AW4 transmission shifting issues?

[JamesHatch]

Hello all, I recently picked up a 1999 jeep XJ with 203K on it. Have done all the basic tune up procedures and fixed any leaks that were present. Running great. Recently however, I took the jeep on a 2000 mile road trip to Yellowstone out of Portland Oregon. I noticed on this trip that when climbing hills and when strong head winds were present, the transmission would shifting between gears rather quickly a few times until it eventually decided on a gear. Cruise control also doesn’t seem to work anymore, but I’m not sure if this is related. Recently replaced TPS so I don’t think that could be causing it. Jeep shifts and accelerates find in street driving and has never slipped in any scenario. It is also worth noting that the highways on the trip were mostly 80mph which I’m not positive the transmission is happy with when it comes to picking a cruising gear. Lastly, my transmission fluid is clean DEX/MERC and is to the proper level (maybe a hair over the max mark on the dipstick). Any thoughts?

thank you,

james

 

[Unclewolverine]

Cruise malfunctioning points to a vacuum leak. At 80 mph it should of been plenty happy go stay in 4th with the tc locked up. Also might try reseting the learned parameters on the computer.

 

[Vince]

My XJ does this at altitude. My 2001 was fine in Most of Utah below 8,000ft but in Colorado above 8,000ft I can't use cruise control. I manually select third gear on hill climbs to avoid continuous up and down shifting. The Jeep is fine again when returning to a lower altitude. Whatever learning my Jeep's ECU has done during daily driving was mostly between sea level and 1,000ft.



I think Yellowstone is around 8,000ft elevation.

 

[Unclewolverine]

Also just because you put in a new tps it doesn't mean it isn't bad. Not likely in this scenario but unless you put in a mopar unit they can be bad out of the box.

 

[RCP Phx]

Double check the tpv cable as it also comes to play.

 

[Unclewolverine]

TV cable adjustment is actually probably your best bet.

 

[Vince]

It is worth checking your Jeep for vacuum leaks and cable adjustments just to be sure but if your Jeep is behaving well back at home, read on...


I should have mentioned earlier that the first time I experienced the effect of altitude on manifold vacuum was when I was following my manual 4.0L TJ up a long gradient in Colorado somewhere near Royal Gorge. The TJ dropped out of cruise control first and wouldn't re-engage. My friend was driving it and told me what happened over CB radio as it happened, a few moments later the same thing happened on my XJ. Both Jeeps returned to normal cruise control function at lower altitude.


Engine power also drops noticeably for a normally aspirated engine at altitude. Friends towing their Jeeps on trailers behind turbo diesel pickup trucks would wait for me to catch up on the other side of the mountain. I would have to drop to third gear and just chug along until I could get over the mountain pass.


The effect of altitude on vacuum

 

[lawsoncl]

One common point between shifting and cruise control is the brake pedal switch. The trans computer uses the brake sensor to know when to unlock the torque converter, and the cruise will disengage when the brake pedal is pressed.

On the older Renix, there is an extra separate switch higher up on the pedal and less obvious when that is the problem since it doesn't affect the brake lights. Later HO setup uses the normal brake pedal switch.

 

[8Mud]

TV cable adjustment is actually probably your best bet.

A common problem in models a couple of years older, no idea if they managed a fix or not. In the earlier models pre 97 there was an aftermarket TV part for the tranny to fix this. It may just be the programming. A TV adjustment is quick and easy and may help.

 

[JamesHatch]

One common point between shifting and cruise control is the brake pedal switch. The trans computer uses the brake sensor to know when to unlock the torque converter, and the cruise will disengage when the brake pedal is pressed.

On the older Renix, there is an extra separate switch higher up on the pedal and less obvious when that is the problem since it doesn't affect the brake lights. Later HO setup uses the normal brake pedal switch.

I have now given the TV cable an adjustment and would like to add that when the cruise control doesn’t work and the transmission shifts erratically, I can make the cruise control work again by tapping the brake pedal a few times. And sometimes it will hold and sometimes it will go out of cruise control when I try to accelerate. This leads me to believe the brake pedal switch is causing the problem here. There is also a very dirty ground on the engine block right by the oil filter (the one holding the oil dipstick). I will clean this as well to remove a variable. Any other ideas?

 

[techno1154]

Did you mention tire size and gear ratio? I may have it.

My XJ have been doing that for years,...3.55 gears and 30 inch tires and again with 4.10 gears and 32 inch tires albeit at lower altitude. When it start gear hunting, I manually shift to 3. I have never lost cruise control, matter of fact, that is the only time it happens.