P0720 CEL Won't shift in 4LO

I have a 2000 Cherokee with a Aisin AW4 Trans. I recently replaced the motor with a 4.7 Stroker, while the engine was out I sent the trans out for a rebuild because at 160,000 miles I didn't want to have to pull it a second time. Turned out it was a good idea cause the bell housing was cracked in multiple locations and needed to be replaced. When the New Bell housing, rebuilt trans and new motor were put in place, I started getting the infamous P-0720 Rear Output Speed Sensor error.


However the vehicle was driving fine in 4HI and 2HI, but in 4LO it would not shift out of first gear. The vehicle has since been back to the shop with the trans in place where he did all the usual continuity checks looking for a bad wire or short to ground and found nothing. I have swapped both the input and the output speed sensor front to rear and also with a donor trans of unknown origin and the fault does not follow the sensor. My trans guy is baffled since without a rear output sensor, he thinks that the trans should not shift out of first in either. I think the TCM is somehow getting a redundant signal from the T-Case and using that to compensate the missing signal from the Output sensor. He thinks the TCM is going bad which is why it's still managing to shift somehow but still throwing up the fault code.


I am curious if any one has been inside these transmissions to know what the Output sensor uses to get it's signal. Is there a dedicated tone ring on the output shaft that fires the sensor or does it just run off one of the other gears inside the trans? Is it possible this tone ring was not reassembled and that is why the signal isn't present? Could the trans have been improperly reassembled that the tone ring or gear is not aligning with the sensor and thereby not activating the sensor. The shop admitted they have not done many of the AW4 Transmissions because he thinks they are very reliable units nor has he had to replace to many TCM's for the same reason. I trust the shop as they have been in business for decades and the owner is a close personal friend who I believe to be honest, knowledgeable, and quite frankly; not out to rob me or any of his customers. So I am curious what is going on with the signal and where I look to repair it next. Any input is greatly appreciated.

According to what I have read, the speed sensor detects the revolution of the idler gear parking pawl lock gear and emits a pulse signal. The pulse signal is sent to the Transmission Control Module (TCM) which converts it into vehicle speed. It is unlikely that the T-case is doing anything other than its job. If not wires or sensors, a good used TCM shouldn't cost much.

Your transmission doesnt know or care about 4lo vs 4hi vs 2

OP,

See item 16. This is what triggers the OSS sensor.

I ran a diagnostic on the TCM harness according to information found on another Jeep Forum, the results are listed below. I have since removed my TCM and replaced it with a matching TCM from the Junkyard. I am still experiencing the same shifting issue with either TCM and still triggering a P0720 OSS Code. I am about this close to swapping out the trans and using a the trans from a donor vehicle I picked up last year, unfortunately I don't know anything about that unit or it's condition.

Section2: Specific Resistance Checks (Ignition =OFF,Battery Neg = Disconnected, TCM = Unplugged)
1. CAV1+2 =619 àSpec is 620, acceptable is 500-700
2. CAV3+4 =617àSpec is 620, acceptable is 500-700
3. CAV4 -Not applicableà1997 transmissions only
4. CAV6+7 =61.3àSpec is 60 Ohms
5. CAV11+?24?= 21.8àSpec is 11-15(Solenoid C)
6. CAV12+?24?= 21.9àSpec is 11-15(Solenoid A)
7. CAV13+?24?= 22.0àSpec is 11-15(Solenoid B)


Section4: Specific Voltage Checks (Ignition = VARIED, Battery Neg =Connected, TCM = Unplugged)
1. CAV25+GRND= 12.12àIgnition = LOCK
2. CAV26+GRND= 11.72àIgnition = ON
3. CAV18+GRND= 10.56àIgnition = ON, Trans = REV
4. CAV22+GRBD= 11.83àIgnition = ON, Trans = DRIVE
5. CAV9+GRND= 11.85àIgnition = ON, Trans = THREE
6. CAV21+GRND= 11.87àIgnition =ON, Trans = ONE-TWO
7A. CAV17+16 =0.99 at IDLEàSpec is 0.26-0.96Ignition = ON, Engine = OFF, Throttle = NotDepressed
7B. CAV17+16 =3.18 at WOTàSpec is 4.49 or lessIgnition = ON, Engine = OFF, Throttle =WideOpen
8. CAV6+GRND= 2.43àSpec is 2.43
9. CAV7+GRND= 2.45àSpec is 2.45


Okay, so according to the numbers, my resistance on solenoids A, B & C were all a little high, not sure if that makes adifference or not. In addition, my idle voltage on my TPS was higher than spec while my Wide Open throttle was a bit low. I do not remember my exact idle rpms, but maybe it needs to be adjusted down a bit which might explain the higher than spec voltage at idle. Everything else tested good, Wiring didn’t appear damaged, Sensors,voltage and resistance for the most part all seemed reasonable. Unfortunately all tests were done with theTCM unplugged, so I am still not sure the TCM isn’t going (or gone bad). Is there any way to test the TCM itself? Any thoughts or recommendations on where to go next are appreciated.