I can see this has been covered a lot, but my problem seems to be different to most i can find
My 96 2500 with 5.9L magnum and 46RE wont shift into overdrive, i do get the TC lockup and 200ish rpm drop at OD speed but it stays in 3rd
with the OD off switch off it stays in 3rd but when coasting revs drop to idle, and when you put your foot down again it picks backup at normal 3rd gear rpm
with the OD off switch on it stays in 3rd, no TC lockup and engine RPM as normal
have checked/cleaned OD/TCC solenoid and was about to pull OD unit off and disassemble but thought it ask first as the common problems (broken snap ring/spring) dont seem to give my symptoms?
trans operates perfectly otherwise, other than not starting in P sometimes and have to move to N to start
any help greatly appreciated. thanks

 

Are you getting any codes? Does the light in the switch operate properly? May want to check for pending codes as well..... (need a good scanner for that though....)

 

hey, no codes on my basic/cheap ODB scanner, do i need to look for a specific scanner type?
OD off switch light seems to operate correctly, and does switch TC lockup correctly etc
thanks

 

In that case, may want to plug in with a better scanner, and see what PCM thinks trans temp is. If it isn't setting a code, that implies it thinks there is something else going on, that it shouldn't shift into O/D...... basically, the only things it pays attention to are vehicle speed, engine load, and trans temp.... If the PCM were actually requesting O/D, and then not getting it, it would whine about that......

 

Could a failing PCM cause this? Maybe it is constantly grounding the OD which turns it off.

 

thanks, that makes sense. is there any way of testing if the PCM is trying to trigger the OD solenoid? its obviously triggering the TC lockup, is that similar criteria? thanks so much for your help. i currently have the valve body out and was trying to decide whether to put it back together for further testing or keep going and pull the OD and disassemble/check it. thanks again

 

I don't know if O/D request is a parameter you can read....... You can install a test light in the circuit for the O/D solenoid, to see if the PCM is actually triggering it, but, if it were, and it didn't see the RPM drop it expected, it would set a code right quick. So, I really suspect that it simply ISN'T requesting O/D...... Since the light in the switch operates as intended, I don't think the switch is the issue. Gotta be something else going on.

 

A high end scanner will show the command. What are you thinking? That the PCM thinks the temp is to high?

 

Yeah, or maybe output speed isn't reading right, and is too low for O/D.... or something equally as fun.

 

ok thanks, ill put it back together and use the test light to make sure its not being triggered, would it be a pulse or on while in overdrive?
that should confirm i should be looking outside the trans, then will look at trans temp and output speed etc
thanks again guys