2000Jeep$1

Hello,

I have a 2000 Grand Cherokee, 4.0 Inline 6, I believe the tranny is a
42RE. (Only one speed sensor)

Inititally the check engine light came on with a code of P0720.
Replaced the output speed sensor, reset the error, check engine light
stayed off until 15 mph was reached, then it came back on. Also, the first time taking off after vehicle was started, there would be a hesitation shifting from 1st to 2nd gear. AFter that, it would be fine.

Replaced sensor again, same results. Tested sensors with a DVMM and magnet and they both appear to be fine. Checked wiring harness and all
connections, everything appears to be OK. Checked all fuses releated to
tranny, PCM, etc, and all are good. Yesterday the vehicle would only operate
in "limp mode". (Starts off in second gear, would not shift into any higher gears). I shut it off and disconnected the battery for a couple hours. Fired her back up and it would shift again, but still have engine light and P0720.

Any ideas? From what I have read, possible the Governor sensor and soleniod inside the tranny? Or maybe the PCM? I don't want to guess and aimlessly start throwing money at parts for this. Any troubleshooting tips or suggestions would be greatly apprieciated.

ct67_72

check the wiring very well. get an ecm pinout and check the oss right at the ecm connector. check it for square wave while rolling, just like you did checking it with the magnet. You are losing signal somewhere inbetween the sensor and pcm.

2000Jeep$1

Ok,so I ohmed out the wiring harness from the connector on the sensor end to the connector at the PCM, checked out fine. I cleaned both ends with electrical connector cleaner, applied some dialectric grease to both ends, put it all back together. First time out it shifted through all gears, with a hesitation from 1st to 2nd, got above 15 MPH and CEL came back on. Shut of and restarted, stuck in limp mode again.

I was thinking of replacing the governor pressure sensor and soleniod, but
why would I be getting a P0720? I would think if those were bad I would get a different error code.

I am leaning towards this being an issue with the PCM. I have another JGC,same year, model, engine, tranny, etc. Can I swap the PCM out temporarily for troubleshooting purposes, or is there some type of security
feature that will render the PCM dead?

Any and all advise would greatly appreciated, beyond frustrated at this point.

2000Jeep$1

Surely someone has had this issue before.
Please help, just bought this for my daughter
and she hasn't been able to drive it yet.
My head is ready to explode!!!

2000Jeep$1

So I have found new info on the web.
I have seen a few posts saying this problem could be
caused by either corroded battery terminals, or possibly
a bad alternator that is creating unwanted RFI within the
wiring harness containing the OSS wires. Sounds like a stretch, but
worth checking before I spend $200 on PCM or $180 on the Gov. pressure
sensor and soleniod. I will clean up the battery terminals and also try driving with the alternator disconnected to see if it makes any differrence.

I will still take any thoughts or suggestions anyone might have.
I ain't giving up on this. I can't afford to!!

2000Jeep$1

I knew it was a stretch.
No luck. No one out there has any thoughts on this?
I have seen plenty of people have a similar issue
but no resolution. Surely someone must know the cause?

2000Jeep$1

Now when I shut the vehicle off, get out and shut the door, after 3 min. the doors automatically lock, horn sounds and the lights flash. Never did that before. I am starting the think the PCM may be spent.

ct67_72

hmmm, maybe. with all the messing around you could have changed some features too. does it have factory keyless?
The only other thing that can happen, and I have seen quite a few times is the snap ring that retains the O/D section sill break, and the O/D section will move around. it could be coming forward and loosing pickup from the speed sensor. remove the sensor and if you can see the pickup in the hole take a long screwdriver and try to pry it back and forth.
Or just see if its lined up in the hole to start.

2000Jeep$1

Pulled the sensor, looked inside and I can see the pick up. Tried moving with a screw drive, it is firmly in place. appears to be in alignment.
I don't know where to go. As a said in an earlier post, I have another identical jeep, can I swap pcm's to test, without any reprogramming?

ct67_72

Yup swap em out.

2000Jeep$1

I swapped PCM's, but the jeep won't stay running unless the PCM
is programmed with the correct VIN number.

I am stumped. I am going to try running a couples wires from the OSS
directly to the PCM, thinking there may be an intermittent short
or open somewhere in the harness. After that I am out of ideas.
Anyone? Beuller? Beuller?

2000Jeep$1

Well, I put in a temp wiring harness, still the same result.
I am leaning towards replacing the Gov Pressure Sensor & Solenoid,
along with a new filter and fluid.

I would think if it was the PCM that failed, it wouldn't have done so
gradually.

All the parts are going to cost $227, plus a 1/2 day on my back .
I really hope this fixes it.

ct67_72

dont waste the money. governor isnt going to set codes for a speed sensor.
You have something going on in that circuit. you need to get someone with a scanner that will stream data and watch the output speed. or datalog it and see if its dropping out.

2000Jeep$1

Well, I have verified everything in the circuit, other than the PCM.
I think I am going to order a new PCM for it. If that doesn't work,
I am setting fire to it!

2000Jeep$1

Upon further research, it appears as though this could be an issue
with the throttle position sensor as well.

I found the specs for input and output voltage, as well as an adjustment
procedure. I am going to check this before ordering a new PCM. With my luck, I would order it, and that wouldn't be be the problem.

I will keep ruling out all the cheaper stuff first. (although the pressure sensor and soleniod was $220 in parts)