MGB Overdrive

[MarkD1961]

Hi, I have a 1965 Mk1 MGB. The overdrive appears not to be a Laycock type, from what I understand it is a "D Type". The OD will operate in all four forward gears. My questios are; does this type of OD usually have a lock out switch for low gears? If inadvertently left in OD while reversing will it damage the unit similar to the Laycock? Thanks.

[George Wilder]

There was only ever one type of overdrive originally fitted to the 3 syncro gearbox of a Mk1 MGB.

Some MK1 cars have had 4 syncro boxes fitted by owners for a number of reasons which has a different overdrive unit.

In both cases the overdrives were by Laycock though quite different models

Which gearbox is in your car?

Both types of overdrive have a lock so that they can only be used in 3rd and 4th gears - see parts books for details.

[Vic Butler]

I believe early cars had a vacuum device which prevented the overdrive disengaging on full throttle when switched off. My 1967 one had it.
The isolator switch isn't easy to access. It's on top of the gearbox on the left hand side.
Reversing with the overdrive engaged will destroy the uni directional clutch in the overdrive. Also it may not be able to withstand the torque in the 2 lower gears.

[MarkD1961]

Mine is a 1965 MGB with 4 speed gear box. 2-4 are synchromesh. The O/D definitely operates in 2-4, not sure about 1st. So if I left the switch on by mistake would I smash the O/D in R? The manual does not show an inhibit switch for gears other than 3rd and 4th.

[Vic Butler]

All MGB overdrives had the inhibitor switch so only 3 & 4 had overdrive.
As I said in my previous post: DO NOT REVERSE WITH OVERDRIVE ENGAGED. You'll destroy it.
There is a vacuum operated switch so the overdrive disengages only on a trailing throttle to prevent a thump on disengagement. Only 3 synchro boxes had it fitted. My previous 67 3 synchro BGT had it.
Edit. I've checked my genuine BMC issue parts list and the overdrive vacuum switch, BHA4282 was fitted to the 3 synchro B's. GHD3 & GHN3.
I don't know what would happen if the vacuum switch failed. Would it have a failsafe where it would not allow the overdrive to engage or would it fail leaving the overdrive permanently engaged irrespective of the dashboard switch position?

[Peter Cresswell]

Firstly all MGBs used a Laycock Overdrive. Early 3 synchro cars used the type D and later 4 synchro cars used the type LH. The main distinguishing difference is the type has an external solenoid and the type LH has an internal solenoid. For both types the isolation switch is located on the nearside of the gear lever remoted control housing, which is bolted to the top of the gearbox and this switch prevents the overdrive being activated in 1st, 2nd and reverse gear. It points forwards on the 3 synchro box and rearwards on the 4 synchro box. I always thought the overdrive only worked of 3rd and 4th gears because of the torque of the B-Series engine, but interestingly, the Triumph TRs used the same overdrive units but it worked on 2nd, 3rd and 4th gears and the TR engine develops more torque, so I now think it is more to do with the location of reverse gear also being on left of the gate. On the TRs reverse is to the right of 3rd and 4th gears and you have to lift the gear leaver to engage reverse. There is no similar protection for engaging reverse on the MGB.
Turning now to the isolation switch failing. If it has failed because the operating plunger is stuck up inside the switch the overdrive will be operating all the time, so in theory it will work on all gears including reverse. The plunger can be stuck because it is rusted up or has become bent for some reason. What ever it is it needs investigating urgently as you will (as Vic says) destroy the Overdrive unit if try to reverse with the overdrive engaged. To change the switch is not easy because of where it located on top of the gearbox but there note on Brown and Gammons website might be useful guidance. See https://www.ukmgparts.com/technical-tips, and go down the page until you find 'MGB - MGB GT – Overdrive problems'.
It is worth remembering that you can't just remove the gearbox and overdrive as a unit on the MGB. This is because the overdrive is too big to fit over the fixed crossmember. You have to remove the engine and gearbox as a unit.

[MarkD1961]

I’m still a bit confused, the overdrive switch mounted on the dash works fine it’s just that the overdrive operates in gears 2,3 and 4. Not sure about first. As above mine is a D type. Does this mean the switch which inhibits o/d in reverse is bypassed? Is there an easy test without trying to reverse and blowing the o/d. By the way I think at some point I did reverse with the o/d switch on with no damage. Very confusing.

[Vic Butler]

Mark, when you disengage the overdrive do you have to lift off the throttle? You should do to operate the vacuum switch. On my previous MGB GT which had a vacuum switch the overdrive wouldn't disengage on full throttle even though I had switched it off with the dashboard switch until I lifted off the throttle.
You couldn't have reversed with the overdrive engaged. It has a unidirectional nu clutch and reversing destroys that clutch and the overdrive then spins in its housing and there's no drive
There is no reverse gear overdrive inhibitor switch as such. As reverse is the same side of the gate as first and second then the overdrive cuts out when the gearlever moves to that side because of the inhibitor switch for first and second and of course reverse irrespective of the dashboard switch. It re engages when third is selected if not cancelled by the dashboard switch.
Check whether the overdrive operates in first. Accelerate slowly then switch the overdrive in. If it does then the inhibitor switch is not working and there is the danger of reversing with the overdrive engaged. It could be the switch or the wiring to it. It's not an easy thing to check as access is very limited.