hey i know this has topic has popped up heaps of times though im running out of ideas on mine 1999 cummins converted to right hand drive , i have replaced both ecm and pcm with remans from auto computer exchange, replaced transmission sensors solenoids.. speedo sensor on diff with new 5mm wires overkill i know, up to plug behind left wheel ive had all harness on engine stripped when i was diagnosing ecm issue, just recently was when i changed the pcm hoping that'd be the overdrive issue prior to that i had the switch on the dash mod so i know the transmission itself isn't the issue i spliced into wire for the o/d at the pcm so i know the wiring down from there is ok and i put in a new transmission plug, interestingly enough the key fob is slow to react and doesn't flash indicators and immobilizer not working the over head console no longer works nor does the o/d off switch i probed both terminals at the o/d off switch with earth and nothing happened and dash light does work, my thermostat is stuck open however ive had it up to about 75-80 degrees c with no luck would it need to be warmer than that?  new battery's earths cleaned and run a flap disc over them wrapped some foil round the wire over the alternator, new tps sensor map sensor oil pressure sensor transmission relay,  any idea's?

You've told us all the things you've done to try an fix a problem with the overdrive but not what the problem is in your post..

Just now, IBMobile said:

You've told us all the things you've done to try an fix a problem with the overdrive but not what the problem is in your post..

no overdrive , o/d off button not working , overhead console not working , immobilizer not that im worried about that too much

6 hours ago, ryanhawkins said:

i probed both terminals at the o/d off switch with earth and nothing happened and dash light does work,

You should have earth (ground) when you probe the BL/LG (black/ light green) of the O/D switch.  This wire is connected to earth at the inside left cowl near left hand drive steering column. 

14 minutes ago, IBMobile said:

You should have earth (ground) when you probe the BL/LG (black/ light green) of the O/D switch.  This wire is connected to earth at the inside left cowl near left hand drive steering column. 

Yeah I got a wire on battery earth and pulled out the switch and probed both of the contacts with no od off on dash come up, I did flap disc that earth when I did the others

Do you have a factory service manual with wire diagrams? 

This wasn't converted from factory been using the diagrams posted on here everything is pretty much the same other than wires being extended 

OK,  The O/D switch is a momentary type,  when pushed it switches a transistor relay in the PCM, then the ground for the O/D light is completed and the ground for the O/D solenoid is removed (open circuit) push it again and the ground for the light is removed (open circuit) and the ground for the O/D solenoid is closed.  The light get its power from fuse #14 in the junction block.  If you have good power and grounds for the PCM, the wires from the PCM to the switch, instrument cluster, and trans mission all test good the it has to be the PCM.