Hi All,

Awhile back my truck had burned up some wires that caused it not to start or even crank over.....I eventually had to take it to a shop to find the source of the problem (turned out there were 3 wires that had burned up...appears engine vibration eroded the protective wire covers and some wires shorted out).....

Well I got the truck back and noticed that it ran louder than usual....I finally determined the problem to be the truck not going into OD......The shop looked at all the wires and said all were fine as far as continuity.....Now before the wires burned upthe tranny did have OD BUT I did notice that it would still shift into OD when the engine was cold (I do remember reading here that there is supposed to be a switch or something that keeps a tranny from going into OD until the engine warms up....could this be the reason for the no OD now?.....I did drive the truck for alont time so the engne was up to op temp.....Any other tricky little things like a TPS or something?......The first 3 gears work fine.......Thanks in advance for any help.

Andrew

 

Mine goes into OD when cold, didnt know this was a problem, Unfortunalely I dont have an answer to your question, sorry

 

There is a switch located in the Transmission line that says if it isn't about 190 degrees or so, it will not allow the shift into OD. I am not sure of the exact temp on that. But I would either check that switch or check your TPS with a meter to make sure it is getting juice to the TPS and that the settings are correct. I think at least 1.0 volts at idle (motor can be off) and then another 2.75 added when WOT.

 

There are 2 temp switches that are in loop with the overdrive.

Coolant - Open circuit below 65F / Closed above 65F
Trans - Open circuit above 270-275F / Closed under 270-275F

The coolant sensor is under the intake manifold on the side of the engine about where the 2 are on the top of the intake.

The trans sensor is in the 'out' coolant line of the trans.

They are in a loop and if either sensor opens the cicuit is broken and won't let 4th gear (OD) engage.

If the TPS is set correctly, it "supposedly" doesn't allow it to shift at over 70% throttle. I think this one is flexable...because it will shift at full throttle under certain conditions like speed sensor readings showing your at the proper speed. TPS info in the Tech Facts section of this site.

Check the dark gray connector on top of the engine. Make sure it's clean and everythings making a good connection.

In my 91 manual, the wires trace out as follows.

SBEC ( computer) pin 10 - > dark gray connector ( RED/WHT )
-> Trans switch ( RED/WHT)
-> Trans Switch ( OR/BK ) to Coolant switch
-> Coolant switch (OR WHT) -> grey connector
-> grey connector (ORG/WHT) -> inside cab to overdrive module

Pin 55 SBEC (ORG/LG) -> grey connector
-> grey connecotr (OR/LG) -> over drive lockout solenoid ( 2 pin yellow connector on top of trans/drivers side ) -> ODLS (DB) -> common connection ( prolly ground)

Andrew you have a PM

Den