MGB Overdrive

MGB OVERDRIVE SOLENOID ADJUSTMENT

The overdrive in a 1966 MGB Roadster had been playing up for months. Generally it would go in on the first application of the day and then be reluctant to operate again. However, it would always come out alright.

The unit had been replaced two years previously so I didn't anticipate anything drastic. The oil level and wiring were checked all in order. The solenoid appeared to be operating and showing a 2 amp discharge on the ammeter.

MGB overdrive unit Consulting Haynes workshop manual I decided to check and adjust the operating lever. On earlier B overdrive units the solenoid plunger operates a lever which in turn via a linkage operates a valve. The solenoid is on the right hand side of the overdrive unit and just forward of it is the manufacturers name plate mounted vertically. Beneath which, links the operating linkage. The plate disappears up between the overdrive and gearbox tunnel. This is enough to put one off for a start so a quick re-check on the manual yes, it clearly states the adjustments can be made with the overdrive in-situ. So it's back underneath the car and the plate removed (actually remove the two lower 5/16th bolts and swivel the plate out of the way on the top bolt). To check the setting the car should be in third or top with overdrive in. Now the solenoid plunger should be attracted into the solenoid body. The operating lever will now move and a 3/16th diameter rod can be passed through it and into a hole in the overdrive case all is well. Alas, all was not well and the holes were out of line, but only by a small amount. The book says hold the solenoid plunger with a spanner and 'tweak' the stiff nut on the other end to afford adjustment. I found this impossible because access is so bad. (Small right hand spanners might do the trick.) I opted to remove the solenoid, then clean and test it to see if matters improved. This I did but the overdrive fault persisted.

By now I was sure that the small amount of adjustment needed to align the holes in the operating lever and case would have to be done. To gain access to the linkage I calculated the position on the inside of the gearbox tunnel and cut out a rather small eliptical hole. Hey Presto! There was the linkage right before our eyes. Tweak, tweak, adjusted, checked with the 3/16th rod, overdrive selected in, yes, a further dozen or so times, yes, everytime. To complete the job an access panel was self-tapped over the access hole.

So to conclude the operating linkage set up on early B overdrives is very critical. This particular unit was less than a quarter of a hole out and would not operate every time until perfectly set up.