TR2/3/3A A type overdrive hard engagement

[tr3a356]

I have an A type overdrive transmission in my TR3A that I believe came from an ex racer. The overdrive engages immediately and harshly. It's quite a jolt. Much more so than a previous TR4 or the current Healy I own. Was there some standard modification used in racing to speed up the overdrive engagement? If so, I'd like to back it out and get a smoother engagement for a street car. Thanks

 

[Hamish Racing]

Hi
Normally it runs at a much higher pressure for comp use. But I’m afraid I can’t say what the original pressure is. Sorry.

 

[JHaydon]

The manual says 420-445 psi for the TRs (and 470-510 psi for the Vanguard and Renown).

Of course the manual doesn't address harsh engagement, just clutch slippage. But one potential cause of clutch slippage in OD is an incorrectly adjusted actuator arm... could setting the arm too far in the other direction cause harsh engagement?

 

[Simmo]

I have always found the "A" type OD to slam on and off with a jolt much more than "J" or "LH" I have owned.

 

[CJD]

All you can do is check the accumulator pressure. Someone may have shimmed the regulator to high.

 

[Graham H]

Buckeye Triumphs cover this issue very well and I think there are two things that will change how hard it will engage. One is how strong the accumulator spring is which can be changed by adding washers and two is the pressure of the oil pump relief spring which can be increased similarly. It could also be a heavy-grade gear oil which Triumph recommended but Austin Healy did not. Most complaints about the A-type OD are for soft engagements rather than hard. You may need to adjust your driving technique to compensate it's all good fun with 7 forward gears

Graham

 

[tr3a356]

Buckeye Triumphs cover this issue very well and I think there are two things that will change how hard it will engage. One is how strong the accumulator spring is which can be changed by adding washers and two is the pressure of the oil pump relief spring which can be increased similarly. It could also be a heavy-grade gear oil which Triumph recommended but Austin Healy did not. Most complaints about the A-type OD are for soft engagements rather than hard. You may need to adjust your driving technique to compensate it's all good fun with 7 forward gears

Graham

Thanks for the replies. Graham, thank you for an informative answer and for pointing out Buckeye Triumphs - great resource. The transmission is out as I am doing some engine work. It shifted and sounded fine but I will go through it to inspect everything and to re-seal it but wanted to take care of the engagement issue and was looking for a hint as to where to start. As I've mentioned, I've owned other British cars with the A Type OD but this engagement jolt is different and is sudden and fierce. I don't believe it is normal and should be addressed for the sake of the rest of the drivetrain. Currently, it makes me not want to engage the overdrive.

 

[JHaydon]

If everything else checks OK, then I wonder if it's worth playing with the gear lube?

Specifically, the consensus seems to be to run Red Line MT-90 (shared by both the transmission and overdrive). The MT-series of lubricants were formulated to be less slippery than other Red Line synthetic lubricants -- standard Red Line gear lube is SO slippery, it doesn't provide enough friction for synchros or clutch-type LSDs to engage at all. But Red Line encourages mixing MT lubes with their gear lubes to "fine-tune" the slipperiness to your needs.

Perhaps adding just the right amount of Red Line 75W90 to MT-90 would allow the clutch to slip just a little during engagement, without becoming so slippery that the synchros no longer engage?

Note, Red Line also makes 70W90NS -- the NS standing for Non-Slip. 75W90NS is even less slippery than MT-90, so adding NS would (could) make the clutch grab even harder.

(Full disclosure, the company I work for sells Red Line lubricants)

 

[CJD]

Your first step is to install a gage and test the operating pressure. That will tell you what has been done as a starting point. From there we can determine what needs a shim adjustment.

 

[tr3a356]

Your first step is to install a gage and test the operating pressure. That will tell you what has been done as a starting point. From there we can determine what needs a shim adjustment.

Thanks, I will need to make a bench test fixture to spin the output flange. After finishing the engine work, I'll start on that and report back in a few weeks.

 

[Sarastro]

Do you use the clutch when engaging or disengaging it? It's probably a good idea to do that, in any case. Mine gives a healthy jerk in all but top gear if I don't clutch it.

Th good news is that, if necessary, I think you could adjust the pressue without pulling the transmission. You would have to remove the transmission-tunnel cover and, in all likelihood, make a serious mess.

 

[CJD]

+1...clutch is not necessary, but with the age of our cars, it is a good idea to clutch when operating to reduce the engagement/disengagement loads.

Just a note when you make a test fixture. The pump works off the output shaft, not the input. That means that all you need is a 1/2" drill with a socket to match the yoke nut, and spin the yoke. It makes no difference if the tranny is in gear or neutral.

 

[Jim_Stevens]

I baby my OD by always clutching it.

 

[Hamish Racing]

They are quite tough. Mine has uprated pressure and this is a top gear full throttle overdrive engagement at about 95mph (4.1 diff).
See about 1:30 in

Aintree the home of the 1959 British Grand Prix 5 months before my car was built.

 

[Graham H]

+1...clutch is not necessary, but with the age of our cars, it is a good idea to clutch when operating to reduce the engagement/disengagement loads.

Just a note when you make a test fixture. The pump works off the output shaft, not the input. That means that all you need is a 1/2" drill with a socket to match the yoke nut, and spin the yoke. It makes no difference if the tranny is in gear or neutral.

But don't spin it in reverse if you are going to engage the OD bad things will happen

Graham.

 

[CJD]

Good point, Graham. I meant to mention the 1/2" drill needs a reverse switch.

 

[KVH]

I baby my OD by always clutching it.

Me, too. Why let it slam if you can feather it.

 

[mctriumph]

The A type OD is usually "briske" when it engages. It is after all a heavily sprung clutch that is getting "dumped "
by the 400PSI+ held in the accumulator well . Be happy it works for heavens sake!!!
mad dog

 

[Jim_Stevens]

Yeah, but Hamish is a racer, and every second counts. For us mortals (and I’m a fan of his) it’s good to know they’re tough enough though. They’re Rube Goldberg enough plus sixty years old for me to continue to baby it.

 

[Hamish Racing]

I do use mine the road. OD for 2nd, 3rd, 4th but with the 4.1 diff 3rd OD is the same as 4th.
mine is not that harsh. I have been advised by the people that rebuilt the g/box that it should easily cope with full throttle changes.

but racing starts again for me 29/4/23 fig 8 sprint.
So fingers crossed it all holds together.