ZJ Not shifting into Overdrive - JeepForum.com
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post #1 of 2 Old 07-18-2013, 06:39 PM Thread Starter
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1995 ZJ 
Join Date: Jul 2013
Location: garfield
Posts: 2
ZJ Not shifting into Overdrive

Recently bought a 1995 grand cherokee, 112k on the odometer. I drove today about 85 miles and the truck wasn't going in over drive . The light above the OD off button turned on too. Once the truck cooled down, light came off and went into overdrive. Ps. The truck was sitting for about 10 months from the previous owner. From reading on the forums, i have comprehended that is cause from the trans overheating

1. The trans fluid level is fine
2. The fluid does not look dirty
3. I am thinking that my trans cooler is clogged or one the lines

Any suggestions, things to check and test would be appreciated, Thanks!

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post #2 of 2 Old 07-18-2013, 07:59 PM
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1997 ZJ 
Join Date: Dec 2008
Posts: 9,000
The OD doesnt work when the PCM thinks the transmission is overheated.

Could be clogged cooler, or the thermistor in the gov pressure sender could sending a wrong signal to the PCM.

Based on the symptoms going away after a cool down period, I would flush the cooler.

(1) Disconnect the cooler lines at the transmission.
(2) Using a hand suction gun filled with mineral
spirits, reverse flush the cooler. Force mineral spirits
into the From Cooler line of the cooler and catch
the exiting spirits from the To Cooler line. Observe
for the presence of debris in the exiting fluid. Continue
until fluid exiting is clear and free from debris.
(3) Using compressed air (under 40 psi.) in intermittent
spurts, blow any remaining mineral spirits
from the cooler, again in the reverse direction.
(4) Pump one (1) quart of automatic transmission
fluid through the cooler before reconnecting.
(5) If at any stage of the cleaning process, the
cooler does not freely pass fluid, the cooler must be

You can check the thermistor by disconnecting battery and pulling the two connectors closest to the engine. Use an ohmmeter to check between pin 4 of white connector and pin 1 of black connector for approximately 1000 ohms at room temperature.

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