AW4 trans sensor problems

[News09]

So I have a 2000 Cherokee 4x4 with the 4.0, AW4 and 231 t case. It throws a P720 (output speed sensor circuit malfunction) code and the transmission shifts weird. Long story short I was just going to change the output sensor, but decided to check all the wires at the pcm first using the write up I found on here. However, when I tested pin 1 (input speed sensor ground) to pin 2 (input speed sensor signal) I get no resistance. When I check pin 3 (output speed sensor ground) to pin 4 (output speed sensor signal) I also get no resistance. All of the other resistance tests are in spec. I'm just curious if that means the sensors are bad, broken wires/poor connection or what else it could be? I was going to try to test the sensors themselves, but can't seem to find any specs for them specifically. Any help is appreciated.

 

[News09]

So I ended up pulling both the input and output sensor off the transmission and checking resistance across them individually. Both show no resistance or continuity. I then tested the circuit going to the sensors by putting a jumper in the TCM harness from pin 1-2 and the circuit tested good the did pin 3-4 and the circuit tested good also. So with those tests I'm leaning towards both sensors being bad which I find unusual since I've had quite a few Cherokees and never even had either of the sensors go bad before. Has anyone else ever had this happen or heard of it?

 

[lawsoncl]

By "no resistance" do you mean it looks shorted or not connected? They're essentially pickup coils. https://www.cherokeeforum.com/f2/p0720-trouble-code-99-cherokee-classic-aw4-219427/ says 500-700 ohms is normal. If you unplug one and measure the sensor directly, you should see a/c voltage spikes 4 per revolution from the rear sensor with the shaft spinning. You can put the xfer case in neutral to test that. The voltage will be rpm dependant, and getting up to 30-volts at higher rpm as I recall. Measure from the TCM and at the sensor to help troubleshoot any wiring issues.

The two sensors are the same part number. SC-121 https://www.rockauto.com/en/partsearch/?partnum=4897734AA. You could swap them to troubleshoot, but given how much of a pain the rear one is to get to and how cheap the sensor is, I'd probably just install a new one.

 

[lawsoncl]

I agree. It's a rare to have one go bad, much less both.

 

[News09]

By no resistance I mean I unplugged the sensor and used my multimeter to check for resistance (ohms) across the sensor and it showed 0 where spec is 500-700 ohms. I don't think there's any wiring issues since I checked continuity from pin 1 in the sensor connector back through pin 2 in the same connector by jumping the wires together at the tcm plug.

 

[lawsoncl]

I just measured the front input sensor on mine (I was under there anyway). It's a 98 AW4 living in an 89 MJ, and the front sensor isn't even hooked up. It measures 645 ohms, so I think that 500-700 ohm spec is correct.

 

[News09]

So it would be safe to assume that both my input and output sensors are shot since they measure 0 ohms. That would also explain why there is no continuity across the sensors.

 

[lawsoncl]

So it would be safe to assume that both my input and output sensors are shot since they measure 0 ohms. That would also explain why there is no continuity across the sensors.

If they are measuring dead short, I would say they are both dead. Its really odd though, and I'd wonder what killed them.

 

[News09]

I really have no idea since I bought the jeep that way. However, the transmission only had 1 bolt in it and it was loose. I removed it, put factory bolts in it, made sure everything was tight and checked for leaks. Then I installed both new sensors and so far so good. Maybe the trans being loose and apparently driven that way caused it?