I kept getting 2 transmission codes and a check engine light after driving at highway speeds on my 1997 XJ (AW4 auto tranny, 4WD) for 20 to 30 minutes. My XJ has 89,000 miles. The check engine light would not go out until I reset the codes with a code scanner. P1898 Manufacturer Control would be in the transmission control unit and P0700 Transmission Control System (MIL request) would be in the PCU (main computer). When the transmission control unit (TCU) has an error code, it communicates with the PCU and throws a P0700 to indicate that the TCU has a problem.
Recently I also started getting P0441 Evaporative Emission System Incorrect Purge Flow. and other Evap Emission codes (indicating a slight leak). I tried replacing the gas cap with a new one, changed the canister and purge valve, and checked the fuel vent lines and vacuum lines for leaks but didn't find any.
This has been happening for about 2 years and I had done the following without any positive results: change tranny fluid, clean tranny ground, replace cracked ignition coil (next to tranny ground), new neutral safety switch and new brake switch. I also checked the clockspring for continuity since I was having problems with my cruise control not working (was a bad brake switch). Some people have indicated that the 5 volt signal to the TCU or the tranny goes through the brake switch and the clockspring, but I have not verified that info.
This whole time my tranny and torque converter were working perfectly except for the codes.
What was wrong? The bundle of wires from the transmission comes up the firewall by the tranny dipstick and the ignition wire for the rear #6 sparkplug. The wiring harness was resting against the ignition wire. There was some electrical interference being transferred from the ignition to the tranny wires which caused the TCU to throw a code. There is also a bundle of wires in this spot that has the wires for the charcoal cannister purge valve. The spark plug wire had good insulation and a good boot.
I unplugged the wire harness connectors and moved the harnesses to the other side of the tranny dipstick, 2.5" away from the spark plug wire. No more tranny codes! I will have to wait longer to see if this has solved my emission code problems as well. The emission codes take much longer before they appear compared to the tranny codes.
I hope that this helps someone. I had searched online many times and never seen this solution posted anywhere. There are many reasons that the TCU can throw a code. If the tranny works fine and fluid levels are good, then check for this. It is a very quick repair.
Dave
Recently I also started getting P0441 Evaporative Emission System Incorrect Purge Flow. and other Evap Emission codes (indicating a slight leak). I tried replacing the gas cap with a new one, changed the canister and purge valve, and checked the fuel vent lines and vacuum lines for leaks but didn't find any.
This has been happening for about 2 years and I had done the following without any positive results: change tranny fluid, clean tranny ground, replace cracked ignition coil (next to tranny ground), new neutral safety switch and new brake switch. I also checked the clockspring for continuity since I was having problems with my cruise control not working (was a bad brake switch). Some people have indicated that the 5 volt signal to the TCU or the tranny goes through the brake switch and the clockspring, but I have not verified that info.
This whole time my tranny and torque converter were working perfectly except for the codes.
What was wrong? The bundle of wires from the transmission comes up the firewall by the tranny dipstick and the ignition wire for the rear #6 sparkplug. The wiring harness was resting against the ignition wire. There was some electrical interference being transferred from the ignition to the tranny wires which caused the TCU to throw a code. There is also a bundle of wires in this spot that has the wires for the charcoal cannister purge valve. The spark plug wire had good insulation and a good boot.
I unplugged the wire harness connectors and moved the harnesses to the other side of the tranny dipstick, 2.5" away from the spark plug wire. No more tranny codes! I will have to wait longer to see if this has solved my emission code problems as well. The emission codes take much longer before they appear compared to the tranny codes.
I hope that this helps someone. I had searched online many times and never seen this solution posted anywhere. There are many reasons that the TCU can throw a code. If the tranny works fine and fluid levels are good, then check for this. It is a very quick repair.
Dave