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Unread 01-26-2014, 05:56 AM   #1
streetzj
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Cold weather overdrive trick

My overdrive hadn't been kicking in during this cold weather, but I realized that after I had been parked and then got back on the freeway that it would kick in. So now when it doesn't kick in after sitting all night, I'll coast along the freeway or whatever road I'm on, put it in neutral, turn the ignition off and then back on, put it back in drive, and sure enough, the overdrive kicks in.

It's probably safer to pull over and park and do this, however.

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Unread 01-26-2014, 06:43 AM   #2
ZeeJay1997
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The PCM is designed to hold the transmission out of overdrive if the transmission fluid is below 50 degrees.

But what you're saying, if I'm reading it correctly, is that it stays in third gear no matter how hot it gets unless you turn the engine off and back on?

If that's the case, the PCM is suspect.
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Unread 01-26-2014, 06:46 AM   #3
streetzj
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Yup, that's exactly the circumstance. I can live with it for now, because it only happens when the temps are like -10. How difficult is the PCM to replace and how much does it cost?
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Unread 01-26-2014, 07:29 AM   #4
ZeeJay1997
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It may not be the PCM, but it could be. You are looking at around 200 bucks to replace it. To be honest with you, I've never heard of a PCM causing this problem, but it sounds like the PCM is not controlling the transition from cold to hot tranny.

If you're so inclined, you may want to pull the pan and look around. Take a read of the following info from FSM.

TRANSMISSION FLUID TEMPERATURE
THERMISTOR
Transmission fluid temperature readings are supplied
to the transmission control module (in the PCM on a 96-98) by the thermistor.
The temperature readings are used to control
engagement of the fourth gear overdrive clutch, the
converter clutch, and governor pressure. Normal
resistance value for the thermistor at room temperature
is approximately 1000 ohms.
The PCM prevents engagement of the converter
clutch and overdrive clutch, when fluid temperature
is below approximately 10C (50F).
If fluid temperature exceeds 126C (260F), the
PCM causes a 4-3 downshift and engage the converter
clutch. Engagement is according to the third
gear converter clutch engagement schedule.
The overdrive OFF lamp in the instrument panel
illuminates when the shift back to third occurs. The
transmission will not allow fourth gear operation
until fluid temperature decreases to approximately
110C (230F).
The thermistor is part of the governor pressure
sensor assembly and is immersed in transmission
fluid at all times.

3-4 SHIFT SEQUENCE
The overdrive clutch is applied in fourth gear only.
The direct clutch is applied in all ranges except
fourth gear. Fourth gear overdrive range is electronically
controlled and hydraulically activated. Various
sensor inputs are supplied to the powertrain control
module to operate the overdrive solenoid on the valve
body. The solenoid contains a check ball that opens
and closes a vent port in the 3-4 shift valve feed passage.
The overdrive solenoid (and check ball) are not
energized in first, second, third, or reverse gear. The
vent port remains open, diverting line pressure from
the 2-3 shift valve away from the 3-4 shift valve. The
overdrive control switch must be in the ON position
to transmit overdrive status to the PCM. A 3-4
upshift occurs only when the overdrive solenoid is
energized by the PCM. The PCM energizes the overdrive
solenoid during the 3-4 upshift. This causes the
solenoid check ball to close the vent port allowing
line pressure from the 2-3 shift valve to act directly
on the 3-4 upshift valve. Line pressure on the 3-4
shift valve overcomes valve spring pressure moving
the valve to the upshift position. This action exposes
the feed passages to the 3-4 timing valve, 3-4 quick
fill valve, 3-4 accumulator, and ultimately to the
overdrive piston. Line pressure through the timing
valve moves the overdrive piston into contact with
the overdrive clutch. The direct clutch is disengaged
before the overdrive clutch is engaged. The boost
valve provides increased fluid apply pressure to the
overdrive clutch during 3-4 upshifts, and when accelerating
in fourth gear. The 3-4 accumulator cushions
overdrive clutch engagement to smooth 3-4 upshifts.
The accumulator is charged at the same time as
apply pressure acts against the overdrive piston.
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Unread 01-26-2014, 07:34 AM   #5
ZeeJay1997
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One thing to try. Turn OD off while it's cold. Then when it warms up, turn it on and see if it will engage.
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Unread 01-26-2014, 09:36 AM   #6
BackRoadSeeker
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I always hit the button whenever I'm gonna set off on a cold morning just out of habit. It also seems to get better mpg's when locked out of OD crawling around town at less than 40 mph.
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Unread 01-26-2014, 10:56 AM   #7
ZeeJay1997
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Quote:
Originally Posted by BackRoadSeeker View Post
I always hit the button whenever I'm gonna set off on a cold morning just out of habit. It also seems to get better mpg's when locked out of OD crawling around town at less than 40 mph.
i've started doing that since global cooling has hit Alabama
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Unread 01-26-2014, 11:15 AM   #8
torchd
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I read this "theory" on here, and please correct me if I'm wrong.

I heard that you can warm up the tranny by letting the Jeep idle in N and that will warm up the tranny???

True or False
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Unread 01-26-2014, 12:01 PM   #9
ZeeJay1997
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i cant answer true or false cuz the bottom line is IDK.

i looked at the fsm and it is unclear on what the fluid is doing in park. no fluid is moved in the tc in park, but i'm not sure what the rest of the hydraulic system is doing. fsm is very clear what the fluid does while in the different gear positions. it stands to reason that since the pump is being driven by the tc hub at all times, the pump has to move some fluid.

It looks to me like that in neutral it moves more fluid than in park. the extent of the difference is unknown to me. So my guess is that the fluid being moved in park would warm up faster than the fluid moving while in neutral. But the second you put it in gear, the cold fluid from the TC is added to the mixture and cools it back down.

So you still have a theory... of someone who's guessing
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Unread 01-26-2014, 01:24 PM   #10
MaintMech
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My O.D. works normally until the temp is -5-8 Deg. F. After that it doesn't want to shift at all. However if I drive somewhere, park, and come back after 15 min. all is well. My theory is that the trans needs to "soak" a little and have the heat open the thermistor. I've driven 30 miles with no O.D. and sometimes even stopping doesn't help. I think it's just too dang cold at -15 and the trans is conducting heat away faster than the fluid will warm up the thermistor.

I don't worry about it too much as long as everything is normal above -5. With a 15 minute commute it's asking a lot for everything to get up to normal temps.
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Unread 01-26-2014, 01:36 PM   #11
ZeeJay1997
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Couple of things I read today that may come into play:

SHIFT VALVE OPERATION
The shift valves are moved by a combination of
throttle and governor pressure. The governor pressure
is generated by electrical components.
The conditions under which a shift to fourth will
not occur are:
Overdrive switch is Off
Transmission fluid temperature is below 10 C
(50 F) or above 121 C (250 F)
Shift to third not yet completed
Vehicle speed too low for 3-4 shift to occur
Battery temperature below 5 F.

GOVERNOR PRESSURE CURVES
There are four governor pressure curves programmed
into the transmission control module. The
different curves allow the control module to adjust
governor pressure for varying conditions. One curve
is used for operation when fluid temperature is at, or
below, 1C (30F). A second curve is used when fluid
temperature is at, or above, 10C (50F) during normal
city or highway driving. A third curve is used
during wide-open throttle operation. The fourth curve
is used when driving with the transfer case in low
range.
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Unread 01-27-2014, 06:32 AM   #12
jeepjeepster
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These temperatures have only happened a couple times in this area that I can remember, but when its 10F or below, my 94 42RE has a very hard time shifting into 2nd. Ill keep it in first for as long as I can leaving work, then when I let it shift into 2nd the jeep almost stops. Sometimes it will shift back into 1st for a few to get going again then shift back into second. After that all is well, it typically goes into 4th and locks right away, even when the temps are near zero.
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Unread 01-27-2014, 08:43 AM   #13
bigchieftaco
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Whenever I go out and start my ZJ to warm up in the morning, I put the e-brake on and shift into neutral. This does engage the tranny and it will help warm it up faster.
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Unread 01-28-2014, 10:58 AM   #14
djtricia
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OD disable switch?

I've got a 98 and I've never seen a switch for disabling the OD. Never had a problem towing but it would be nice to know for future reference. Can anyone help?
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Unread 01-28-2014, 11:07 AM   #15
970001zj
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square button next to the seat heater button if equipped forward of the key ignition
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