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Old 08-16-2013, 06:50 AM
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P0988 Transmission Issue - Need Help

I have a 2005 WK Grand Cherokee with Hemi and 4WD. Recently I've been getting the P0988 DTC Code and my transmission goes into limp mode after about 20 miles of driving. When I clear the code, the transmission shifts normally, until the code appears, then eventually limp mode in 3rd gear.
P0988-4C PRESSURE SWITCH RATIONALITY only.
No other codes show up.

Things I've done so far.....
Took my Jeep to the Dealer in Aurora Ohio. They held my jeep for two days and said they need more time troubleshooting. When they started to give me the BS that something was wrong with my MSDS system in the Hemi I asked for my jeep back. Extremely poor service.

I have a factory manual on CD and an OTC Nemisys scan tool. Reviewing the manual, and since no other codes were present, I replaced the solenoid valve pack in the transmission. The 4C pressure switch is in the solenoid pack. Still have code.

I disconnected the wiring harness at the PCM and verified the 4C wire is shorted to ground at the PCM from the transmission. This is why the code is set. I disconnected the harness at the transmission, and the short is removed. Thus the wiring harness is ok.

Now I am stumped. What could cause the 4C line to short to ground and stay shorted? I checked the 4C pin on the removed solenoid pack, and its open as it should be. Something on the valve body is causing the 4C line to short to ground. I am determined to drop the transmission pan one more time before I take it to a transmission shop. Any ideas?
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Old 08-16-2013, 08:32 AM
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Re: P0988 Transmission Issue - Need Help

So the 4C pressure switch is closed with the engine off? Did you check this after several hours engine off? Maybe eventual residual pressure keeps the pressure switch closed. Each clutch pack has a very small orifice with a small filter screen which should let the fluid pressure drain away after the respective solenoid has cut the fluid supply to the clutch pack. When that orifice is closed by debris, it could still keep pressure to the pressure switch, eventhough the solenoid itself should let drain away the fluid as well.
Attached a hydraulic schematic as a reference.
Attached Files
File Type: pdf 5-45RFE hydraulic schematic.pdf (110.5 KB, 4 views)
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Old 08-17-2013, 08:11 AM
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Re: P0988 Transmission Issue - Need Help

Thanks for the help. I checked the switch after over 24 hours and it is still shorted. Today I plan to drop the pan again and remove the valve body. Then I can check if the switch opens when I remove the solenoid pack.

Is there any specific place to check for the clogged oriface without removing the transmission?
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Old 08-17-2013, 11:20 AM
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Re: P0988 Transmission Issue - Need Help

Update...

Before I dropped the pan again I checked the wiring harness again and found it was not shorted to the sensor ground, just the frame ground. I went underneath and pulled on the wiring harness and the short went away.

I will drive it for a while and if the short comes back, I will splice a wire for the 4C line.

Thanks again for the help.
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Old 08-17-2013, 11:37 AM
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Re: P0988 Transmission Issue - Need Help

You could not find any shave marks on the wire bundle? When you removed the connector from the transmission the wire was open, so the wire is probably shaved through close to the transmission and making ground to the same part which caused the shaving.
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