Transmission Control Module?

Hello, I have a 1989 cherokee that is completely stock. We just replaced the engine with a new 4.0 that now has around 80 miles on it. The problem is that it doesnt shift until 4500RPM. We tried adjusting the kickdown but it didn't solve the problem. So we took it into the local mechanic shop because we were clueless. They think it is the Transmission Control Module. The price of this module is $780. Does anyone have any other ideas before we make this move?

We checked the local junk yard but they don't think they have one. I am assuming this jeep is equiped with the AW4 tranny but I'm not sure. It has Full Time, Part Time and 4LO selections.

Any help would be greatly appreciated!

Thanks

They're available for alot less than that(maybe <$100)-- you just need to find one.

Do a search on here for TCU ( might have to use JeepFreak21's search method).

There is an inline fuse -- yellow wire -- common problem.

Shift problems that amount to shifting at the wrong time are almost always due to a faulty Throttle Position Sensor. The second connector on the TPS supplies a signal used in setting shift points. The mechanic you are dealing with does not know the AW4 transmission. Try a search here, to get the full picture.

didn't the 4.0 change to a different TPS in the early 90s? one goes from 0 to 5 volts and the other goes from 5 to 0? that would explain it if you have the wrong one.

The tranny cable is not a kickdown cable like non-electronically shifted trannies use. It controls internal hydraulic pressure in trans and therefore shift firmness. The Transmission Control Unit (TCU) decides the shift points based on the throttle (TPS) input and the output shaft speed signal.

I would double check that you have the TPS connected. On an 89 Renix setup, the second set of wiring from the TPS for the TCU is the square connector. If you have a meter, you can check that it's working. The TPS signal for the TCU, as Brett pointed out runs backwards from the normal TPS signal that the engine computer needs. On the HO engines, the second signal is generated by the engine computer instead of a seperate output from the TPS.

I would also verify that the wiring from the tranny is properly connected as it may have been damaged or unhooked during your swap. There are two larger square connectors on the passenger side of the firewall that go down to the tranny (one black and one grey as I recall). One is for the NSS and the other is for the shift solenoids and output shaft speed sensor on the drivers rear of the tranny. It's possible that the output speed sensor or its wiring got damaged and the TCU is not seeing the output shaft spinning. I don't recall which pins, but you can check the output speed sensor with an ohmmeter. On the older AW4s, it is a magnetic switch that closes once per revolution.

A messed up NSS sensor could cause problems, but that would not be my first guess. I almost forgot to ask, this problem is happening with the stick in (D) right? If it's trying to start in 4th gear in (D) then check if the TCU has power.

If all else fails, I have a spare 1988 TCU you can have for $20 plus shipping. PM me if you're interested.

lawsoncl said:

I almost forgot to ask, this problem is happening with the stick in (D) right? If it's trying to start in 4th gear in (D) then check if the TCU has power.

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Yes, the shifter is in Overdrive. It starts and runs fine until around 27 mph then it wont shift until 4500RPM and 35mph. I does the same around 55mph, but after each shift it runs smoothly.


Hasta

Limited89XJ said:

Yes, the shifter is in Overdrive. It starts and runs fine until around 27 mph then it wont shift until 4500RPM and 35mph. I does the same around 55mph, but after each shift it runs smoothly.


Hasta

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Check the TPS, thats what controls the shift points.

I have been reading up on the TPS and from what I have read, when they fail the engine light is suppose to light up. In my case it is not. The only light that is on, is emissions maint. Which is going to be taken care of very soon.

all this trouble for and outdated 4x4 lol

Limited89XJ said:

I have been reading up on the TPS and from what I have read, when they fail the engine light is suppose to light up. In my case it is not. The only light that is on, is emissions maint. Which is going to be taken care of very soon.

all this trouble for and outdated 4x4 lol

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You don't have a check engine light btw on your 89, your computer doesn't store codes. It could be the TPS or TCU but it definitely sounds electronical to me.

I feel retarded now lol

Limited89XJ said:

I feel retarded now lol

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You shouldn't, you're still pretty new at this. You'll learn quick over time the best way to diagnose a Renix XJ is with a multimeter, for Jeep didn't have a CE light.