Bobble Posted March 3 Report Posted March 3 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 Quote
RogerH Posted March 3 Report Posted March 3 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 Quote
BlueTR3A-5EKT Posted March 3 Report Posted March 3 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. Quote
Drewmotty Posted March 3 Report Posted March 3 (edited) 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 March 3 by Drewmotty Quote
Lebro Posted March 3 Report Posted March 3 (edited) 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 March 3 by Lebro Quote
gloide69 Posted March 3 Report Posted March 3 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 Quote
RobH Posted March 3 Report Posted March 3 (edited) 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 March 3 by RobH Quote
Lebro Posted March 3 Report Posted March 3 Yes, but as pointed out above, the one way clutch should prevent the output shaft rotating slower than the input shaft. Bob Quote
Bobble Posted March 4 Author Report Posted March 4 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 Quote
stuart Posted March 4 Report Posted March 4 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. Quote
Bobble Posted March 5 Author Report Posted March 5 Stuart I haven't been in touch with TRGB for many years. Bob Quote
stuart Posted March 5 Report Posted March 5 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. Quote
Ralph Whitaker Posted March 7 Report Posted March 7 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 Quote
Bobble Posted March 10 Author Report Posted March 10 Ralph I think the last time it was topped up (a while ago probably) it was Castrol Hypoid 90. I am thinking of having the gearbox drained and refilled following advice from earlier posts. This seems to be the simple option before thinking about stripping down. Any advice on a particular brand? Thanks Bob Quote
RogerH Posted March 10 Report Posted March 10 Hi Bob just out of interest, why are you using a hypoid oil The OD does not have hypoid type gears. The 90 grade is not a big issue. Roger Quote
RobH Posted March 10 Report Posted March 10 Not a good idea to use hypoid oil in the gearbox. It's too slippery and stops the synchromesh working properly. Maybe that contributes to your OD slipping as well ? Quote
james christie Posted March 10 Report Posted March 10 Hum! I doubt it. I've been using Hypoid 90 for over 50 years in my gearbox and overdrive and haven't noticed any clutch slip or otherwise. Doesn't stop the drips though.... james Quote
Lebro Posted March 10 Report Posted March 10 (edited) For years I used Penrite GB40 in my O/D box. Last year the O/D threw it toys out of the pram, & I got it rebuilt by Hardy engineering. They poo poo'd the idear of using a "specialist" oil, & said the box's & O/D's were designed to use EP 80W90. So that's what is in there now. (They also recommended dipping the clutch when going in or out of O/D. Bob Edited March 10 by Lebro Quote
RogerH Posted March 10 Report Posted March 10 Hi Bob that is very interesting as the OD OEM way back forbid the use of EP oils as the heat generated by the clutch could break down the components that form the EP and block the very small oilways in the case. As for dipping the clutch that is for beginers and learners Oils are made for a purpose. Use them where their purpose lays. Roger Quote
rcreweread Posted March 10 Report Posted March 10 Penrite GB40 was highly recommended but somewhat difficult and expensive to find these days after which Dynolite gear 40 seems to well thought of cheers Rich Quote
Ralph Whitaker Posted March 10 Report Posted March 10 (edited) I`ve been using Morris Golden Film 40 which it says is ideal for classic gearboxes requiring SAE40 oil. But there`s the confusion as the original Triumph workshop manual says SAE30 oil, Haynes says Castrol GTX (20/50 multigrade), and TR4 on says Castrol Hypoy. Nowhere have I seen a specific listing for cars fitted with an overdrive which suggests something different. From my own experience I originally used EP90 and found the car hard to get 2nd gear until fully warmed up and on advice on here I changed to Morris`s GB40 which improved things enormously. I would be interested to know if anyone has tried auto gearbox oil, as that it designed to work with band brakes so not dissimilar to the way an o/d works. Ralph Edited March 10 by Ralph Whitaker Quote
RogerH Posted March 10 Report Posted March 10 Hi Ralph the confusion arises because people are comparing apples with pears. Engine SAE is different to Gear oil SAE Have EP oils advanced such that the EP does not eat the yellow metal!! The thinner oils do help the synchros Use what you think works best. I use Penrite GB40 (whatever that is) Roger Quote
Bobble Posted March 10 Author Report Posted March 10 Wow - can of worms or not! All good stuff though and plenty to think about. I may have the "hypoid" part wrong - is there a difference between hypoy and hypoid? Quote
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