[Fot] A-Type Overdrive Slipping

[Don Elliott]

Over on the right side there is a hole that you have to line up with a drill 
of a certain diameter.  Recently, owners of TRs with "A" type overdrives 
have reported that if you move the lever arm a bit one way or the other that 
you can find a "sweet spot" where the problem disappears.  The drill won't 
line up but the overdrive will not slip.

Also you mentioned that the piston rings were broken.  The early TR2s had 
overdrives with smaller pistons and steel rings in the piston grooves much 
like we all find in the pistons in the engine.  Later, Laycock re-developed 
the overdrive with larger pistons (hence more pressure) and these larger 
pistons did not have piston rings - they came with o-rings.

I have 325 psi oil pressure using Valvoline 20-W 30 Racing oil and while I 
have to let up for a millisecond when switching into 2nd overdrive only, the 
325 psi oil pressure I measured is sufficient for the "A" type overdrive in 
my 1958 TR3A.

Don Elliott, Original Owner, 1958 TR3A, Montreal
--------------------------------------------------------

----- Original Message ----- 
From: "M&M Hado" <mdhado at comcast.net>
To: <fot at autox.team.net>
Sent: Sunday, May 02, 2010 10:13 AM
Subject: [Fot] A-Type Overdrive Slipping


>I have an A-Type overdrive in a TR4 that is slipping under load.  Classic
> problem, I know, but bear with me please.  It started out as a standard
> rebuild on a unit that had only 300-350 psi and wouldn't hold.  It is a
> single-spring accumulator type.  The piston rings were broken and it was
> dirty and basically clapped out so I did a standard rebuild of both the OD
> and gearbox, replacing all wearing components in the overdrive including 
> the
> large thrust ring bearing.
>
> When done, I got 420 psi on the bench and it would not hold at all in OD
> when installed and put on the road, slipping out at the slightest throttle
> increase.  I had the pressure gage in place for the road test and the
> pressure was steady except for the usual temporary drop when engaged.  I
> ruled out any electrical issues by powering the solenoid directly by hand
> with a jumper.
>
> I took it out of the car again for inspection and found nothing unusual.
> There is plenty of liner surface left so I cleaned the cone clutch
> thoroughly and roughed up the surface to remove any glazing.  Since the
> slipping point is between the clutch outside liner and break ring, I set 
> the
> brake ring on the bench and lowered the cone clutch slowly to mate them up
> and thought there was a less than even contact.  In other words, the 
> clutch
> looked like it could be slightly oval and therefore would have a two-point
> contact rather than circumferential contact all around.  It seemed to 
> wobble
> slightly instead of being a tight even fit.  It was ever so slight and 
> after
> several attempts, I wasn't even sure it was a problem.  It's hard to 
> measure
> the clutch and break ring diameters because of the tapered surfaces.  I 
> wish
> I had put in a lathe to check the runout.
>
> To make a long story even longer, I added a washer or two under the
> accumulator spring and put it back in the car.  Now I have no less than 
> 525
> psi which should be more than enough.  The overdrive now kicks in and 
> holds
> at steady highway speeds but kicks down like an automatic transmission 
> when
> slightly more throttle is applied.  It's worse in second and third gears
> because of the higher torque.  It has to come out again and my plan is to
> change out the clutch and brake ring since I don't know what else to focus
> on.  Any other ideas or suggestions?
>
> Thanks,
>
> Mike
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