[Sandflea427SS]

Hello all
Sorry I can't ask Hotrod questions all the time, but I have this dang late model a girl just brought me. 1996 Ford Explorer V-8/automatic.
The speedometer needle jumps around and the transmission jumps from drive to overdrive without finding a happy place. This happens at speeds where is should upshift to OD and it just doesn't stay there. Someone around town seems to think it's a speed sensor(bolted to the transfer case). Others think it's a computer problem, wiring harness, burnt wire, or wire with insulation rubbed off. The speed sensor is $28.
Just asking in case anyone out there has been here before
Thanks much....
Jeff

[DeathRattle]

Is the check engine light on(throwing a code)?

[sbchevfreak]

Likely the speed sensor, as I have seen many of these come through the door. Sometimes the plastic gear on the drive end of the sensor gets concaved, and does not have good mesh with the drive gear on the t-case output shaft. Result is bouncing speedo, and erratic/harsh/spongy shifts.


EDIT: Having the PCM scanned for codes is a great way to pinpoint the system(s) you may need to look at, and should be done before swapping any parts. Have this done at a dealership, as Autozone/NAPA/Carquest/Schucks, ect., will only have generic "code readers", which are about as useful for proper diagnosis as boobs on a rock. The OEM scan tool will allow them/you to monitor the induvidual output indicated by the DTC.

[Chazmac]

Erratic speedo = speed sensor problem = shuttle shift, esp at higher speeds.
Check the speed sensor WIRING too.
This vehicle may have a speed sensor on the top of the differential. If so, replace that one first; double check the wiring.
ANY corrosion in the connectors will cause weak signal; the corrosion also moves up the wires (like a wick!)

THe best diagnosis will require a scanner to view the speed sensor signal...

[bates_k]

If this is happening between 35 and 45mph, turn
the OD off, or shift it to D, it is hunting because
the engine isn't in its happy rpm range...

K

[Crosley]

check ( quality scanner) speed sensor input or replace the speed sensor......... If the computer thinks the vehicle is slowing down ( erratic speed-O) the computer will down shift the trans.


Quality scanner is one that can read a live data stream as Chazmac has pointed out

[Sandflea427SS]

Thanks for the ideas everybody. Last week I was in an accident so I'm on crutches, so it's hard to work under the vehicle, but my buddy and I found the wiring harness pulled tight under the truck with exposed wiring on the (4 wheel drive motor)? on the transfer case. Isolating those wires didn't fix anything so he said there's an AutoZone by his house and we're going to scan the computer and go after what the code says. We really tried to pinpoint it down to the speed sensor... thanks again.