'93 ENGINE SPEED SENSOR

[ED MEEK]

HAVE A CODE 39,68,81 TRANNY WILL SHIFT THRU ALL GEARS NICE AND FIRM, BUT DOES NOT GO INTO LOCKUP. WHEN IN 4TH GEAR, SOMETIMES IT WILL JUST DOWNSHIFT TO 3RD, AND NOT SHIFT BACK UP. YOU CAN GO TO NEUTRAL,TURN KEY OFF THEN START IT BACK UP AND IT WILL SHIFT FIRMLY UP TO 4TH AGAIN WHILE STILL GOING DOWN THE ROAD, BUT WILL STILL DOWN SHIFT TO 3RD IF YOU ARE UNDER 65 MPH. I WONDER IF IT COULD BE THE ENGINE SPEED SENSOR? THE ONE ON THE BACK OF THE BLOCK. IT DOES HAVE A CRACK IN THE TOP OF IT. NO TRASH IN PAN, AND FLUID LOOKED O.K. ANYONE HAVE AN IDEA?

[JohnC]

First thing I'd check is the power to the trans fuse. The ignition switch controls it separately and it is a common failure mode. when it fails you can get all sorts of shifting problems.

[ED MEEK]

thanks for the reply, J.C.
do you think that it could be an intermittent problem, as it does it worse when truck is cold.

[JohnC]

Originally posted by DIESEL45@HOU:
do you think that it could be an intermittent problem, as it does it worse when truck is cold. Yes. Others haver reported the same thing: showed up first or was worse when cold.