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On two occasions I had the rear speed sensor replaced.  The 1st time both the check engine and ABS lights came on.  The repair shop replaced the sensor at the differential.  About 1 year later the same symptoms occurred.  These included an erratic speedometer and going into and out of gear.  However, this time the check engine light did not come on.  Another mechanic again replaced the differential speed sensor, plus changed the fluid as it contained metal filings.  The problem initially was resolved but within days the same symptoms occurred.  The mechanic attempted to locate the cause by driving the vehicle for a long while without the problem happening again.  I have been driving the vehicle for several weeks.  The problem recurred several times for a short period of time, but quickly resolved itself.  To date the check engine light has not come on.  Any idea what the problem could be?  Thanks for your advice.  Charlie

you probably have green cheese growing in the wiring harness, what year are are we dealing with

if there is a connector underneath the drivers seat take it apart from under the vehicle, 10mm socket

Thank you for your reply.  The vehicle is a '99 Ford Ranger - 6 Cyl. - 3.0 L engine and has an automatic transmission.  I will attempt to locate the connector you speak of, and see if there is a corrosion problem.  Thanks for your courtesy.

Hi. I just came across your post and was wondering if your problem ever got resolved. And if it did, what was the final fix? I have a 2000 Ranger XLT 3.0 V6 and I'm having similar problems. At first my speedometer just plain quit working and I received an ABS light along with the check engine light. I replaced the speed sensor on the differential and now all the lights are gone and the P0500 code is gone but my speedometer begins getting erratic when I reach speeds of 60+ MPH. Also the transmission begins to shift in and out of gear, sort of down shifting and up shifting. The problem only occurs at 60+ MPH.

Hi Bob,

 I'm having the exact same problem as you - speedo quit, lights came on, etc.  I replaced the ensor on the differential and everything is fine unless I drive over 65 mph, then I get the downshifting, etc.  Did you ever figure out what was causing your problem?  Any help would be appreciated!