My legacy has recently began losing 3rd and 4th gears after driving beyond 20 miles or so. The ATF is good.  I believe the problem is electronic and replaced the vehicle speed sensor (top passenger side of AT).  The code I retrieve is for the output speed sensor circuit. (I am beginning to see my solution as I type) Has anyone else experienced this problem?  I am hoping that it is a simple wire harness fix or sensor swap.  Are all Gen 1 sensors interchangeable in the AT?  (i.e. can I swap a sensor from a '99 AT to my '97?)  

 

 (i.e. can I swap a sensor from a '99 AT to my '97?)

this is a yes.

but i don't understand how you came to this conclusion.

 

 

The code I retrieve is for the output speed sensor circuit.

what was the code Pxxxx number?

 

do you have a flashing AT Temp light at start up?

this would indicate an electronic problem in the trans.

 

if no. then look else where, like TPS.

the code retrieved was P0720-  "output speed sensor circuit."

 

The AT Temp light only flashes after I attempt to retrieve AT codes through "secret handshake" method.  (drive, depress brake,put car into first gear, turn off ignition, turn on ignition, put transmission into 2nd back down to 1st the up through drive. it does not give me codes or anything at that time; but after sitting when I return to the car it flashes rapidly...I have read that this is a power supply or grounding issue code.)

 

By TPS you mean throttle position sensor?  

 

The AT Temp light only flashes after I attempt to retrieve AT codes through "secret handshake" method.

i don't think the handshake works on obd2 cars.

you have to gorund out a pin on a diagnostic connector under the dash.

 

 

 

it does not give me codes or anything at that time; but after

sitting when I return to the car it flashes rapidly...I have read that

this is a power supply or grounding issue code.)

 

drive the car as best you can, and then stop and  restart it.

if the AT Temp light flashes 16 times,

it means there is a trouble code stored in the TCU.

 

the flashing indicates there was an electrical fault in the trans the LAST time it was driven.

Thanks for clarifying about the handshake method and OBD2-- I had been wondering.  Typically, it the AT Temp light does not flash when the vehicle is started.  But if it is not immediatly on the CEL will light up after a couple miles. The returned code is P0720. 

So I just checked the TPS according to the Haynes by backprobing the harness with ignition on.  V-meter showed voltage but it was unchanged when the throttle was moved.  It did make a humming noise ,of which I could not positively identify the source, as the throttle was moved. Is it common for a TPS to fail and cause this problem?  

It is confirmed that the solution to my transmission problem was in fact the TPS,  After installing one from a junkyard for $17.50 ($99.50 for a new one) the car has no shifting problems or dropping gears.  Now to catch up on the rent because of lost economic opportunity.. Thanks Johnceggleston and USMB!!! I really don't think the solution would have occurred to me without your input!

Can't express my regret that my previous post was incorrect.  The used TPS TEMPORARILY FIXED the problem!!! What is going on with this crazy car?  It must be electronic in nature.  I think I am just going to go through all the connectors and detach/re-attach them because this intermittent problem with the trans crept up again after about 50 miles of perfect driving/shifting.  Although this time it was giving me an AT trouble code the first couple times I drove it.(?)  Also I will look for this illustrations which show the AT trouble code connector cause I could not figure out which one it was.

I have attempted to locate the connector under the dash for the diagnostics described as "black" but cannot find it.  Am I looking for a connector with nothing connected to it or will it be connected to more wires?  I am using this diagram: