Overdrive Switch?

[Vince Reynard]

Firstly, hello to you all.

I am interested in buying an MGB GT but the vendor is not the owner and does not think it has overdrive fitted.

However there is a plain switch to the right of the fuel gauge in the normal position.

If not overdrive, then what would this switch be for? Car is '72.

[johnbuckley]

I guess the switch could be for any accessory a previous owner had cared to fit.
What year is it?
Can he look underneath to see O/D unit?

[Dave Linkson]

Turn the ignition on, operate the switch and see and listen to what happens.

Testing:

With the ignition on but engine stopped and the gearlever in 1st, 2nd or reverse turn the manual switch on and off a few times. You should hear the relay click as it operates and releases with the manual switch.
With the manual switch off, select 4th gear, then operate the manual switch. You should not hear the relay click at any time.
Move the gear lever into 1st and you should hear the relay click once as it operates.
Move the gear lever into and out of 4th a couple of times and you should not hear the relay clicking.
Switch the manual switch off and you should hear the relay click once as it releases.

[John Barnacott]

Quote:-
"With the ignition on but engine stopped and the gearlever in 1st, 2nd or reverse turn the manual switch on and off a few times. You should hear the relay click as it operates and releases with the manual switch.
With the manual switch off, select 4th gear, then operate the manual switch. You should not hear the relay click at any time.
Move the gear lever into 1st and you should hear the relay click once as it operates.
Move the gear lever into and out of 4th a couple of times and you should not hear the relay clicking.
Switch the manual switch off and you should hear the relay click once as it releases."

Not necessarily a good test. I've just tried this on my '72 B with functioning o/d and didn't hear solenoid at all (ignition on/engine off)......having said that, I have heard solenoids clicking in and out on some other Bs and Cs. I also seem to remember the solenoid being heard when 3rd and 4th were selected but not in 1st, 2nd or rev. as stated?

As previously mentioned, a visual inspection beneath the car will divulge all, assuming that the car has its original type g/box.

[Dave Linkson]

Sorry about that, it's not a good test, have just tested my working overdrive, and heard no clicking sounds except for the switch clicking.
I shall contact the author of the statement who has a well known and usually reliable and recommended website.

[ROGER COOPER]

I would certainly look UNDER the car to see if you can spot the overdrive unit bolted to the back of the gearbox - it's a fairly obvious lump of aluminium encased machinery with a flat access plate bolted underneath ( this is the magnetic filter which collects any particles of metal circulating in the oil). This will confirm whether the car has an overdrive but will not tell you if it is working.

My overdrive switch is on the right hand side of the dashboard (1974 GT)and has a circular metal label round it with the word "NORMAL" in black at the top (the off position) and "OVERDRIVE" in red underneath. I find the operation of this LAYCOCK unit smooth with upward changes but too abrupt when switching the overdrive out of use so I have to partially de-clutch when switching it out to avoid a jolt. This could be a fault on my unit which needs attention - I just don't know but it is a point worth remembering when test driving a prospective purchase!

Later models of MGB have the overdrive switch built into the top of the gearshift knob.

The overdrive should be inhibited from operation in any other gears than 3rd and 4th. There is a second switch on the gearshift mechanism - not easily visible somewhere on top of the gearbox, which disconnects the overdrive automatically when you downshift from 3rd. to 2nd, 1st and reverse, but will automatically switch the overdrive back in again (provided the switch on the dashboard is still in the "on" position) when you change up again into 3rd and 4th.

It is inadvisable to have the overdrive working on all four gears because the torque reaction in 1st and 2nd is too aggressive and can do damage to the transmission. It can also make reversing extremely scary!!

[Dave Linkson]

The overdrive only works on 3rd & 4th gear, and can't inadvertently engage OD in 1st & 2nd or reverse gears, I have left the OD in by mistake a few times and reversed, albeit not far, and had no problem, that's what the inhibitor's for.
There shouldn't be any need to declutch when going out of OD, just back off the throttle very slightly.