Slipping in Overdrive

[Bobble]

Good morning

I have an issue with my overdrive (1965 TR4A).  If I try to accelerate while in 3rd or 4th overdrive I get a very severe clutch slip - revs shoot up with no increase in speed.  This only happens when overdrive is engaged and I get no slip at any other time.  I have lived with this isuue for several years (since TRGB fitted a reconditioned overdrive unit) and always accelerate gently or drop out of overdrive if I want to gain speed.  However, during a recent service our mechanic suggested that the overdrive unit had a clutch of its own.  I am not familiar with the workings of overdrives so I am quite clueless.  The main clutch should be OK as it was replaced around 5,000 miles ago.

Any thoughts from those of you with greater knowledge of the workings of a TR (around 99% of the forum).

Thanks

Bob

[RogerH]

Hi Bob

indeed the OD does have its own clutch.

If you are experiencing clutch slip (and it does sound like it) then it is almost certainly the OD clutch that is the culprit.

The OD needs removing and sorting.  Mention it nicely to TRGB and they may be kind to you.

 

Roger

[BlueTR3A-5EKT]

Humour me here.

Has the oil level in the gearbox, which is shared with the overdrive, been checked?   I think I would do that job first before embarking on an overdrive removal.

[RogerH]

I wish I had said that  

[Drewmotty]

If the overdrive cone clutch is slipping it should stop slipping at the direct drive ratio as the  overdrive uni-directional clutch should take up the drive. 
Sounds like an o/d strip is needed to me. 

Edited Tuesday at 04:11 PM by Drewmotty

[Lebro]

That's true, but Bob (the other one) did say it did not slip when overdrive not engaged.
perhaps a test is needed to see if the revs go beyond what you would expect in direct drive.

Bob

Edited Tuesday at 04:28 PM by Lebro

[gloide69]

I had similar slipping symptoms when I refitted my A-Type overdrive after some gearbox work. I had fitted the 8 big springs incorrectly, resulting in the overdrive not being able to fully engage the clutch. Initially I'd used the length of the springs length to identify them, but this is flawed as the length can change with age. It is was the spring's coil thickness (or compressible length) that is important.

Though, in this case I sure TRGB wouldn't make the same mistake as me.

Paul

[RobH]

The OD internal clutch works in two directions.    When OFF it locks the  annulus of the sun & planet gears  to the sun gear, so the sun and planets cannot move and the input and output shafts rotate together. That bit sounds to be working.  

When ON it releases the annulus and locks the sun gear to the OD case so it cannot rotate, which makes the drive go from the input shaft through the planet gears to the annulus and output shaft. 

It sounds to me as though the annulus is being released  but the outer brake doesn't bite,  so the sun and planet gears are both rotating freely instead of driving the annulus, meaning no drive is transmitted to the output shaft.  

 

Edited Tuesday at 07:59 PM by RobH

[Lebro]

Yes, but as pointed out above, the one way clutch should prevent the output shaft rotating slower than the input shaft.

Bob

[Bobble]

Thank you all for your advice.

I will try the simple stuff first (ie oil levels) although I doubt that this is the problem as the issue has existed for a few years and levels have ben checked on a regular basis.  I also doubt that TRGB will be interested after such a long period - I have stupidly lived with it rather than sorting it earlier.  It has come to the fore due to the mechanic that looks after the car mentioned it after taking it for a test drive.

I have copied the advice offered here and will pass it on when I take the car in again.

Thanks again

Bob

[stuart]

2 hours ago, Bobble said:

Thank you all for your advice.

I will try the simple stuff first (ie oil levels) although I doubt that this is the problem as the issue has existed for a few years and levels have ben checked on a regular basis.  I also doubt that TRGB will be interested after such a long period - I have stupidly lived with it rather than sorting it earlier.  It has come to the fore due to the mechanic that looks after the car mentioned it after taking it for a test drive.

I have copied the advice offered here and will pass it on when I take the car in again.

Thanks again

Bob

I would still be having a chat with TRGB, they can only say no.

Stuart.

[Bobble]

Stuart

I haven't been in touch with TRGB for many years.

Bob

[stuart]

2 hours ago, Bobble said:

Stuart

I haven't been in touch with TRGB for many years.

Bob

Time to get reacquainted then I would say.

Stuart.

[Bobble]

⚠️

⛔

Edited Thursday at 04:35 PM by Bobble

[Ralph Whitaker]

On 3/4/2026 at 8:53 AM, Bobble said:

Thank you all for your advice.

I will try the simple stuff first (ie oil levels) although I doubt that this is the problem as the issue has existed for a few years and levels have ben checked on a regular basis.  I also doubt that TRGB will be interested after such a long period - I have stupidly lived with it rather than sorting it earlier.  It has come to the fore due to the mechanic that looks after the car mentioned it after taking it for a test drive.

I have copied the advice offered here and will pass it on when I take the car in again.

Thanks again

Bob

Out of interest what oil are you using .

Ralph