2004 4.7L H.O. WJ transmission problem - P0988

[Backwoods Rambler]

As the title says, I'm getting an intermittent P0988 error.

When the code goes away the transmission operates perfectly, but when the CEL comes on, it goes into "limp mode" and I have to shift it between 2 and D in order to drive properly and prevent over working the transmission.

After doing a bunch of research online and reading the FSM, I decided to have a mechanic friend check it out. He informed me that there are no shorts to ground, no loss of voltage, or any other issues that he could find with the wiring or voltage signals. In fact; after he verified, then cleared the CEL, the problem didn't come back for him the entire day.

I decided to get a few repair quotes. They ended up ranging from a $900 "we think that will fix it" quote, to a $3,000 "rebuild the whole thing" quote.

That didn't sit well with me and the consensus seemed to be that the transmission WAS the problem so I decided I would just swap the entire transmission myself. I found a used one through LKQ with 30k less miles than my Jeep.

After the swap, which included all new fluid (a flush) & new filters, everything seemed fine for just over 2 weeks, which was the longest the CEL went away prior to the swap, then the code came back. Then went away and came back again twice. Like I said; it's intermittent.

I did a little more research and decided to replace TCM. After that the issue went away again, but only for a day and a half.

I can swap the PCM, but I have the 4.7L H.O. and sourcing a PCM takes a bit more time since they need to ship one from out of state.

I really don't mind doing the swap, but I just don't know if going down the wrong road, or if the PCM could really be the cause of the problem.

Could my replacement transmission have been ruined by my old TCM being bad?

What are the odds that the replacement transmission has the SAME exact error code?

I'm thinking of taking it back to my mechanic friend (not free) and telling him to keep it until the code pops up for him, then check the wiring again.

Sincerely,

Frustrated and very-close-to-broke... :REOutShootinghunter

 

[Backwoods Rambler]

I got the following advice from Digger on CO4x4.org. He's an ASE certified master mechanic and always gives excellent advice.

Don't replace the PCM. That will only drain your pocket book.

After replacing the TCM and Trans, the only thing left is a problem in the harness.

My professional Advice would be first inspect the connectors for corrosion, then to isolate (clip) the 4C sense circuit and overlay a wire between the TCM and trans connector, and see if that cures it.

I can definitely do this, but first I want to inspect the contacts and maybe add some type of electrical contact grease to try to improve the connection.

I was told that NAPA sells this Stabilant 22A so I thought I would use that, but at $103 for what looks like a 1oz tube, I'm looking for other options so if you know of something I can use safely, I would appreciate the lead.

Also, I'm wondering if I do use a contact grease, do I need to make sure that I only get the grease in each individual pin so that the electrical circuits don't cross, or can I just apply it without worrying about electrical signals getting crossed?

Thanks for your input.

 

[theksmith]

normally contact grease is "dielectric grease", which helps insulate the contacts from corrosion, but does not itself conduct nor improve conductivity. i'm not familiar with the product you linked to though.

 

[Backwoods Rambler]

Yeah, I've used that before to fix a shifting problem with my old XJ and it worked, but that connector was also very dirty so it required cleaning as well. I wasn't sure if it was conductive or not. Stopping corrosion is good of course, but I'm hoping to also improve connectivity, but for less than $104 per half-oz.

Thanks

 

[Backwoods Rambler]

Update: I inspected the connections and they all appeared to be free of corrosion. I then overlayed a wire between the trans and TCM and tapped it into the 4C circuit wire as close to the connectors as possible. Less than 1" from the connector at each end. This was incredibly difficult at the transmission end.

Results: Code returned :-(

New information: Prior to today I thought the code was intermittent. That it was only present when my CEL lit up. Today I learned that the code is present every time I start my jeep. Or at least it did during the 15 minutes I spend cycling the engine on/off and clearing the code every time. It will verifiably clear until I start the engine again.

More new information: The past couple of day the CEL has been on constantly, but the transmission has been acting normally.

I suppose it is possible that the transmission I installed could have the same issue that the one I removed had, but I'm having trouble believing that's the case.

 

[theksmith]

what all codes are you getting now and how are you getting them (key trick, scan tool?) just curious cuz my FSM doesn't even list a 0988

 

[Backwoods Rambler]

Just P0988. Using a scan tool.

 

[theksmith]

Just P0988. Using a scan tool.

does the key trick report the same thing? my FSM has a list of TCM 2-digit codes and the standard 4-digit DTC's as well, but no 988 in there.

 

[Backwoods Rambler]

Currently the CEL is not on. The key trick just says: done

I'll try this again next time the CEL lights up and report what I get.

 

[Backwoods Rambler]

That code is not in my FSM either (not that I would be if it's not in yours, but I checked anyway). I asked the mechanic that I've been dealing with if this means anything.

Do you think that the message I get from the key trick will be more specific than an auto parts scan tool?

 

[Backwoods Rambler]

I requested some more details from the mechanic (Digger) that has been giving me feedback on another forum. Here's what he had to say:

Yes, that is what I meant. You must clip the wire out of the circuit. So long as it is in the same circuit with the overlay, it still can short the circuit.

The key trick will not work in your vehicle. A scan tool is always the preferred method.

Edit to clarify... transmission codes in a vehicle that has a separate TCU will not show up with the key cycle, other than P0700, which may or may not show, as an indicator for the scan tool operator to query the TCU for codes.

 

[theksmith]

I requested some more details from the mechanic (Digger) that has been giving me feedback on another forum. Here's what he had to say:

ah good to know that key trick does not give the specific TCU codes, only regular DTCs.

i don't know which scan tool you are using, perhaps it's interpreting the Chrysler TCU codes differently than an official DRB III tool would? maybe worth an hour or 2 of time at the dealership for diagnostics only the next time the MIL is one - if they would agree to provide you with exactly what they find including the TCU codes even if you don't want them to fix it - that might be a hard dealership to find i guess.

do you have the separate transmission booklet/pdf with your FSM? i didn't read through it all, but it lists several diagnostics tests you can specifically run through with a DRB III.

anyway, sorry to be of no help, was hoping maybe to get a code that matched something in the FSM that would give you better information.

 

[Backwoods Rambler]

No worries. Thanks for the effort. :ylsmoke:

He also said the code description and diagnostic chart will be found in the transmission diagnosis portion of the FSM.

My FSM does have a transmission section, but that section does not include diagnostic steps or codes.

 

[theksmith]

sent you an email with some diagnostic info... but for future people that might need this thread, turns out i was previously looking at the diesel tranny manual, the correct manual does have p0988 which is a problem with "4C pressure switch sense circuit", as you already knew.

but it also mentions that "Solenoid/TRS assembly may also set codes P0988(88) and/or P0987(A8)" - so maybe still a wiring issue, but a different circuit(s). the info i sent you also tells you some specific resistance and continuity checks to make for both the 4C pressure switch and 4C solenoid circuits.

 

[Backwoods Rambler]

Thanks for the update.