Welcome to our Site for MG, Triumph and Austin-Healey Car Information.


MG parts spares and accessories are available for MG T Series (TA, MG TB, MG TC, MG TD, MG TF), Magnette, MGA, Twin cam, MGB, MGBGT, MGC, MGC GT, MG Midget, Sprite and other MG models from British car spares company LBCarCo.

MG MGB Technical - Overdrive Transmission?

You'd think that after having my 80 Limited Edition for about 5 years, I'd know the car thoroughly. In the process of preparing the transmission for removal to replace the front seal, I noticed that I have 2 solenoids on the transmission. According to the Moss catalog, the second solenoid is for the Laycock overdrive and is only fitted to the overdrive transmission, not just a "dummy". I'm used to the early D type overdrive which is quite obvious at a glance. A previous owner replaced the gearshift knob with a cheesy aftermarket and there is no wiring to the gearshift knob which is the location of the switch in that year model. The wiring bundle from the transmission has a pair of disconnected wires under the hood. Do I have an overdrive transmission and hadn't noticed before? I have to pull the engine yet then will get a thorough look, but with the holidays and all, that's likely a week away or so. Just wondering if anyone knows that the second solenoid is a certain sign that it's an overdrive transmission.
Rick Penland

Solenoids or switches?

There is only one solenoid and that is on the overdrive itself. There can be anything from none to three switches on the gearbox on US cars - all but the earliest cars should have a reversing light switch on the right, the OD switch is on the left, and you could have one of two types of switch more central and forwards. One of these was for the seat-belt interlock system, and the other was a short-lived switch for the Transmission Controlled Spark Advance system which allowed OD to be kept on 3rd and 4th. After a few months this switch was ditched and the OD switch changed to operate on 4th only for both OD and TCSA.

The only way you will know if you have an overdrive or not is to look for the large lump between the gearbox casing and the prop-shaft. If there is a tapered tube it doesn't have an overdrive. If you don't have the switched gear knob (which a bit flimsy and can short out anyway) you will have to fit a manual switch elsewhere.
PaulH Solihull


Here is a pic of the overdrive. I don't have a pic of a non-overdrive. Everything under the shifter housing is the overdrive stuff (the large lump Paul speaks of). If yours looks like this, you have an OD.

If you have the plain long tapered tube casting that Paul mentions instead of what is in this pic, then you don't.

If you do have it, that would be a fine Christmas present.


C R Huff

"Switch" is the more correct term. I tend to believe it's not an overdrive transmission, but puzzled by the extra switch. Pulling the transmission is the one dreaded repair that I have to do to the car. A previous owner disconnected the valve cover tube to the charcoal cannister and let it vent into the tunnel. Along with the front transmission oil seal leak, it's an oily mess and I'd like to have my hands clean for the holidays so will wait to pull it after. The second switch is most likely the seatbelt interlock system per your tip. Googling "seatbelt interlock switch" seems to indicate that's what it is. The Haynes manual is very vague about transmission mounted switches and doesn't mention its existence as far as I've been able to find.
Rick Penland

It could be the seatbelt warning lamp and buzzer - gearbox switch. See the workshop manual, page 316.

Thanks to all for your help and thanks for the pic, Charley. That cinches it, it's not an overdrive transmission. I'm familiar with the early overdrive since I had one in a 69 MGC long ago. When I found the second switch, I was puzzled, but had never seen the Laycock style overdrive transmission.
Rick Penland

Oh, well. Too bad Rick. I've always dreamed about buying one from someone who didn't know it had the overdrive, and then discovering it did. It's kind of like those barn find dreams.

C R Huff

This thread was discussed between 22/12/2010 and 26/12/2010

MG MGB Technical index

This thread is from the archives. Join the live MG MGB Technical BBS now