Reverse not working after overdrive stopped working?

[Philip Tibbitts]

A couple of days ago, whilst driving my 1976 Rubber Bumper Roadster with overdrive, I engaged reverse gear, only to find there was a whiring noise in the gearbox - but no travel in reverse.

I thought the clutch had failed, so I engaged 1st gear to check, but it pulled forwards ok, so I tried for reverse again.

As I engaged the clutch, I felt a slight jolt in the gearbox , but once reverse was engaged and the car moved backwards ok.

Up untill this point the overdrive had worked as it should, in 3rd and 4th gears.

The car has recently been recommissioned after none use for many years.

After a full mechanical rebuild and service, the reverse switch on the side of the gearbox/ overdrive unit was replaced in order to get the reverse lamps to work again, during its replacement the overdrive wiring was checked, contacts cleaned and reconnected ok.

Today (about 60 miles after the no - reverse incident) whilst driving the car at about 60 mph the overdrive started to not drop in / out correctly and failed completely (stuck in overdrive) quite close to home.

When trying to manouvre on the drive, I lost reverse motion again, with the same whiring noise coming from the gearbox.

It was put into 1st gear again sucessfully moving forwards ok, but I've completly lost movement in reverse this time.

I must admit I didn't check or change the gearbox oil during the service I did 6 months ago.

Does this issue of losing overdrive and reverse ring any bells, if so I'd be glad to hear of any possible solutions or recommended actions / checks I sould carry out to rectify the problem.

Thought appreciated!

[Vic Butler]

Reversing with overdrive engaged will destroy the uni directional clutch. The whirring noise in reverse would indicate the gears are engaging but the overdrive is spinning inside its housing and the jolt and restoration of motion is the overdrive re engaging.That would probably be the reason why it's dropping in and out of overdrive although low oil level and pressure is another reason or it could be the solenoid.
Did the overdrive disengage when you switched it off on the gear lever switch or did you leave the switch on and did you replace the inhibitor switch? It would be worth reading the MGB overdrive thread.
I've found after the winter lay up the overdrive can be initially slow to disengage so I don't use it until the gearbox oil has warmed up.
I don't want to be a pessimist especially at this time of year but I reckon you'll need a new overdrive but I could be wrong. Sorry for the gloomy news.
When engaging/disengaging overdrive on the move don't depress the clutch but let the overdrive in or out with a thump. I always do that; it stops the overdrive clutch linings from glazing.

[Peter Cresswell]

Hi Philip

Firstly Vic refers to other threads about the Overdrive. One is very near to you thread in the 'Technical Board', but there was a lot of discussion some months ago which gives more info. See viewtopic.php?f=4&t=618&p=3024&hilit=overdrive#p3024

I agree with Vic's diagnosis but if needed, you do have the option of buying another Overdrive Unit or getting yours rebuilt. If you buy another overdrive unit you need the LH type (which has the internal solenoid) with a blue label which is the colour of the label on the solenoid housing cover plate. This has the correct speedo drive gear for cars with the smaller speedo (also known as the Triumph instruments) which I assume your car has. The speedo drive is on the output shaft of the overdrive unit and you need many special tools to dismantle the unit to change this. Buying a secondhand unit risks acquiring another overdrive that doesn't work correctly, so a 'new' unit from one of the main suppliers is preferable. These however are not 'new' but have been reconditioned by one of the 3 companies in the UK that can do this, and as you have the same amount of work to change the unit it makes more sense (to me!) to remove the overdrive unit and take or send it to one of the reconditioners to get it overhauled.

Incidentally, you have to take the engine and gearbox out of the car as one unit. There is a fixed crossmember under the overdrive unit which prevents the gearbox/overdrive from being moved rearwards enough to clear the clutch and drop it down. Also make sure you mark the relevant positions of all the parts of the removable crossmember before you take it off the car. This crossmember has to be removed as it is too wide to clear the chassis rails, and to do this you have to take it apart. There is a mod to the crossmember that makes it easier to put back together. See here for this - https://www.mginfo.co.uk/upgrades4mgs/D ... cross.html

The nearest reconditioner to you in Telford (or me in Stone!) is ODSpares in Rugby who are not far off the M6 at Junction 1 and I have found them to be very helpful. It is easy to remove the overdrive from the gearbox, but not so easy to join them up again so I took my gearbox to ODSpares for them to do it when my overdrive unit had been repaired. Their contact details are on their website - http://www.odspares.com/.

[Philip Tibbitts]

Update:

Drained the oil and removed the aluminium cover plate to expose the filter gauze area.

I Started to remove the filter gauze and pieces of a phospher bronze thrust washer started to drop out .......(exactly as the previous post by Maxi?)

When the parts were best assembled into its orginal shape, there's approx 25% of it missing - (The missing bits may have dropped into the oil catch tank - to be checked later).

Can anyone describe or show a picture of where this thrust washer is fitted?

Is it possible to replace with the overdrive in situ - or is it strip down time?

Assistance appreciated in understanding the ongoing issue......

[Peter Cresswell]

It is item No. 70 in the centre of this diagram from the Moss website:
https://www.moss-europe.co.uk/shop-by-m ... 67-80.html

As I mentioned above, you can't do any work on the gearbox/overdrive with it in the car. All the parts inside the overdrive are accessed from the front (gearbox face) of the overdrive unit, so the overdrive has to be separated from the gearbox. Also you need several special tools (8 I think) to dismantle and rebuild the overdrive, and even then you can't test it out of the car at home.
Best to remove the engine and gearbox as one unit, split the engine from the gearbox to make handling it easier, then stand the gearbox on the bell housing face (use some blocks of wood to stand it on as the gearbox input shaft sticks out the front). Take the overdrive off the back of the gearbox. I highly recommend you then take or send the overdrive to one of the experts to have it checked/rebuilt and tested.
Before you put the engine and gearbox back into the car don't forget to put a new switch into the remote gear lever housing, as this was likely to have been the cause of the problem. I would also put a new clutch in as this is a lot of work to change at some time in the future.

[Philip Tibbitts]

Peter,

Thanks for the words of wisdom.

I called the specialist mentioned and explained what had happened.

He was familier with the broken thrust bearing and gave his thoughts on it......

It appears the phospher bronze thrust washer was originally (on earlier cars / production) a Top Hat design that fitted into a machined recess in the casting body of the overdrive unit, carrying the sun wheel shaft.

These can be replaced as a service part and the unit reused only - on the earlier cars, BUT; on later cars, the design was 'value engineered' to use just a flat washer-type phospher bronze cast into the alloy main case.

Of course, when the washer breaks loose, it renders the case (£35) scrap.

Seems its possible to use the o/d unit without the washer, but the sun wheel shaft eventually drills its way through the soft alloy case and things go bang - probablly just when its most inconvenient! - the timescale for this happening can be upto several thousand miles.

Thats probablly why an identical previous post by SWISSMAXI came up with the idea that he thought the washer was left in there after a previous failure and washer replacement, he registered no aparent issue after refilling with oil and continuing to use the o/d unit after the broken washer discovery on thd gauze filter.

But we now know the issue is 100% going to be terminal.

Drive train strip on the way!

[JohnH]

Couple of tips worth looking into before removal if its the LH type.
Remove the SMALL plate which has the solenoid inside which will drop down. Then remove the plunger it operates and at the top end is a very small 'o' ring which may have come off or become hardened. I recall there is a ball bearing too. Its this 'bobbin' arrangement that diverts the very very high oil pressure so it needs to be operating properly and not sticking.
Always change the gauze square filter held in by the LARGE plate when changing oil. If much debris is found though it will more than likely be broken up linings meaning an exchange overdrive unit unfortunately.