Rare tranny problem...-Newb needs help

Purchased a 1990 xj with the tranny that has the fulltime 4wd option.

As the truck begins to start moving it quickly goes into second gear without much of a fuss, probablly engages too quickly. Then it kind of bogs/slips its way through third gear until about 40mph. At 40-45ish the engine shifts into 4th gear waaay too early, bogging badly. The tranny will not downshift no matter how hard you mash the gas, only downshifts when slowing down or at lower speeds.
According to the haynes manual if the truck will not downshift you adjust the plunger/button thing on the throttle cable, which me and a friend did exactly as stated in the manual with absolutly no change in shiftpoints. I just had the tranny flushed today in an effort to resolve the problem, and they repaired a slow leak so the tranny is now holding fluid just fine, yet after picking it up from the shop an hour ago I find that the problem has not gone away.
Other than the slipping/shifting too early everything is fine. No loud noises and the tranny shop told me that everything looks fine. (guess they didn't look far enough)

atxchris1234 said:

Purchased a 1990 xj with the tranny that has the fulltime 4wd option.

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I am reading this to mean that you have a 90 4.0 renix with aw4 transmision and a np242 transfer case.

Aw4's are computer controlled. that means when you have wierdness wih them it is most likely, bad connection somewhere between a sensor and the tcu, a bad sensor, or a bad connection between the tcu and the tranny. It is theoretically possible to have a bad tcu but i have never heard of it happening with a renix era tcu.

if you are just flat out needing to control these things yourself. Brettm makes a spiffy kit to allow paddleshifting (or use of and arcade shifter). if you are really into that sort of thing fletchs90xj has a schematic that you could build yourself to do the same thing.

in any event good luck tracking the problem down.

cumorglas said:

I am reading this to mean that you have a 90 4.0 renix with aw4 transmision and a np242 transfer case.

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Correct. Is there an easy way for me to begin troubleshooting? I am rather new to this.

The problem is most likely a bad Throttle Position Sensor (TPS). Very common and your symptoms match. If you're handy with a meter you can check to see if the voltage is changing smoothly from 5 volts to 0 volts at the square connection coming off the TPS. A replacement sensor is around $30-40 at the auto store.

The throttle cable you adjusted does not affect the shift points (despite the crappy Haynes writeup), but rather controls the hydraulic pressure in the tranny which affects the shift firmness. Adjusting shouldn't hurt anything and it does eliminate it as a potential problem.

What fluid did the shop refill the tranny with? Some shops and even many dealers assume it's a Chrysler made tranny and use ATF4+ . They should have used Dexron III and ATF4+ is the wrong fluid.

it sounds like you got the little Power/Comfort switch in the Comfort possition.Just a guess!Try the easy stuff first.

atxchris1234 said:

Correct. Is there an easy way for me to begin troubleshooting? I am rather new to this.

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Depnds on what you mean by easy. I find working on renix jeeps easy because you can test anything with a Digital VoltMeter and the manual. DO NOT waste your time with the haynes manual. there is a renix manual available on pirate in billavista's tech section. that won't help with this problem but will for everything else. and get a good fsm. if you need help finding them google helps, or worst case pm and i'll send you in the right direction.

that having been said tps is possible candidate as is no power to tcu. start there.

FWIW the shift points you quoted are right on par for the AW-4 during a normal up to speed. However the not downshifting, and not holding the gears longer when getting on it is not. My 2000 shifts close to redline on a full throttle jount from a stop, and goes straight to second from as low as about 40/45mph. I would only assume your Renix era should behave closely in the same situation.

cumorglas said:

is possible candidate as is no power to tcu. start there.

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No power to the TCU would cause it to try and start out in 4th gear with the stick in (D).

lawsoncl said:

The problem is most likely a bad Throttle Position Sensor (TPS). Very common and your symptoms match. If you're handy with a meter you can check to see if the voltage is changing smoothly from 5 volts to 0 volts at the square connection coming off the TPS. A replacement sensor is around $30-40 at the auto store.

The throttle cable you adjusted does not affect the shift points (despite the crappy Haynes writeup), but rather controls the hydraulic pressure in the tranny which affects the shift firmness. Adjusting shouldn't hurt anything and it does eliminate it as a potential problem.

What fluid did the shop refill the tranny with? Some shops and even many dealers assume it's a Chrysler made tranny and use ATF4+ . They should have used Dexron III and ATF4+ is the wrong fluid.

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I had my tranny flushed about 4 months ago and they used ATF+4 in mine are you saying that is incorrect? I have a 1987 Wagoneer 4.0 auto 4x4.

Yes, ATF4+ is the wrong fluid for the AW4. Your owners manual should specify Dexron IIe or equivalent and in fact the drain plug has IIe stamped in it. Many have posted here with problems that developed after using ATF4+, that cleared up after doing a flush and refilling with the proper fluid.

thanks for the help guys, indeed it was the tps. I took it to jeepmasters in austin,tx and they hooked it up to a voltometer (or something like that) and found that the old one wasn't getting enough juice. Swapped for another one off a junk jeep they had in the back and presto. Works fine and gets the right amount of voltage. Also, they didn't charge me. Sweet.

BTW with the new TPS sensor the jeep has a ton more power. Feels like I got heads and cam compared to how it was.

Thanks for the help, if I hadn't been able to tell them tps first, they may have had to charge me for a diagnostic.

Wow, rare to find an honest place that will swap parts and not charge you. Definitely a place to recommend!