I got my new (to me) W250 about 3 weeks ago and drove it back from Texas to California without any problems, doing about 80 - 86 MPH the whole time.

I installed some Isspro gauges a few days ago - and so I was under the dash screwing around with the wiring a bit.

Got the truck loaded up and ready for the 1000 mile trip to New Mexico and once I got out on the freeway I realized I couldn't get it out of third. Hit the O/D OFF switch - the light in the switch still works, but having the switch on or off didn't make a bit of difference in the trans speed or MPH. Got under the dash to inspect, but couldn't find anything wrong. Checked all the fuses, none were blown. (There didn't seem to be a fuse for the overdrive, anyway) So, I ended up doing 50 to 60 MPH the whole trip, keeping my eye on the newly installed EGT gauge the whole time. But what a pain!

Has anybody had this problem before? Does anybody know if the trans needs to see a VOLTAGE or if it needs to see NO VOLTAGE in order to be able to go into OD? What does the O/D OFF switch directly control? A solenoid? If it is a solenoid, anybody know where that solenoid is? ANY information on this subject will be greatly appreciated!

As far as I understand, this is the A518 transmission.

Thanks!

 

Check the switch on the brake. When you touch the brake, the OD kicks out. You may have bumped it out of adjustment when you were under the dash.
Chris