1971 MGB Roadster Overdrive

[Packman]

Hi,

I've just had my engine bay re-sprayed (Bare Metal) and took the opportunity to have a new gearbox and clutch fitted whilst it was all out. Drove the car home last Friday, (Some 30 Miles), all working well apart from the overdrive. The person who undertook the job for me is not an MG specialist and will now need a detailed list of what to check first and what to check last and of course all points in between.

He and I know that the gearbox (and overdrive) will have to come out of the car again to carry out some of the checks but some help is now required. Nigel - I'm possibly looking for your input here, along with others who have specialist knowledge.

The Haynes Manual shows a relay in the circuit and I thought that this might just be wired up incorrectly but i took a trip to my local MG specialist in Bexleyheath today who advised that the 1971 Roadster never had a relay in the overdrive circuit. He also took some time to scare me witless on potential problems.

So to re-cap I just want to know what to check first and what to check last.

Thanks

Packman

[Geordie MGmike]

The first thing to try is to make sure 12v is getting to the OD solenoid. There will be a wire going in to the OD, disconnect it and measure the volts to ground with the OD switch on and gearbox in third and/or forth. If the volts are there the problem is inside the OD, otherwise follow the cable back to where the volts are.

The OD wiring connects to the main harness to the right of the fuse box (white and yellow wires IIRC). That's a good mid point to check also. Volts here and the problem is likely to be the inhibitor switch on the top of the gearbox, no volts and the problem is back towards the dash switch.

If the volts all all okay and the box is suspected, check the resistance of the coil for an open circuit. If this is OC then it can be removed without taking the box out.

Get on youtube for John Twists vid's on OD checks
http://www.youtube.com/watch?v=r-WUuz4BzTw

Best of...
MGmike

[nta16]

Geordie MGmike and others here know more in their little finger nail than I do but I do know where to point you in the right direction

I'd consider an electrical fault first - but - I'd also want to know if the o/d was originally on your car or whether it was added before or with the new gearbox and if so are both original type and standard type of fitting to car (switch on dash, etc., etc.) (I've seen one person with both types of o/d switch fitted (!?!))

has the o/d unit been serviced or at least oil changed and filter clean

have look here for loads of info - http://www.mgb-stuff.org.uk/gearstext.htm#

yes of course I fully agree with posting the John Twist video 15 on o/d plus these other videos for background info and further servicing work if not already done, as the g/box and o/d are using engine oil the workshop oil change (and filter clean) interval is 24k-mile (2 years)

Overdrive gear ratios (overview of internals too) - http://www.youtube.com/watch?v=3HhjFNnsKr8

104 MGB Overdrive Repair Part 1 - http://www.youtube.com/watch?v=XPSgUeHAy8E

105 MGB Overdrive Repair Part 2 - http://www.youtube.com/watch?v=LCqPu77EWEE

106 MGB Overdrive Repair Part 3 – http://www.youtube.com/watch?v=FHw_o5y0HWE

76 Fitting an Overdrive Unit to the Gearbox - http://www.youtube.com/watch?v=9Xilqrw48bw

if you PM me (if it's still open) I'll give you a link to a free downloadable PDF of a b workshop manual

Edited by nta16 on Tuesday 22 October 00:23

[wildoliver]

Don't worry about pulling the engine, other than a mechanical fault with the OD it won't need to come out.

Mechanical faults are rare, electrical faults are common, lubrication faults can occur but are fairly rare, these usually manifest themselves with overdrive coming in then dropping out or being slow to drop in/out.

A very good fault finding route is listed above, I won't waste time repeating it, I suspect you have either got a case of the wiring being connected up wrong, entirely likely given it's been a rebuild, or a wire being off somewhere, most likely either in the engine bay (yellow and white) or under the car to the solenoid (wire hanging down at rear of box on underneath) this is just a bullet connector and is easy to catch when putting the box back in.

[flashgit]

Packman,
This happened to me when I rebuilt my MGB, also a 71. Check the OD switch first and the connections to it. I know it probably didn't get disconnected but this is Lucas wiring At this point if you have found the fault, you are not too dirty but may have a few skinned knuckles

Time to enter the engine bay and check those connections mentioned above (and the fuse while you are there )

If they all check out OK, then time to chock up the car and get a bit dirty. Go to the connections to the solenoid and check they are free of corrosion and are connected properly.

If all this fails then the solenoid may have been damaged/stuck/lodged (?) during the work you had done.

One other thing - make sure that the oil level in the gearbox is correct. I have a slow leak on mine and if it gets too low, the O/D stops working
Have fun and Good Luck finding the problem

[PoleDriver]

As suggested above check switch and solenoid. Also gearbox oil level. OH had a lot of problems with hers which I cured by cleaning the filter. Never had to remove O/D or gearbox to repair it!

[nta16]

Ian,
as usual two things have come up, one that Lucas products weren't very good this is a load of rubbish, consider how old the items are, many still working very well despite the use, abuse and neglect of the previous and many times present owners - many Bs sit in garages unused for most of their lives this doesn't do even the electrics much good

second is the possible lack of (preventative) servicing, maintenance, repairs and use by many previous and present owners, not check and changing the oils or cleaning the filter, some unless/until there is a problem

a note - the o/d should slip in and out effortlessly and (almost) instantly, the clutch does not need to be dipped or accelerator pedal fully lifted off

[nta16]

I knew I'd seen it somewhere and here it is, a short article by John Twist that summarises the fault diagnosis and advises on the addition of an in-line fuse, see - http://www.mg-cars.org.uk/news/news363.html

[Geordie MGmike]

nta16 said:

Geordie MGmike and others here know more in their little finger nail than I do but I do know where to point you in the right direction

Nigel you are too kind
I'm sure there's a lot I don't know but have yet to come across it

Some good links and pointers from you and the important bit about the OD wiring not being fused as standard (how daft is that) is an area a modification is essential.

[nta16]

I just repeat what I see other knowledgeable people put as far as technical and mechanical stuff goes - I regularly use something I saw the wild one put

that John Twist written article is good for this thread but I couldn't remember where I'd seen it, as well as listing diagnostic it says useful things like o/d faults can be fixed from under the car, solenoids don't fail, repairing wiring faults and cleaning the hydraulic components always repairs the units (plus fitting new 'o' rings)

as I often put much servicing and repairs just boils down to cleaning and lubing even electrics (connections)

Edited by nta16 on Tuesday 22 October 19:39

[nta16]

so Ian what's happening?

you didn't pull the g/box out before reading the replies did you

[Packman]

Nigel, you ask whats happening? - Well nothing as yet. I will be checking the gearbox oil level tomorrow (Thursday). The person that undertook the work filled the gearbox with oil whilst it was off the car and I already knew that the O/D will not kick in and out if the level in the box is too low. Failing that the car will be going back to the person who undertook the work soon. (if it ever stops raining)!!! All of your comments and advice chaps is all good stuff and he will use it to hopefully get it working for me again if it is simply not enough oil in the gearbox. I will keep you all informed but as summer is now over I'm in no rush as long as its fixed before next summer.

When its done and dusted I will post up some photos of the new blingey engine bay. I'm very pleased with it. Very good price too, but I do not want to sing the restorers praises premturely until he has sucessfully undertaken this fix for me.

Packman

[nta16]

best to fill both g/ox and o/d - but not overfill them obviously - then take the car for a run and check level next day

up to you of course but even last autumn and winter (the worst for decades) had over 50% of, at least one, sometimes both days, of the weekends where it was dry, even sun out, where I drove my car with the top down (has to be +5C for that) so I think you're missing many excellent driving days and opportunities, often better than summer days - it was spring that was a wash out

and of course that's just two days opportunity out of seven, some people have a seven out of seven

laying your car up could mean it keeps a lot better to look at than actually drive, like the majority of classics and particularly Bs

on that cheery note, I'm oot

[Expatloon]

nta16 said:

best to fill both g/ox and o/d - but not overfill them obviously

Your comment suggests that they are separate which of course they are not, the gearbox and OD share same oil and the fill point for both on a 71 is via the dipstick hole.

[nta16]

yeah, sorry, have you never seen my poor use of the language in other posts

it's not often I don't make mistakes in each post

I have seen where people have put where they've filled the box(es) with the front end still raised, I'm sure their intention would be to check again when on the level but things do get forgotten

[Packman]

Right, got the car back now, O/D still not working properly although it is working intermittently now that I have filled the gearbox to the correct level. (it took an extra 2 pints to get it to the full mark). I know that there is 12v at the solenoid every time that the switch is operated with the car in 3rd or 4th gear but it does not always kick in, although when it's in it and working it always kicks out when the switch is returned to its off position.

Next job will be to change the solenoid (Coil) and to renew the filter etc as I now suspect that the problem arose when the old gearbox was drained when 42 year old oil sludge blocked the filter and has probably made the solonoid stick too!

Incidently I found out tonight that the rear lights are now not working either. (Brake lights and numberplate lights are). I suspect that will be an easy fix compared to the overdrive!

Packman

[nta16]

er, apart from under filling the g/box/od combo has any other work been done on the o/d unit like cleaning the filter onwards?

I know you engineers like to find technical stuff to get your teeth into but often it’s the basic stuff that counts (like having sufficient oil in and remember to check the level)

in case you missed it before here’s a good article - http://www.mg-cars.org.uk/news/news363.html

and if you want full works see JT vids above

I was recently reminded that as the g/box uses engine oil it should be changed every 24k-miles (2 years?)

I might be telling grandpa how to suck eggs but it’s a common practice - you don’t dip the clutch when operating the o/d and you do keep load on it (i.e. you don’t lift off the accelerator)

my o/d was almost instant engagement and disagreeing with JT I did use 3rd o/d to rapidly gain speed on a slip road or overtake, no clutch, no lifting (well not fully at least) and no o/d thump

I suspect all might be easy fixes it’s the regular servicing and regular year round driving that keeps them fixed better

if you want a free MGB factory Workshop Manual pdf then PM me as I can give you a link

[Packman]

Nigel,

Yes I know that my nice new gearbox should have been filled correctly but as I have said before the person undertaking the paint job was not an MG specialist and only some detective work on my part I deduced that the Gearbox was grossly underfilled. I put that right but I drove the car home first. (Some 30 miles down the road)!

I think that I have now deduced what I need to do next as my previous post and your latest one seem to agree. Coil, (Solenoid) and Filter, however, as the work needs to be undertaken on my driveway I need a good day weather wise and there are now not too many of those that coinside with a Saturday at this time of year! So it may be a while before I can report back.

Interesting what the link says about solenoids!

I do not need the PDF manual as I have a Haynes workshop manual which seems quite good enough to guide me through the work.

Thanks

Edited by Packman on Thursday 31st October 19:42

[nta16]

Packman said:

Yes I know that my nice new gearbox should have been filled correctly but as I have said before the person undertaking the paint job was not an MG specialist and only some detective work on my part I deduced that the Gearbox was grossly underfilled. I put that right but I drove the car home first. (Some 30 miles down the road)!

I'm confused by this but that don't matter - have you physically checked what he partially filled the gearbox with

Packman said:

I do not need the PDF manual as I have a Haynes workshop manual which seems quite good enough to guide me through the work.

Thanks

having had a few Haynes and two currently neither of which fully cover my model despite being the prints that should, like publications there are a few errors and omissions in the ones I've had anyway

I like the confidence you wont need the factory manual for any future work, which Haynes would probably cover anyway

sooner you than me working outside (I've had 20+ years of that), see if you can borrow a garage as the car is drivable, one with a lift would be handy or perhaps there are still places that rent out garage and lift time

best of luck anyway

[Expatloon]

Why make Packman ask for the manual, it's freely available from a number of sources, for instance:

http://www.bmcno.org/manuals/MGB%20Workshop%20Manu...