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MG MGB Technical - Overdrive

How do I determine if my 71 MGB has Overdrive or not? My owners manual tells me that there is a switch on the steering-column that you "move the switch lever towards the steering wheel to engage" and "away from the steering wheel" to disengage". I have a switch (it also operates the wipers)but it moves towards the front of the car and then back. Is there any other way I can tell if this model has overdrive? Is there a ID number, special drain plug on the transmission? Will the tachometer register less rpm's when the overdrive is engaged?

If the car does not have overdrive how does the transmission get fluid? I understand you must top it off two or three times a year, but is there a way (or an necessity) to change the fluid?

Thanks for all the help I've received on this web site.
r seaman

Yes if o/d is fitted , working and engaged you will show a drop of about 20% in rpm . The OD just uses the 20W50 in the gearbox , but I have never owned a 4 sychro one although I think the filer is on the side .
S Best

If you look underneath, the overdrive is a big lump at the rear of the gearbox with a small oblong sump on the bottom with six bolt heads (if it's an LH).
Non O/D gearboxes are smoothly tapered at the rear.
Dave
Dave Wellings

I have overdrive in my 75 mgb. Do not engage od and go in reverse. Damage could result. While driving move lever into overdrive and vehicle should reduce in rpm.... There is a single wire at the top of the overdrive that can get disconnected if you have one and it doesn't work. Crawl under vehicle , reach over top of od and make sure wire is connected. I install a yellow dash light to let me know that od is turned on so I would not go into reverse.
Ronald Milostan

Ronald, if the OD is wired correctly it won't be in OD when in reverse or 1 or 2, even if the switch is in OD position. Different people wire them so that they may use OD in 3 & 4, or just 4.
FRM
http://www.usachoice.net/gofanu
FR Millmore

FRM,
My 1980 original drivers manual states not to reverse the gearbox with Overdrive in. Did they change something because it's been a worry of mine for awhile and I have trained myself to look at the switch before putting it in reverse

Cec
Cec

All I know is every car with the Laycock overdrive I ever encountered had lockout switches to keep it out of OD regardless of driver error. It's just too easy to forget, or know what the last guy did, so I can't imagine the factory leaving the interlocks off. The trouble always happened with badly wired cars or modified ones. The racecar guys would wire the OD so that they could split all gears, then forget or accidently knock it in, and thereby total the OD whilst backing the car on the trailer! Bad day at the races. Wise racers kept the lockout on reverse.
FRM
http://www.usachoice.net/gofanu
FR Millmore

My approach is to bypass the factory safety switches and fit a cutout relay in the reverse light circuit. This allows use of overdrive in all gears but when the gear lever is placed into reverse it cuts the positive power going to the od solonoid and prevents engagement. Ray
Ray

The factory *always* fitted a lock-out switch with OD so it can never be engaged when reverse is selected.

Note that using OD in 1st and 2nd can damage it due to the high reverse torque loadings. I have so say that unwiring the standard lockout switch and using the reverse light switch and a relay is adding to the possibility of the system failing. Also when OD drops out in 1st and 2nd and kicks in again when you go up to 3rd at least you get the confidence it really is working as well as the jolt.

The factory limited OD to 4th only on North American cars from late 76 for reasons connected with the emissions stuff but all other (4 cylinder) cars had it on 3rd and 4th.
Paul Hunt

Paul,
In the fifteen years that I have used this setup no problems have manifested tmemselves. The LH unit is a very strong overdrive and used with a modicum of care should last a lifetime. The torque output of a standard MGB should not pose a threat to reliabity. At present I am running mine with a Moss supercharger and have been quite pleased with the performance and durability. Ray
Ray

Im a new mgb owner (purchased a 1965 restored car today)off E-bay, do you need to de clutch when going in or out of o/d ?
all the best
marc
M C Lovell

MC,
i 'feather' the clutch when engaging OD, just enough to take the jolt out when it goes in. Mind you, I do put the clutch right in when switching it out again. By the way, my OD takes a couple of seconds to engage when switched in - I'm assuming that's much the same for all of them?
Dave Smith

The procedure is not to use the clutch at all. Why cause additional wear on the clutch, thrust and hydraulic components. Try where possible to select overdrive with throttle slightly relaxed and conversely direct drive under slight acceleration. That way the jolt is minimised. Couple of seconds to engage is normal as it takes this time to move the cylinders and engage.
Iain MacKintosh

Ray - I understand Triumphs had overdrive on 2nd as well as they used a stronger unit. For the reverse reason the V8s *didn't* have overdrive in 3rd as the torque in that gear is too much for the unit, which is basically the same one as used in the 4-cylinder cars. The earlier D-type unit is even less strong and had a vacuum switch and relay so it wouldn't disengage under conditions of high manifold vacuum i.e. the overrun.

The handbook says not to use the clutch or feather the throttle, it just isn't necessary. JFDI :o)

What you shouldn't do however is switch overdrive out if the resultant rise in revs would put it in to the red zone.

Both mine switch out instantly, switching in does have a second or so delay, I think the V8 is a bit quicker than the roadster, possibly because of the higher pressures used.
Paul Hunt

You read my mind Paul - I was just about to ask what the owners handbook said about use of the O/D.

Feathering doesn't seem to be a problem Iain, I've had the clutch for 15 years and 70k miles so far (so that's just jinxed that then.....)
Dave Smith

My '75 MGB has od which is playing up. The od engages fine but when disengaged on slowing down etc the od re engages automatically, jumping in and out. This gives the effect of no engine breaking, it even does it when coasting meaning that when you accelerate, the engine picks up revs before engaging drive (sounds like an automatic). This phenomina also happens in 2nd gear which is not good as when the od drops out, the back wheels lock up. Any coments?
D G Pym

Workshop Manual: No engine braking on the overrun, and slip in reverse, is a symptom of a worn or glazed clutch (overdrive clutch, that is) lining or a broken circlip on the sun-wheel. If it really is engaging and disengaging, rather than slipping and gripping which could give a similar effect, then I would be concerned about the electrical connections.
Paul Hunt

My o/d switch is on the way out, if you put it in o/d it dosnt always work till you fidle with the back of the switch. Cleaned all the contacks on the back , put a meter across it and its hit and miss.D G Pym. When in od it some times drops in and out new switch to be fitted at the week end

mc
M C Lovell

This thread was discussed between 21/05/2004 and 26/05/2004

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