer TCM Module so I ordered one without too much troubleshooting since I wanted to perform this upgrade anyway but when I plug the new module in I get the same fuse blow right away.  I have included the schematic and highlighted where I believe the issue may be.  I have circled the fuse that is blowing and traced the fault down to Pin 87 on the TCM control relay.  Pin 87 appears to be grounded but where I am hoping for some advice is working around the TCM relay outputs on the TCM and the Transmission solenoid which is in the tranny oil pan so I am hoping there is a way to confirm I am on the right track before dropping the pan to dig deeper.  I am hoping there may a way to isolate the fault without accidently frying anything inadvertantly.


 


You can see the complete circuit here and I have confirmed that Pin 87 on the Transmission Control Relay is showing a complete ground.  Now I need a good strategy to chase this down but since this line is shared with circuits on the TCM and the Transmission Solenoind down at the transmission I am hoping there is a good way to test without accidently making the problem worse.  

Anyone see this before?  Have any thoughts/ideas?  This jeep has a mild 4.5" lift kit with long arms so it gets wheeled moderately.  I was told the Transmission mount was bad and replaced that with a new Poly-Euro upgrade but maybe the extra movement could have damaged a line along the way.  Unforunately, this schematic doesnt appear to show how the lines enter the Transmission (hopefully I can find a single connector I can remove to isolate the circuit for more testing.  Maybe we have a Jeep guy in here with previous experience.  

I have not been able to find anyone on the internet with the same issue so I guess I get to break new ground on this one so any help is appreciated before I start really poking around.  I want to be sure I protect the other sensitive electronics so thanks again for any help/advice.

Hey Lvumlow! Great to see you back.  Q60 still running well?

So we need to narrow down where the short is. There's a couple options. You noted that you believe pin 87 of the Transmission Control Relay is grounded. How did you test for that, specifically?

I grabbed part of the OE diagram because we'll be wanting to refer to specific circuits:

 

Option one is to buy a pack of 30A JCASE fuses and start narrowing your suspects. You may have done some of these already, so feel free to let me know. First, key off, install a new fuse. Does it pop immediately? Then the problem is somewhere in circuit A30 or the TCM itself. I know it's unlikely that you have two bad TCM's, but lets leave that suspect on the list for now.

If the fuse didn't pop with the key off, now turn the key on. If it now popped, your problem is somewhere after the TCM relay. Circuit T16 or in the transmission solenoid assembly (the valve body in the trans).

You can distinguish between a short in the wiring (circuit T16) or the solenoid assembly by disconnecting the solenoid assembly. Slap another fuse in and see if it pops. Stays intact? Cycle the key a few times. If it's still OK, you've localized the problem to the solenoid assembly.

An alternative to buying a bunch of JCASE fuses would be to buy and install a circuit breaker in place of fuse #5.  Something like this would be fine.  Unfortunately, they don't make circuit breakers that fit in a JCASE socket, so you'd need some female spade terminals and wires to make it work.

Another alternative would be to install a headlight bulb. This is my personal go-to for short finding. I normally use a 9007 socket and bulb. The bulb has two elements, high and low beam. Each draws 3-4A, combined is closer to 8A. The idea is that, in the presence of a direct short to ground, the bulb will light brightly. With no direct short, the bulb either won't light or will light dimly. You won't let the smoke out of anything because the current flow in the circuit will be capped at 8A (the max current of the bulb). You also get a nice visual cue if the short is present or not.

The headlight bulb also gives you a few more options for testing. For example, if your testing leads you to suspect circuit T16 or the solenoid assembly, you can remove the transmission control relay and install the bulb between B+ and pin 87 at the relay socket. If the bulb lights, disconnect the transmission and recheck. If the bulb is out, the short is in the trans somewhere.

Overall, your chances of letting the magic smoke out of something are pretty low. You're working on a 30A fused circuit (so it's built for some current) that's already shorted to ground. There's really not much you can do to it that hasn't been done already. The only place you may go wrong is testing at the TCM relay. Leave the control side alone and you'll be fine.

Hey Lvumlow! Great to see you back.

 Q60 still running well?

Thank you, the Q60 has been running great - we have almost 10K miles since the overlay was installed and while that is not the best fix, it has done the job but I will not be surprised if I dont have to dig into the harness and fix it correctly one day...


So we need to narrow down where the short is. There's a couple options. You noted that you believe pin 87 of the Transmission Control Relay is grounded. How did you test for that, specifically?

I pulled the relay out and tested the 30/87 circuit, the front side (30) routed back to the main 30A fuse that was blowing and the backside (87) was directly grounded.  because the fuse would blow immediately, I kept the main fuse out and utilized my PowerProbe IV since it has a resettable circuit breaker.

I did look at the power distribution diagram (like the one you attached) but I found that it was a little mis-leading as it shows another 15A fuse off pin 87 and I believe that is only on the 4.0L engines whereas the 4.7L may be different?  I did have a 15A fuse in a similar location but it was not blown so it was a little confusing unless the short was before that point so I started chasing the 87 circuit and noticed it went through a connector (C103) so I disconnected the battery and the TCM and checked the 87 circuit wire at the feed side of the C103 connector and it was grounded so I knew it must be somewhere close to the fusebox.  As i started pulling everything out I was baffled because there was no indication that the main wiring harness has ever been touched (no witness marks or any sign of activity or breach) but I couldnt argue with the findings so I started cutting open the harness and sure enough, I found a short. 

 

I had just picked the car up from the body shop following a complete paint job but couldnt find any way that was related - until I got into this harness.  What appears to have happened is that the body shop must have removed the fender liners (maybe for masking or they removed panels to paint them but when they put the fender liners back in, they did not use the factory fasteners and drove self tappers up through the inside of the fender well.  Just my luck; this appears to have lined up directly with the main harness and the self tapper drive right into the (87) circuit that was grounded.  I cleaned everything up and put it all back together and no more fuse popping or P1698 codes (had to clear the DTC) but now I have a new code P1793 (Trd Link Communication Error)?  My theory is that the screw must have hit more than one wire or I have another screw somewhere else so I need to get back in there today.  I checked the wiring diagram and dont see anything to directly point me to a specific wire so does anyone have a clue how to identify any specific wire that would contribute to the P1793 DTC?  At least I had a specific circuit to chase before, now I need to try and figure out what line would lead to the P1793 code?

I feel like I almost have this whipped so I will go back to where the orginal damage was and see if I missed damage to another line, if that is not it I will need to decide if damage occurred during the original short or something along those lines.

Thanks for any advice/guidance so feel free to brainstorm along with me and I cant believe I have now owned two cars with main wiring harness damage, what are the odds but it goes to show you how easy it is for things like this to happen.  I am definately feeling more comfortable digging into electrical troublshooting and just try consider this a new skill to refine.

Thanks again for your help, with a little luck i can get this last code resolved without having to replace any parts.
 

Well, on the one hand, you found it! On the other hand, kinda frustrating to find damage like that... Did it leave the body shop with the dead short present?

P1793 is a new code to me. Had to look it up:
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You're specifically looking for circuit T99, orange/yellow:
 

Guess which connector also has circuit T99 in it.