I just bought a 1993 w250 with the a518. the overdrive does not work on it the switch was replaced and the pcm as well. I went under the truck and the plug was out so i plugged it in still nothing the od on off switch does not light up. I want the od to work like it did from the factory no hard wired switch stuff. any ideas were i should look next?

 

Check the plug to see if you are getting 12V with the key on. one side of the plug should always be hot, the PCM switches ground to activate the OD solenoid. TPS adjustment is important for OD, but the fact that it was unplugged may mean it has a short in the solenoid.

 

Wire in the switch on the OD to validate the solenoid and circuit is good.

Check and make sure the momentary switch on the dash is plugged in correctly and has power.

 

Does everything else work like the cruise and the speedo? If they work then look in the sticky and read how to test the tps and make sure it is adjusted and working properly.