Trans won’t shift to 4th help

he Jeep:
-1990 Jeep Laredo 4x4 4.0 / AW4 6” lift on 33’s stock gears I believe.

Little back story could help:
In 2013 - the PO (a very close friend) went to the junk yard and got a used motor and trans out of a donor XJ same year, completely rebuilt both , pulled original motor and replaced with said Rebuilt motor, transmission, & T case. They all have about 12 miles on them now. At the time of all the rebuilds ALL new sensors for everything were installed He got the Jeep running and driving. He then started the interior, completely gutted the inside down to metal. He lost interest at that time. The Jeep sat untouched since 2014. Recently I bought it from him for a song and continued where he left off. Drained the fuel and added fresh yada yada got it running and driving again. I am new to Renix era stuff but, no stranger around a wrench or volt meter. A little while back I ran into shift issues which I found was simply the TCU was unplugged and still attached to the lower dash. Plugged it in and all seemed fine, HOWEVER, I that time I have not driven outside my neighborhood and assumed it was good to go.


The issue I am having now. Jeep shifts fine through all the gears 1-3 ( I think I counted 3 ) IT SEEMS but, as soon as I get around 40 mph and I think I wants to shift into 4th/ OD, it’s like I got on the brakes hard. It will NOT go into 4th/OD, can not get past 40ish mph at all. I tried to power through it but it just down shifts. I let off the gas it’ll go back to normal. I have searched all the forums I could and I think I’ve gone through most of everything I’ve read that was suggested. There is so much crap still unplugged I am hoping it’s that simple and y’all may have a clue for me.

So far I have: Tested the brake switch, static no pedal shows good resistance and depressed pedal shows OL, seems ok. Checked, adjusted ( square plug ) and even swapped out the TPS with a known good one, no change. Tested all three trans solenoids at the C14,C15 & C16 pins, all show exactly 15ohms which I seem to recall is normal. Checked both ground pins on that plug and they tested solid. Any ground I could find has been cleaned and I replaced the big 7 under the hood with 2g wire, sanded metal and added another body ground with a welded stud to fender. Check all the fuses. Like I said, all sensors are new, albeit sat for 6 years.

I found one, 2 prong plug NOT plugged in on the trans driver side. I see nothing around that looks like the missing half of said plug.

What else can I check?

Link to the evidence : https://photos.app.goo.gl/t2gaa8p7Ai4vpB9GA

Thanks




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Eah, I don’t know how to edit with this dang app.

Update. I just unplugged the TCU and attempted to shift it manually. 1/2 only gets me 1st no shift to 2nd, manually shift to 3rd, then attempted to shift to D. It does the exact same thing with or without the TCU.

TCU fried maybe???


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Is the throttle-body to trans cable adjusted correctly? If you're not getting 4th gear with the TCU unplugged and the shifter in (D), then it's a mechanical issue. Possibly an error made during the rebuild?

Just curious, have you tried to see what happens with the xfer case in low? I can't tell from the video if you're just losing power or the transmission is jamming up in 4th.

lawsoncl said:

Is the throttle-body to trans cable adjusted correctly? If you're not getting 4th gear with the TCU unplugged and the shifter in (D), then it's a mechanical issue. Possibly an error made during the rebuild?

Click to expand...

In regards to the trans cable. I tested to reset it like I have done before on Chevys but seems it’s retracted all the way and does not “pull out” with throttle movement. As I understand it it just controls line pressure and shift timing? I also thought I remember reading somewhere this era of Jeep wasn’t adjustable and controlled by the TCM?

Funny I just found a read last night about just exactly that. Well S**T.. My buddy took it upon himself to rebuild the trans, by no means a mechanic but I had full confidence he did a good job. He had a good manual and had a BUT load of pictures as he took everything apart, was very organized. I certainly wouldn’t have attempted this. He said it drove fine with no issues once he got it all back together and see no reason for him to lie about that. Have no idea what could have happened. Very frustrating!

Fortunately, I still have the original motor and transmission that were both in good working order when pulled. Looks like this turned into a trans swap. Guess I’ll be yanking a trans this weekend. I’ll transfer all the new components/sensors from the “rebuilt” trans to the original.

Thanks for the reply






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lawsoncl said:

Just curious, have you tried to see what happens with the xfer case in low? I can't tell from the video if you're just losing power or the transmission is jamming up in 4th.

Click to expand...

I have not tested anything with the transfer in low. As soon as it try’s to shift into 4th you see the RPM’s jump up the go low, it like I’m riding the brakes real hard. If I try to power through it at all it just down shifts to 3rd.


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Yeah, the trans cable just controls the hydraulic pressure, and thus shift firmness. Too far out of whack can cause overly-firm shifting or sloppy shifting or even not shifting at all. Sounds like you're binding up though.


The AW4 Service Manual isn't that helpful:

Condition:

- Drag or Bind on Upshift
Causes:
- Shift Linkage out of Adjustment
- Valve Body Faulty
- Transmission Faulty
Corrections:
- Adjust Linkage
- Repair Valve Body
- Disassemble and repair transmission

Yes, not much help at all lol

I talked to a transmission guy today. Going to take the truck to him Saturday and see what he thinks real quick.

I’ve read that rebukes in the transmissions don’t hold up as well as factory. Inferior parts I assume. I guess worst case I’ll throw the old one back in. Kinda like the idea of having a near zero mile Cherokee though!

Thanks for your input. I’ll post back results Incase it helps anyone else


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Years ago, i decided to upgrade my renix brake booster to a 96+.
Long story short, i didnt have O.D. after that.
Took me about 3 days to realize that there were 2 functions of the brake switch. 1 for the brake lights, and the other for telling the trans that its ok to shift into O.D.
There may have been 2 switches.....its been a while since i fooled w it.
Both were on the brake pedal.

uglyrenixxj said:

Years ago, i decided to upgrade my renix brake booster to a 96+.
Long story short, i didnt have O.D. after that.
Took me about 3 days to realize that there were 2 functions of the brake switch. 1 for the brake lights, and the other for telling the trans that its ok to shift into O.D.
There may have been 2 switches.....its been a while since i fooled w it.
Both were on the brake pedal.

Click to expand...

Man I must have tested that switch like 5 times, but tomorrow I think I’m going to try to bypass the switch with a jumper, just to doubly know for sure


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You already tested with the TCU unplugged, so it can't be related to the brake switch. The usual symptom if the switch is misadjusted or bad, is that the TCU thinks you're hitting the brakes and unlocks the torque converter.


I believe the 1990 is like the 1989 and there are two switches. One white switch on the brake pedal for the TCM which also has a vac line to release the vacuum on the cruise control solenoid - it closes when the brake isn't pressed. The other is for the brake light itself which closes when the brake is pressed.

The 1 with the vaccuum is the 1 i was thinking. I was beginning to think i made the whole thing up.

lawsoncl said:

You already tested with the TCU unplugged, so it can't be related to the brake switch. The usual symptom if the switch is misadjusted or bad, is that the TCU thinks you're hitting the brakes and unlocks the torque converter.


I believe the 1990 is like the 1989 and there are two switches. One white switch on the brake pedal for the TCM which also has a vac line to release the vacuum on the cruise control solenoid - it closes when the brake isn't pressed. The other is for the brake light itself which closes when the brake is pressed.

Click to expand...