MG-Cars.net

Welcome to our resource for MG Car Information.

Recommendations

Parts

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 in all gears!

My 67 roadster began life as a non-overdrive car and was retro-fitted by the previous owner's garage some time during his 21 year ownership. By accident, I have recently discovered that the overdrive operates in all four forward gears - naturally I have not tried reverse! Can anyone shed any light on why this may be? Have they put it in wrongly, or left something off? Will it do any part of the car or overdrive harm if I accidentally leave it engaged in 1st or 2nd?
Thanks for your help.
Martin

Yo, Martin. It is critical that you do not go into reverse gear with the OD on. Your tranny is guaranteed to disintegrate if you do. There should be a switch (that is probably bypassed or non-functioning) that will allow use of OD only in the top two gears.

Please look into this right away. If you accidently leave the OD on and hit reverse, nasty consequences will occur.

Luigi
Luigi

No harm in using it in forward gears, the part that is missing is a 3-4 lockout switch, it looks just like the reverse light switch and mounts on the shifter extension. hard to get to with the tranny in the car.
Jake

Thanks Jake - I figured it was something like that so I'll leave as is until I have cause to take the transmission out for other reasons. Luigi - , unless I'm mistaken, even if it had the lock out switch in place it still wouldn't stop it from destroying the whole thing if I reverse with overdrive switched on, but in the years I've owned MGs I've always taken great care to disengage and double check before backing up.
Thanks for the advice...
Martin

The switch uses a plunger contact to cut the power to the overdrive when the gearstick is moved to any gear to the left of third/fourth, reverse included. If the switch is there, the plunger my just be stuck.
Steve Postins

Martin, Look under my posting on the General board about overdrive and reverse. Ray
RAY

The overdrive can be damaged by excessive amounts of torque, which can wear out the clutch in the O/D and cause it to slip. Uprated overdrive components have been used in the past to get O/D 2nd (it was standard on the TR6) but I wouldn't reccomend using the O/D in anything under 3rd gear.
Dave

I had a failed interlock switch once. o/d 2nd is a great gear , esp on the 3 synchro box with the gap between 2nd and 3rd . I was careful not to use full throttle in o/d 2nd , and not engage it in reverse , until I could fit the new switch.
S Best

Martin

Just been chatting to another member at this month's meeting of MGOC Oxford branch who had EXACTLY the same problem. Turns out a PO had gearbox and overdrive rebuilt but got the wiring for the lockout (inhibitor) switch mixed up with the reverse light switch - even changing some of the spade connectors which didn't match and stretching the solenoid lead to its limit to make it reach! Result was both switches were effectively passing current all the time - i.e. the O/D could be switched in and out in any gear, including reverse, and the reversing lights were on all the time as long as ignition was switched on (how often do we turn ignition on, engage reverse and get out to see if the reversing lights are working correctly?).

My R/B factory V8 has a version of the same problem - gearbox & O/D rebuilt for a PO in 1994 after which O/D now operates on third and fourth gears whereas it should only work on FOURTH gear on a V8 (except for a few very early cars). I've yet to work out what's wrong with mine as the inhibitor switch must work in a different way to that on a 4 cyl car. Meantime I only use O/D third when cruising around in 30 or 40 mph zones as I can leave the car in third and just use the column mounted O/D switch and thus saving a lot of clutch operating!

BTW Martin, look out for my car in Newbury as my daughter and her husband live near to "The London Apprentice" pub and I visit from Grove at least once a month.

Laurie Webb
75 factory V8 Teal Blue
L Webb

Laurie
The problem you describe ie. overdrive in 3rd and 4th while you should only have it in 4th can mean that the shifter extension on your gearbox was swapped with an earlier unit. the later shifter extensions has a small piece cut off on the part that moves the plunger in the lockout switch to make contact. all you need to do is replace the shifter extension with an earlier unit, I believe all MGBs with 4 synchro transmissions, v8 included, used the same extensions.
Jake

Laurie - funny the efforts some people go to in order to get something so wrong! I think I'm safe from that point of view as my reverse lights are wired up to a toggle switch floating under the dash (those damn previous owners again!) - and to be honest with you, I have never used them anyway!
With regards overdrive, I rarely use on 3rd gear - mostly just on 4th as a 5th gear. Having just had the engine rebuilt and head overhauled, I tend to just enjoy the bags of torque in each gear I now have available and use overdrive as an aid to cruising up the Motorway/dual carriageways!
I'll keep an eye out next time I'm passing the London Apprentice ... mines a 1967 Racing Green roadster with a tan roof - black hardtop instead if I run out of space in the garage to store it!
Martin
Martin

Martin - you say "if it had the lock out switch in place it still wouldn't stop it from destroying the whole thing if I reverse with overdrive switched on". The whole purpose of the lockout switch *is* to prevent damage to the OD if you reverse with the manual switch on.

UK 4-cylinder cars always had OD on 3rd and 4th, it was only later US spec cars that were restricted to 4th only. OD in 1st and 2nd *can* cause damage, from excessive torque reversals in the lower gears, which is why V8s were restricted to 4th only very near the start of production, hence different extensions at different times.

Paul Hunt

Jake, Paul

Thanks for suggesting the probable reason why I have O/D in 3rd AND 4th on my V8. The "wrong" remote being fitted is very likely as the 1994 PO bought a reconditioned gearbox and then paid a garage to fit it - reusing the original O/D unit. The rebuilt box could have either been a very early one (I'll have to find the serial number) or maybe just had the remote from a 4 cyl car fitted because they had a good one lying around!

Martin

Is it possible the dashboard switch could have been added to correct the problem of "always on" reverse lamps which I detailed in my previous posting? Is the O/D a "D" or "LH" type and is it fitted to a 3 or 4 synchro box? Lots of questions and having looked at both my MGB manuals there are, as I see it, two or three possible answers for non-operation of the inhibitor switch.

Laurie Webb
L Webb

So, is there something wrong with my O/D in that it will only engage in 4th gear. It works great, but if you try to turn it on in 3rd gear, nothings happens.
Just wondering?
Robert Browning

Try adjusting the lockout/inhibitor switch. It uses fibre spacer washers and is adjusted in the same way as the reverse lamp switch.

Laurie Webb
L Webb

Robert - like I say late North American cars (from late 76) did only have OD on 4th gear as the gearbox switch was also used to control vacuum advance. As this was limited to 4th only the OD had to be similarly restricted. Normally if the lockout switch is worn it affects 4th gear first, and can sometimes be made to switch in and out by pulling the lever about whilst in gear.

AFAIK the D-type OD can only be fitted to the earlier 3-synchro box, which was phased out in 67 for the MkII, so Martin could have either. But as the lockout switch is in the same position in the circuit on both D and LH systems it shouldn't make any difference ... PO garage wiring frolics notwithstanding.

Paul Hunt

Hi Folks:
I seem to remember that you can change or add the lock-out switch to a mk1 MGB. Remove chrome ring, gear lever, carpet and screws holding the cover in place. Suggest you fix it now, since Murphy's law is always with us.

Good Luck: Rich Boris 67B roadster
Rich Boris

Martin - Two things, first it is not all that difficult to change out the 3rd/4th lockout switch in situ. As Rich suggests, by removing the entire cover over the gearbox, you gain almost unrestricted access to the top of the gearbox. Second, one time of very careful trying out reverse with the O/D engauged will not destroy the unit. The reverse lockout switch on our car was misadjusted (by the stupid current owner) and I put the car in reverse after having forgotten to disengauge the O/D switch. Upon release of the clutch, I got a ratchet type noise from the gearbox and the car stood still. I shoved the clutch in very quickly, realizing what had happened, disengauged the O/D and drove home fearing the worst. Out on the freeway, I tentively engauged the O/D and happly found that it was working fine. Needles to say, I fixed the problem immediately and have been using that O/D unit ever since (about 15 years). My point is that you might want to try reversing gently with the O/D engauged to see if the reverse lockout switch is working and replace it if it isn't. Cheers - Dave
David DuBois

No need to risk your o/d by engaging it in reverse . Just measure the current flowing in the solenoid circuit . You do not even need to get under the car to do this . If you do not have a meter that can handle the current a light bulb will work. This is best done without the engine running . Another check is just to listen for the solenoid clicking in as you engage reverse. Engine off ,ign on o/d switched on, as you move the gear lever around you will hear the solenoid go in .Careful you do not burn out the ign coil of course .
S Best

Only the D-type solenoid clicks, but that also has a relay which clicks, so you have to differentiate between the two. If you think your gearbox switch is permanently closed checking for current flow is definitely much better than driving in reverse to see what happens. This can be done by connecting a test-lamp, voltmeter (a voltmeter will register 12v if the solenoid circuit is completed by the lockout switch) or ammeter across the contacts of the manual switch, which should be in the off position, with the ignition on.

David seems to have been very lucky, especially as it made expensive noises. Someone else may not be so lucky. Even when the solenoid is operated the OD ratio will only selected when sufficient hydraulic pressure has built up to move the clutch against return spring pressure, and that only happens once you have started moving. Judging by the number of times people recommend using the clutch or feathering the throttle (both of which are specifically stated as *not* being required in the handbook) to soften the shock of engagement I would imagine that once sufficient oil pressure has built to start moving the clutch it does so pretty quickly.
Paul Hunt

Hi Folks:

I think the hydraulic pressure on the "D" type is always present, unlike the "LH" type where the solenoid when engaged allows the pump to pressurize the system. The solenoid on the "D" type shunts the existing hydraulic pressure to the cone clutch pistons.

Just info: Rich Boris 67 B roadster
Rich Boris

Although the solenoid mechanical arrangements are different, which is why the D-type clicks but the LH-type doesn't, from the solenoid-operated hydraulic valve onwards the two are evry similar. In both types the pump is pumping all the time the gearbox is turning, whether in OD or not. In direct drive a valve is open allowing all the flow to be used for lubrication, and any excess above that is dumped back in the 'sump'. With OD engaged that valve is closed allowing the pressure to rise and be applied to the clutch pistons, then a high-pressure valve opens allowing the excess to be used for lubrication, and the excess after *that* to be dumped via the low-pressure valve.
Paul Hunt

This thread was discussed between 05/01/2005 and 11/01/2005

MG MGB Technical index

This thread is from the archive. The Live MG MGB Technical BBS is active now.