Speed Sensor Question

[tomeeg]

I have a 97 Explorer with Control Trac and have a question about the speed sensor. After reading countless posts, I'm wondering if this sounds like a speed sensor issue.

Since I bought the vehicle (about 2 years and 20k miles ago, just over 100k now), the speedometer has read fast, about 10 MPH over at highway speed, less in town.

Now, I'm having intermittent problems with the 4WD staying engaged and getting the 6 flashes of the 4WD lights.

Also, intermittently, the O/D light comes and stays on.

Ignoring the speedometer issue, I did first clean the contacts on the shift motor thinking it was a shift motor issue and it seems to be shifting in and out of 4Lo, but sometimes sticks in 4 HI with the flashing light issue. However, factoring in the speedometer issue, does it make sense that the speed sensor is bad??? Also, is there 1 or multiple speed sensors. Different posts I read had different information.

Thanks.

Tom

 

[tomeeg]

Well, I replaced the speed sensor today and things seemed fine at first with the truck engaging normally into 4HI and 4LO, but drove the thing for about half a mile and started getting the 6 blinking 4WD lights. Any guesses on where to turn now.

As for the speedometer, it still seems to be off the same amount. I did test the resistance on the speed sensor I removed and it was at 1124 ohms, so much higher than specifications which I believe should be in the 150-250 ohm range, so I thought this might be throwing the speed off, but that doesn't appear to be the case either. So, I'm still at square one on the issue. Do I need to reset codes or does the computer take some time to recognize and calibrate to the new speed sensor?

Thanks.

Tom

 

[jp450]

I dont know if things changed much from 97 to 99 with the speedo sensors. I dont think they did. If they are the same then you actually have 3 sensors. One in each front wheel hub and one on the rear end. The info is sent to the ABS module first then to the PCM then to the speedo... Anyways I dont think your problems are related. The speedo calibration is done in the PCM. It is set with the computer based on your tire size at the fdactory or dealer.

 

[410Fortune]

woah wait a second.

There are two speed sensors in your 97-99 truck
the VSS and OSS.

The OSS or outputshaft speed sensor is lcoated inside the transmission

the VSS or vehicle.... is lcoate din the rear diff.

The sensors on your front wheels are ABS sensors, not speed sensors.

The dealer will re-calibrate your PCM to match your gear ratio and tire size.
This is important to propoer vehicle funtion but I dont think it has anything to do with your flashing 4WD light.......

 

[tomeeg]

I checked the codes and had a "P1729 4x4 Low Switch Error". Since the switch seems to work fine switching from 4Lo to 4Hi, does this seem to be a PCM problem? I couldn't find much info on the P1729 code on the site.

Thanks.

 

[jp450]

The following has been taken from Alldata DIY:

"SENSOR, VEHICLE SPEED SENSOR (VSS)
The Anti-Lock Brake System (ABS) provides the GEM with vehicle speed information."

"SENSOR, FRONT AND REAR OUTPUT SHAFT SPEED (OSS)
The externally mounted OSS sensors are located at the rear of the transfer case. The sensors provide the GEM with front and rear driveshaft speeds to control and adjust the transfer case clutch duty cycle.


Also when studying the wiring diagram it would appear the the ABS system reports the speed to PCM/GEM.

So when I disconect the ABS module then why does my speedo stop working?

 

[jp450]

Tomeeg:

I was able to locate a TSB about your condition. I have cut ans pasted it here for you.

"ISSUE
The 4X4 indicator lamps may flash on some vehicles. This may be due to oxidation buildup and/or premature wear on the sense plate of the shift motor which can send incorrect signals to the 4X4 controller. Terminals on the transfer case connector may also develop a resistive film, leading to faulty signals to the 4X4 controller.

ACTION
Install a new shift motor and/or speed sensors with new terminal material. Refer to the following Service Procedure for details.

PART NUMBER PART NAME

XL2Z-7F293-BA Front Speed Sensor
XL2Z-7A786-AC Jumper Wiring Harness
xL2Z-7F293-AA Rear Speed Sensor
XL2Z-7G360-M Transfer Case Shift Motor"

I would inspect the both speed sensors and the transfer case shift motor connections for corosion. Also as indicated the sense plate may be worn out on the shift motor. In this case then you may need to replace it.

Jeff

 

[tomeeg]

Jeff:

Thanks for the info. I will probably start with the shift motor. I did previously clean the connectors on it, but I'll try removing it and giving it a thorough cleaning and inspection as detailed in other threads.

Tom

 

[tomeeg]

Well, I replaced the shift motor and that wasn't the problem. It does shift a lot smoother though with the new motor (which I got at Rockauto.com for about $55). I tested the speed sensor on the transfer case and found it to be at 224 ohms which is within range. I'm at a loss where to look now.

 

[tomeeg]

Well, as a last resort, I changed the transaxle fluid and everything seems to be working fine. I've shifted in and out of 4wd lo and hi several times and no more flashing 4wd, so things are back to normal. Thanks to all who responded.

Tom

 

[jaymir]

I was wondering if anyone had a reference I could see to how I could access this TCase OSS referenced in this post? I actually have a 98' explorer XLT 4.0L v6 4x4 and purchased the OSS and need to figure out how to gain access to it to change it out. Thx!