If the PCM is commanding O/D, and not seeing the RPM drop it expects, it really should set a code...... That is kinda puzzling.... I have no clue what RPM der diesel would be at in O/D at 55 mph....... don't play with the oil burners much.....

 

Mine did the same thing when I bought it. I traced the problem to the TCC relay not being energized by the computer. Check to see if it's charging because the same wire also turns the field windings in the alternator on or off. The computer was replaced just before I bought it, but still did the same thing. I connected a switch in the dash to energize the relay manually.

 

How do I check that?

 

Start the engine, look at the volt gauge.

 

Could a vacuum issue cause it not to shift properly?

 

On the left side of trans kinda close to the rear cooling line is a pressure tap....that is 4th clutch pressure.....attach a gauge and see if pressure is there when 4th is commanded....no pressure at all means the shift valve did not stroke and this could be a stuck valve faulty solenoid or no voltage.....low pressure means a hydraulic problem..the orifice plug is prone to come out of the fourth clutch piston resulting in no 4th....if you have full pressure then it is a mechanical issue inside the I'd housing....

 

overdrive housing that is....

 

Nope. Trans is not vacuum modulated at all.

 

I found the problem with my transmission. It was the governor sensor and pressure switch. My buddy fixed it and changed the trans fluid and filter for 150.00

 

I changed the relays around and it worked with it not changing gears , now it does.