MGB Overdrive

[Davidwill]

I sold my 1974 MGB roadster a couple of years ago, and missed it so much I have bought a 1971 model that looks almost identical.

I have a problem with the overdrive not engaging, I first established there was power to the dashboard switch, so far so good. Next was there power to the solenoid in 3rd & 4th gear, ignition on, no. I found that the connection to the terminal had come off the gearbox switch, any one who has had experience of this switch knows it is a right B------ to get at. After jacking the car up and putting it an stands I spent almost all day on my back, getting the terminal back together by dropping the gearbox as far as I could then using a very long – long nose pliers getting it back on, only to find after putting everything back together and testing it, it had come back off, (lots of bad language and a pint of beer required). Next day I decided to cut a 3 x 4 inch hole in the side of the body bell housing about 4inches in front of the gear stick just to the passengers side (UK). This means taking out the passenger seat and removing the carpets, to gain easy access to the switch. This I done and found the spade connector was very loose on the spade. Adjustment made to make a tight fit.

Time to re test the electrics.
I now find that the electrics are as they should be i.e. ignition on, power to the solenoid in 3rd & 4th gear only. Ignition off, no power at all. Sounds good.

Road test.
I drove the car down the road got to 3rd & 4th gear, flicked the switch,,,,,,, nothing happened. (back home another lot of bad language and another pint of beer).

After looking at John Twists videos on You Tube about MGB overdrives I decided to take the solenoid apart and see if there is anything in there going wrong. Took it apart cleaned it all, put it back together. Road test, just the same.
Next step I am going to drain the gearbox take out the overdrive oil relief valve and oil pump clean them up replace all o rings, clean it all and re assemble.
After that I am stumped.

Anyone with any ideas??

[v8250]

David, you're right to do this - drain gearbox oil, flush, clean OD parts and reassemble. Frankly, if you're completing this much work it would be wise to buy and fit an OD overhaul/service kit whilst you're at it. Interestingly, many OD's don't operate correctly simply because the 'box oil is old and at a low level. Come back when all this is done if the OD's still not working correctly. Good luck.

[nta16]

as I was reminded the other day if you're using engine oil in the g/box & o/d then it's a service replacement every 2 years(?)/24k-mile which makes sense with engine oil in a gearbox

(I think there are 3(?) JT vids on o/d)

if you don't know already know it then on this site you'll find more info (in 'Spanners') - http://www.mgb-stuff.org.uk/hometext.htm

v8250 said:

Interestingly, many OD's don't operate correctly simply because the 'box oil is old and at a low level.

ETA: just out of interest are you one of those that dips the clutch and/or fully lifts off the accelerator pedal when engaging/disengaging the o/d?

Edited by nta16 on Wednesday 14th August 19:36

[jagracer]

nta16 said:

ETA: just out of interest are you one of those that dips the clutch and/or fully lifts off the accelerator pedal when engaging/disengaging the o/d?

My O/D sometimes sticks in and I have to lift slightly to get it to disengage.

[Spitfire2]

The OD in MGB is basically a J-Type (although I think it got a different name).

The site below is a mine of info

http://www.buckeyetriumphs.org/technical/jod/JOD1/...

Also useful is below - Triumph focused so ignore reference to EP oils which aren't needed in the MGgearbox.

http://www.canleyclassics.com/?xhtml=xhtml/infodat...

[nta16]

jagracer said:

My O/D sometimes sticks in and I have to lift slightly to get it to disengage.

wouldn't that suggest a slight fault(?)

I know the B has two different o/d units so they may vary(?) in their operation but certainly for the later type you don't (fully) lift off the accelerator and certainly don't dip the clutch but many insisted on doing one or both

I had 73 model BGT and the o/d operation on it was almost instantaneous and smooth in or out and I can't remember lifting at all

plus the Driver's Handbook as far as I'm told just says flick the switch

[Spitfire2]

There is a marginal benefit in terms of reducing wear on the cone clutch. But a big disadvantage of dipping clutches and lifting off when engaging OD is that it misses the best thing about OD in sports cars.. The instant changes between 3 and 3OD on a fast twisty road is just fabulous fun.

Of course I am talking about a Spitfire and B's rarely keep up,on the twisties ;-p

Midgets however leave me behind......

[Gramrugby]

If your switch is on the top of the gearstick have you tried taking it apart and checking the wiring ?.
I had that problem with mine, sometimes worked, sometimes didn't. I changed the steering wheel to a Mota Lita
and after changing started to get shocks from it when the overdrive kicked in or out. Problem was loose connections
in the switch on the gearstick.

[Davidwill]

Thanks for the interest, an overall of the lower parts of the overdrive will take place in a weeks time. I hhave looked at all 3 of JT's videos on od. Very interesting. I'll have a look at those other sites suggested.
I was taught about 40 years ago never to dip the clutch while using the od.
Thanks, David.

[jagracer]

nta16 said:

jagracer said:

My O/D sometimes sticks in and I have to lift slightly to get it to disengage.

wouldn't that suggest a slight fault(?)

Yes, It's sticking probably through lack of use, the car had sat in a garage for 7 years before I bought it.

[nta16]

jagracer said:

Yes, It's sticking probably through lack of use, the car had sat in a garage for 7 years before I bought it.

if you've not already done so checking the g/box oil level and better still flushing and changing the oil and cleaning the o/d filter - all can make a big difference

the o/d changes in and out should be almost instant and not jarring

as I was reminded recently, if using engine oil in the g/box then it's change it at 24k-miles/2 years(?)

and yes plenty of use will help the whole car

[ReformedPistonhead]

This thread was useful to me so despite it being a 7-year resurrection I thought it worth posting what I did...

I bought a 1971 BGT for my son to learn to drive in and then use.

The O/D interlock wasn’t working, I established this by reversing into a space in a motorway services and making nasty noises.... Then I saw the switch was down.

I investigated and someone had bypassed the interlock.

Then I bought a new interlock switch.

Then I looked at how to get to it, very hard. I was about to drop the gearbox crossmember but then I decided to just put a relay off the reverse light that would cut the power feed to the overdrive switch when you engage reverse. Would mean you could have OD 1,2,3 & 4 but that is fine.

Big advantage is that the reverse light wire is in the engine bay near the fuse box instead of being under the car...

Simple relay wiring so that when the reverse light is off and the relay is unpowered (5 pin relay) the OD power reaches the switch. When it is in reverse the relay flicks over and the power is dropped.

I also put a little red LED in parallel with the OD solenoid so you have a discrete red light under the cigar lighter when it is powered.

Should be 17-year old (and 48-year old) proof.

Hope this helps someone else.

[matnrach3]

I don't think it is fine to not have the inhibitor switch for 1st/2nd. I can tell you even with a light, occasionally it is quite easy to forget to disengage the unit and this will obviously then put too much torque into the planet gears. I guess you will get away with it if you don't put a high load into it but I would fit it when you can

[ReformedPistonhead]

I have heard that 1st or 2nd can destroy an overdrive too.

But, I have also learned that fitting the O/D interlock switch results in much swearing and experimenting with washers to get it to engage and disengage just right.

I also see that if it is not just right then you could be in O/D 4th at maybe 75 mph and if you nudge the gear lever slightly it could disengage the O/D. Then you suddenly jump from a nice 4k rpm to 7k rpm and either blow up the engine or something else not nice for a new driver.

So I chose the take a risk in 1/2 O/D rather than a risk of it jumping out of O/D.

The fact that fitting the O/D is a massive PITA helped ;-)

In any case when he is older and insurance is cheaper we will junk the 4 cylinder and stick in a V8 with a 5-speed box anyhow ;-)

[ReformedPistonhead]

PS just looked at your garage. Much respect!

Have just bought a lathe which looks identical to yours!

[matnrach3]

I must admit that I have had no issues at all with my overdrive. My car did not have one when I bought it so I got a secondhand one , changed all the O rings, fitted new inhibitor switches (no shimming required) tested it on a rig I made to make sure it worked before putting it in the car.
Works instantly when switched on or off but I still put a tell tale light on as well.