Overdrive query

[Pete Arnold]

Coming out of a roundabout in my MKI Vitesse today when I noticed that I had left overdrive on.
Switched it off and the revs went up - but I was in 2nd gear!  Funny, I thought it only worked in 3rd & 4th gears.

Reached home and selected reverse to back into drive but all I got was a whirring sound (almost as though teeth were not quite meshing properly) and car would'nt move enough to get up slight slope into drive.  

Decided there must be a gearbox or clutch problem so drove car in forwards.  Sat there telling wife, who was in garden, what had happened and tried reverse again.  Worked fine so went back out to road, turned round and reversed in as normal - all seems ok now!

Has anyone any idea if this is an indication of trouble looming and, if so, what?  
All sensible suggestions would be welcome.

[Mark Hammond]

Sounds like a fault with the overdrive inhibitor switch that stops it engaging in 1st, 2nd and reverse.  Probably you weren't quite in reverse which is why it wouldn't go backwards.  I find engaging 1st then reverse again helps line the teeth up for the reverse idler when this happens.   Whatever you do, don't engage the overdrive when in reverse as it will wreck the overdrive for sure and cost you dearly!

M.

[Freebird]

Have you tried the OD since the reversing incident? Eg, does it still work?

If the OD was engaged in reverse, it's not good news at all I'm afraid.

[Nick Jones]

Overdrive won't engage when stationary or at very low speed as it needs hydraulic pressure - which is generated by a pump driven by the gearbox mainshaft.   You'd need to be going fairly quickly backwards to engage it. Alternatively, it may possibly be mechanically stuck in.  That is bad news as reversing will destroy the one-way sprag clutch inside the overdrive.  Hopefully this is not the case here.

Nick

[Deleted User]

A bit more info...

The electrical circuit to the overdrive solenoid requires the car to be in 3rd or 4th, so that a switch is closed (to complete a circuit). Being in second AND having overdrive activated suggests the switch that should only activate on the 3rd/4th plane is not deactivating properly.

[Pete Arnold]

Many thanks to those who have replied.

I've taken the car out this morning and (much to my relief) the overdrive still functions in 3rd/4th gears.  However it does still function in 2nd gear as well - and I certainly do not intend trying it in reverse again after reading your advice!!

Presumably I do need a new inhibitor switch, which both Canley & Rimmer have in stock, old part no 127380, now BAU1074.  Am I correct in thinking that this has to be fitted from the top of the gearbox through the inside of the car rather than from underneath?

I assume that in the meantime I can carry on driving the car without causing any damage,provided I use the overdrive as intended, until I get the new switch.

[Jason]

Before you go buying new switches it's best to take the old one off and verify it is indeed defective - because of the way these things wear it's very easy to get a few more years out of them with a clean and fettle approach. Often removing the spacing washer under the inhibitor switch will give you that little extra space to operate. I had one play up on me on a 10CR once, it was the mounting bracketry that was at fault and after some percussive maintenance at 2am in a German car park, all was well - that fault caused me to "lose" overdrive at speed, very disconcerting!

Overdrive in reverse is usually fatal unless you're lucky - I was lucky once, sounds like you have been too.

[Freebird]

912 wrote:

Many thanks to those who have replied.

Am I correct in thinking that this has to be fitted from the top of the gearbox through the inside of the car rather than from underneath?

I assume that in the meantime I can carry on driving the car without causing any damage,provided I use the overdrive as intended, until I get the new switch.

Correct, it is located on top of the gearbox close to the bellhousing, so easy access from inside the car after removing the tunnel cover. I think it's the same part as the reversing light switch, so you could swap them over short term to prove it's the switch at fault. Been wrong before though.

I would worry about using reverse, so my advice would be to disconnect the wires at the overdrive solenoid until you have fixed the inhibitor switch.
You will have no OD at all, but better that than a scrap one.

[mikeyb]

Yes it is the same as the reverse light switch, although you usually have to unbolt the remote linkage setup to allow it to unscrew fully.

Easy to test without driving the car when the tunel is off - you can hear the click when the solonoid operates, just move the gearstick though its relevant positions and ensure it clicks when it should and doens't when it shouldnt!

[Stodders]

As Nick said earlier it is very unlikely that an electrical fault would cause the overdrive to engage in reverse. Remember that the pump is driven off the gearbox main shaft so there would not be any pressure as soon as the car is stationary so the overdrive should drop straight out.  You would then need to drive at about 15 mph in reverse for a couple of seconds to build up enough pressure to engage it again. To me it seems like it is sticking for some reason.  If you want to prove whether it is electrical I would rig up a warning lamp connected in parallel with the solenoid to warn whether there is power to the solenoid.  Good luck