Have been reading the site for years and always learned a tremendous amount by reading old posts. I have a 2000 F350 7.3 4x4. It began giving intermittent tow/haul flashing light, when going up or down hill but would clear and drive next time truck was cycled off/on. No ABS lights and code reader generated P0720 for the output speed sensor, now I can get it to generate just driving slowly fwd and rev a few times. All of this has been in the last few days. Truck will stall if I take it up a slight incline with the fault. I replaced the sensor on the top front of the 4r100 transmission and also replaced the sensor on the rear differential after giving up on looking for an output sensor on the transmission. I know the 2WD transmissions have an output sensor but is there one on the 4WD version? I have read several past threads discussing the P0720. I am hoping to confirm what sensor I need to be looking at and where it is located in this application.

 

It should be on the transfer case.

 

Attached a picture. There isn't a sensor there on mine?

 

A picture of the sensor replaced on top of my rear differential. If this sensor was going through the ABS controller I assumed that I should be getting an ABS fault which I am not. Since I couldn't locate an OSS sensor where I thought it would be I tried this. There was no change in behavior. /// This morning I disconnected the sensor and drove back and forth and was able to generate an ABS fault and code specifically indicating that sensor from the ABS module. Also I was getting the output shaft speed on the trend. Still getting the 720 code and truck will shut off if I try to "gun" it or go up a hill. Still not sure where my OSS sensor is located.

 

Ok just additional info on the diff sensor. I have excerpt from a chiltons book. The sensor on the rear diff is the VSS vehicle speed sensor. Description is in the attached pic and related fault codes P0500 and P0503. Not related to my issue which is OSS Output Shaft Speed with a corresponding P0720 fault code.

 

I just crawled under my 2002. The sensor is mounted on top of the transfer case adapter, between the adapter and floorboard. I tried to take a picture, but hard to get in there. In the picture, you can just see part of the sensor.

 

Went looking with inspection mirror and found a sensor identical to the input speed sensor. It is located on the top of the transmission just aft/behind the end of the pan. I have attached a couple pics, hope they help. I did try to put the old sensor I removed from the front of the transmission into this location but still got the fault. Although the behavior was a bit better. The truck did not shut off. Will get another new sensor at the dealer and if problem remains begin looking at circuit/harness. There was no obvious damage to wiring on the harness to the sensor.

 

Eric thanks for your help. The dealer will have the part this afternoon. I'll install and see where I am at. I appreciate your help.

 

Picked up sensor DY-1229 F81Z-7M101-AA installed and had same issue.

 

You could have a problem with the wiring between the pcm and the output speed sensor. The wiring diagram shows an inline connector C103. I would start looking at that connector. IIRC, that connector is around the air cleaner. The wire color is dark blue with yellow @ pin 3. If that looks good, then pin 59 at the pcm connector should be looked at.

 

Excellent thanks, I was just searching for just this info for tomorrow.

 

Ok. Still have the fault. Disconnected the connector at the OSS and disconnected connector C103. Read with multimeter through the signal line and it was solid .4 ohm which I attribute to the resistance of the wiring I had set up. No change pulling and wiggling the harness. With C103 hooked up I was able to read through the common connection on the connector to the OSS to chassis 0 Ohm no impact from pulling and wiggling wires. I cleaned the C103 connector with contact cleaner and applied a little dielectric grease. I had the batteries disconnected. Reconnected the batteries and started the truck. Got two faults initially, the 1670 and 603. I put the truck in drive and began moving fwd and got the 720 again. I am going to pull C103 again with the connector connected to the OSS and try to take a resistance reading through the sensor. On subsequent restart of the truck the 1670 cleared.
Did note that I was able to get the fault 720 to appear by revving the engine while in park.
I have a question. If I find nothing with the next planned test I intend to take a reading from the connection to the PCM. I have not pulled the PCM before. There are three connectors inside the engine compartment which I assume I pull loose, disconnect battery first, then pull PCM from inside the cab assuming I actually need to pull the PCM, perhaps I can check at the connector inside the engine compartment..

 

acapital, I've been doing some research on your problem also, what I've come up with is that the trans temp sensor also communicates with your speed sensor and if the temp sensor goes bad then symptoms like you're having show up also showing 720 code,

Also I found an old thread linking your problem to bad alternator.

https://www.ford-trucks.com/forums/6...ode-p0720.html

Hope some of this helps.

 

Update checked the resistance of the OSS from C103. It read about 1.1Kohm. Reconnected C103. This resistance is consistent with the now spare sensor I have. While I had the batteries disconnected I decided to check them as I had a batt low voltage earlier. One of the batteries checked good with the load tester. The other battery dropped off badly when load tested. I had a spare battery, slightly smaller, which was used for something else, it tested good so installed it. Started the truck and proceeded to do everything which caused the fault to appear earlier but this time no fault. Up hill down hill revving from standstill, revving in park....no fault. I remember reading someone having an issue with an alternator causing this to generate the fault. I will drive the truck to town today and see how it goes, and get a new battery, maybe both just to be safe.

 

Mike, I think trans temp was something I could read on the diag software. Will try to add it on next check and see what it says.