MGB Overdrive


My wife has a 1977 MGB w/ overdrive. The overdrive is activated by the flick of a switch on top of the gearshift. Lately it has been going into overdrive after activating the switch, but it kicks out and causes the car to sputter between the overdrive and 4th gear. It is obviously malfunctioning.

My mechanic says it isn’t the electric switch but a part in the transmission and he says to replace the part, he needs to drop the transmission and do a major job costing almost $3000.00 I have trusted this guy up until now, when I find myself questioning his solution to this problem, and particularly the cost.

What does anyone know about this situation, and is it likely the transmission needs to be torn apart to fix this problem?


The overdrive on the MGB is activated electrically but what it is hydraulic unit. It uses the same oil as the gearbox. The unit has it’s own oil pump to make it work. It is a simple system but one must really know what he is doing in order to fix it. Usually all parts in have to be replaced including the pump nad check valve onece it fails. Actually that is the best way to go with it rather than having to get in there again.

I find that $3000 is a bit high for this. Try to see if you can find a complete unit someplace and just swap it out. It is not that difficult seeing that it is bolted to the rear of the gearbox and the latter does not have to be dissasembled in order to separate them.(I hope I am making sense, I think a lot faster than I write)


The MGB overdive if identical to Jaguar, therefore I know this system reasonably well.

A few comments…

  • The MGB uses a Laycock overdrive unit, this is not integral to the gearbox but is bolted to the rear of the gearbox in place of the tailhousing. The overdrive unit shares the same oil as the gearbox, low gearbox oil can cause the overdrive to intermittently drop offline.

  • The overdrive control circuit is very simple comprising of a switch and the actuation solenoid mounted on the overdrive unit. The solenoid energizes when you flip the switch and pushes an adjustable control rod to actuate the overdrive. You could have a faulty switch, solenoid or the solenoid control rod could be out of adjustment. There was also a recall in 1978 to fix chafing wires from the switch to the solenoid, these shorted on the base of the gearstick causing the overdive to drop offline.

  • finally, the 4th gear & reversing light switches on the gearbox can create problems, these are designed to prevent overdrive engaging when in reverse or any gear other than 4th, if they are faulty those switches will also cause the overdrive to malfunction - note some earlier boxes allowed overdrive to engage in both 3rd & 4th.

To chase throush these problems you really need the workshop manual, but if you plan to try this yourself, let me know and I’ll dig up the troubleshooting process and solenoid adjustment specs.



Scudder is right. I will also add, from direct experience in owning two MGBs (B and GT) that there is also a vacuum operated switch on the intake (near firewall) that can also fail. $3k is way too much. You can get a recon unit and put it in yourself from Moss Motors (check the Internet). Do NOT use it in this condition. The O/D is electro-magnetically activated by both solenoid and electro-magnets. “Jumping” in and out of O/D will cause it grief-- to say the least. Also, check all grounds, etc. Remeber, Lucas, the prince of darkness.




Thanks to all. I suspected as such. Your answers are all greatly appreciated here. I have a service manual but no lift so I usually don’t do the work where a lift is required to fix the problem. I will start with the most simple solutions and work my way through to the top hoping we will get the fix somewhere before the major cost of the worst case scenario.

This is my first visit to the site and it has proved a most satisfying experience. Thanks, again.


73 MGB Reverse Problem

My 73 MGB recently started acting up when I place it is reverse. the car barely creeps in revers or when on even a slight incline will not move backward at all!

The car is equipped with the standard 4 speed transmisson and overdirve. Can anyone give me any ideas where the problem may be or how to fix it? I would really appreacie any help. My email address is;

I live in the Nashville TN area





Can you be more descriptive? Does it go into gear? Is the clutch slipping? Is it making a noise?