Failure of output shaft speed sensor on AW4

If your output shaft speed sensor on your AW4 fails, it will set not just a code, but several that point fingers at two of the solenoids as well as other false stuff.

Don't ask me how I know -- but it did take me a couple of weeks to learn to ignore the false codes. And was there a need to drain the oil and drop the xmission pan: Well, no.

halmca said:

If your output shaft speed sensor on your AW4 fails, it will set not just a code, but several that point fingers at two of the solenoids as well as other false stuff.

Don't ask me how I know -- but it did take me a couple of weeks to learn to ignore the false codes. And was there a need to drain the oil and drop the xmission pan: Well, no.

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The output speed sensor failing should only prevent upshifting out of 1st. Interesting that the TCM threw codes for something at all, much less failing solenoids. I kinda wonder if you had a wiring issue too.

The power into the speed sensor is on the same circuit as the CPS and cam sensor and other sensors depending on model year. A partially shorted sensor that causes a voltage drop in the power in circuit for other sensors can cause all sorts of wacky faults. If the voltage drop gets bad enough it will cause a bus fault. Garbage in garbage out.
Just an idea.

8Mud said:

The power into the speed sensor is on the same circuit as the CPS and cam sensor and other sensors depending on model year. A partially shorted sensor that causes a voltage drop in the power in circuit for other sensors can cause all sorts of wacky faults. If the voltage drop gets bad enough it will cause a bus fault. Garbage in garbage out.
Just an idea.

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He's got a 98. The aw4 trans output speed sensor on a 98+ isn't powered. It's a magnetic pickup that generates 4 pulses/rev - no power to it like the cps. Prior to that it was a magnetic switch that closed 1/rev. A partial short on either would just make the sensor not work.

His other post about wierdness with the ABS and rear wiper makes me think a partial short or bad connection somewhere though.

lawsoncl said:

He's got a 98. The aw4 trans output speed sensor on a 98+ isn't powered. It's a magnetic pickup that generates 4 pulses/rev - no power to it like the cps. Prior to that it was a magnetic switch that closed 1/rev. A partial short on either would just make the sensor not work.

His other post about wierdness with the ABS and rear wiper makes me think a partial short or bad connection somewhere though.

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Thanx for the info, learn something new every day.

lawsoncl said:

The output speed sensor failing should only prevent upshifting out of 1st. Interesting that the TCM threw codes for something at all, much less failing solenoids. I kinda wonder if you had a wiring issue too.

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I wondered that too, to the extent of my replacing the transmission wire harness that runs from the firewall to the transmission; i.e., to each solenoid as well as the input and output shaft speed sensors. The $500 O'Reilly scanner showed failures of each of the three solenoids, in addition to one that I had begun to suspect: the "output shaft speed sensor circuit"

That was a bit tough to swallow, since the transmission shifted perfectly, as well as engaged the torque lockup. Maybe it's worth noting that my sole concern was to extinguish the MIL. The Jeep drove nicely, and still does, but now w/o the MIL illuminated.

Replacing the entire harness had no discernable effect.

Balking at O'Reilly's $106 price for the output shaft speed sensor (OSSS) I came home, fished around in my junk bucket and located a couple of OSSS's and tried one out. I then killed the stored codes with my cheap-ass scanner, and about 80 miles later, no codes of any kind and more importantly, no MIL. (as an aside, the output and input shaft speed sensors are identical)

I have a sneaking suspicion that the AW4 control module -- and/or perhaps, the vehicle ECM is smart enough to rely on the VSS when the OSSS circuit fails, but still illuminates the MIL. Just a wild guess, but consistent with my observations and experience. And with full realization that the VSS is downstream from the transfer case, meaning that the VSS and the OSSS would read the same rpm only when the Xfer case was in 'strait through' operation.

Lastly, every electrical component on the Jeep works perfectly with the following exceptions: The airbag light is almost always on and the rear gate window wiper does not work when the airbag light is on. But (and this is a big butt) the airbag light and the failure of the window wiper are perfectly in sync. Just to be clear: When the airbag light goes off (as it sometimes does) the wiper will work. When the airbag light is on, the wiper won't work. LOL! The transmission marches to its own drum, completely independent of the airbag and rear wiper. So far.

But the MIL is no longer staring me in the face.

8Mud said:

Thanx for the info, learn something new every day.

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Without going into detail, I learned the difference between the '97 AW4 and the 99 AW4 the hard way. I say '97 -- 99 (omitting '98) because I have never nailed down the point at which Chrysler -- or maybe Toyota -- made the change.

Bottom line: installing the later version in a vehicle that had the earlier version (and the other way around) is easier said than done. The matter is addressed in the "Everything you ever wanted to know about the AW4 thread"

Sorry: double post

halmca said:

I have a sneaking suspicion that the AW4 control module -- and/or perhaps, the vehicle ECM is smart enough to rely on the VSS when the OSSS circuit fails, but still illuminates the MIL. Just a wild guess, but consistent with my observations and experience. And with full realization that the VSS is downstream from the transfer case, meaning that the VSS and the OSSS would read the same rpm only when the Xfer case was in 'strait through' operation.

Lastly, every electrical component on the Jeep works perfectly with the following exceptions: The airbag light is almost always on and the rear gate window wiper does not work when the airbag light is on. But (and this is a big butt) the airbag light and the failure of the window wiper are perfectly in sync. Just to be clear: When the airbag light goes off (as it sometimes does) the wiper will work. When the airbag light is on, the wiper won't work. LOL! The transmission marches to its own drum, completely independent of the airbag and rear wiper. So far.

But the MIL is no longer staring me in the face.

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The trans computer has no connection to the VSS, and I'm skeptical that it's smart enough to query the engine computer over the OBD bus if it's own sensor goes belly up.

In some ways I love the challenging problems like your ABS/wiper issue, but only when they're repeatable. I hate the ones that only happen when the wife is driving and doesn't happen when you're actively trying to diagnosis it.

The worst example was the 98 ZJ that would die halfway through the community whenever my wife drove it. No codes or anything, and it would always start and run after I towed it home. Never happened to me. It finally stayed dead after the 5th time. After a few hours with a meter, I noticed the crank sensor had voltage on both the power and ground wires. Turns out all of the sensor and ECM ground wires merge together inside the ECM connector and run back to the battery on a single single dedicated ground wire to the battery. That wire got pinched under the battery when Firestone replaced it two months earlier. Why only the wife though? Guess who doesn't slow down for the potholes?

Edit: It was a 93 ZJ that was dying on me.

lawsoncl said:

The trans computer has no connection to the VSS, and I'm skeptical that it's smart enough to query the engine computer over the OBD bus if it's own sensor goes belly up.

In some ways I love the challenging problems like your ABS/wiper issue, but only when they're repeatable. I hate the ones that only happen when the wife is driving and doesn't happen when you're actively trying to diagnosis it.

The worst example was the 98 ZJ that would die halfway through the community whenever my wife drove it. No codes or anything, and it would always start and run after I towed it home. Never happened to me. It finally stayed dead after the 5th time. After a few hours with a meter, I noticed the crank sensor had voltage on both the power and ground wires. Turns out all of the sensor and ECM ground wires merge together inside the ECM connector and run back to the battery on a single single dedicated ground wire to the battery. That wire got pinched under the battery when Firestone replaced it two months earlier. Why only the wife though? Guess who doesn't slow down for the potholes?

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GREAT story!

A pal who knows that I have been wrestling with a "check engine" lights sent me this. Hoping for some slack from admins, since it's not strictly XJ related . . .

I had the same issue on one of my '98 XJs. It was flaring the 1st shift. I think it got worse from there. I did ohm check the solenoids and looked at other stuff. Really looked at the manuals etc. I had a neighbor code read it with a scanner from the dealership. He couldn't determine if it was the speed senor or not. But everything else looked good.
Bolt snapped etc. Really a pain. I had to drill out the bolt and epoxy in an insert. It ran OK.
Also, I think you can test them with an ohm meter. I believe it has to be a manual one, not digital. Attach to the leads or connection and rotate the rear wheels. Its a Hall effect sensor. So, an interrupted magnet. So, ON, then OFF repeat.