I've been putting a 2003 Grand cherokee drivetrain into a 94 YJ for a buddy of mine. The drivetrain is a 2003 4.0 engine, a 2003 42RE automatic transmission, and the original NP231 transfer case from the YJ.

Here's the problem: The tranny is not using 1st gear. When you start out and shift into Drive, it starts off in 2nd gear not 1st. When you manually shift it down to 1st, it still starts off in 2nd. Accelerating from a deadstop WOT won't make it downshift to 1st.

I'm thinking the PCM has the tranny in limp mode. Any ideas on where to start the troubleshooting as the FSM isn't much help.

All sensors are hooked up from the PCM except for these:
1. C1,14 - Transfer Case Position Sensor Switch (Nowhere to hook up on a 231)
2. C2,19 - A/C Pressure Signal
3. C2,27 - Vehicle Speed Signal (Used with the ABS control module)
4. C3,1 - A/C Clutch Relay Control
3. C3,4 - Speed Control Vacuum Solenoid Control
4. C3,5 - Speed Cotrol Vent Solenoid Control
5. C3,8 - Oxygen Sensor 1/1 Heater Control
6. C3,9 - Oxygen Sensor Downstream Relay Control
7. C3,10 - Leak Detection Pump Solenoid Control
8. C3,11 - Speed Control Power Control
9. C3,14 - Leak Detection Pump Switch Sense
10. C3,16 - Oxygen Sensor 1/2 Heater Control
11. C3,20 - Evap/Purge Solenoid Control
12. C3,32 - Speed Control Switch Signal

Is it a 42RE or 42RLE? The 42RLE was the auto from 03+TJs.

It's a 42re.

Well, after further review and some good FSM searching, I have pretty much narrowed the problem down. The 2003 grand cherokee PCM controls the tranny. But we are using a 2000 Cherokee PCM which does not control the tranny (external TCM), specifically no PCM "governor pressure solenoid control" output.

Unfortunately, the 42RE tranny is only offered with the grand cherokees. So a grand cherokee PCM is what we are gonna have to find in order to properly control the tranny.

PATC was working on a stand alone controller for the RE transmissions. You may want to see if thats available before you source out a new PCM/ wiring harness.

EDIT: Forgot, if your trans wont go into first gear even when in manual low, you have internal trans problems as well.

these transmissions are nutorious for eating governer pressure transducers.so you probably ought to put one in it anyway.I've seen it many times while working in local shops.

just had this happen on a cherokee at my shop. the fluid was low on startup after a service. computr put the tranny into "limp home" mode and stuck it in second. toped off the fluid and cleared the code and it was fine... not sure if this will help