TR Question but may apply to a Stag too

[Andrew S]

I am trying to help a friend who has an unusual gearbox problem

TR6 with J type overdrive fitted

From cold he can pull away in 1st then 2nd no problem at all but when he puts it into 3rd there is a vibration under the drivers seat at around 2000-2200 revs but as soon as he out the overdrive in, it goes. Its the same in 4th but the notable thing is that the engine revs are the same.

He doesn't feel that its a slipping clutch

When gearbox is warm and engine is at normal running temperature, there is no problem at all and all's fine, its just from cold and warming up

Very odd

Any suggestions much appreciated

Cheers
Andrew

[jonfey]

The only thing I can think of is the overdrive solenoid. Had a similar (ish) thing on a Sunbeam Rapier some years ago.

Fine when hot ( Solenoid expanding with heat and freeing up perhaps?)

Maybe the vibration is the solenoid rapidly trying to switch in or out when cold?

I'm just taking a stab at it. I'm certain there are those on here who'll know much more than I.

[stag8manod]

Hi jonfey, On the J type remove the little sump and in there are some screwed plugs and behind one is the high pressure filter. This could be worth taking out and cleaning. A pin wrench is required to undo. It may not be the problem just a thought.
Cheers Ian A

[flying farmer]

I wonder if it is the inhibitor switch on top of the gearbox that is barely making contact causing the solenoid switch to jump in and out rapidly.

A bit like the problem you can get with starter solenoids and bad battery connections where the starter solenoid clatters rather than engaging properly.

Sometimes the terminals come loose on top of the inhibitor switches where they are crimped on. The change in temp as the car warms up probably improves the connection, and the correlation to engine rpm is possibly a resonance factor on the loose connection.

Neil

[Andrew S]

Thanks all, this is very helpful as I had ruled out the overdrive being that the problem is there before it is engaged but if the solenoid sticks or the switch allows power to the solenoid, then the overdrive would be trying to engage and being that its only in 3rd and 4th, it makes sense.

If I remember from when I had my 6, the solid yellow wire to the solenoid is teh constant power and the yellow and purple wires comes from the switch which earths out the circuit out by the selector shaft allowing the switch button to pop out and complete the circuit.

Is the selector switch on the side of the box or on top up by the tunnel in an non accessible place

Thanks for all the advice
Andrew

[flying farmer]

On the top in a non accessible place of course!

If it is the usual cardboard or fibreglass tunnel I generally cut a hole in it if I can locate the faulty switch from underneath, saves removing the H piece and tunnel.

Neil

[TR5convalescent]

I am trying to help a friend who has an unusual gearbox problem

TR6 with J type overdrive fitted

From cold he can pull away in 1st then 2nd no problem at all but when he puts it into 3rd there is a vibration under the drivers seat at around 2000-2200 revs but as soon as he out the overdrive in, it goes. Its the same in 4th but the notable thing is that the engine revs are the same.

He doesn't feel that its a slipping clutch

When gearbox is warm and engine is at normal running temperature, there is no problem at all and all's fine, its just from cold and warming up

Very odd

Any suggestions much appreciated

Cheers
Andrew

Andrew, I assume the OD will engage and disengage ok once everything is warm?

[Andrew S]

The the judder or vibration felt under the seat when in 3rd or 4th goes straight away when the overdrive is engaged whether the car is cold or hot. The problem is only from cold, in 3rd or 4th, at between 2000-2200 revs in either gear until the overdrive is engaged.

[trunt]

Heres a theory..



It may be the wiring to the OD switch or the switch itself shorting. This could be at a particular rpm when it "resonates" to the engine vibration. I think there is a relay involved on the TR6 and that could definitely be vibration sensitive.. It would only happen in 3/4 (when the inhibit switch allows the OD to engage) and when the OD switch is OFF (otherwise its ON so shorted anyway)

http://tvrna.tvrccna.org/tech-m06.html

remove the 12v power to the OD, if the problem goes away its electrical


Terry

[Andrew S]

Thanks Terry, good idea as a starter
Cheers
Andrew

[Guest]

The problem is only from cold, in 3rd or 4th, at between 2000-2200 revs in either gear until the overdrive is engaged.

This is a unidirectional clutch problem, maybe mixed up with the relief valve dashpot playing up.
Most people have no idea an overdrive is always engaged whether it's in or out of overdrive.

Laycock made stacks of overdrive units with defective Unidirectional clutches, and the relief valve is known to stick or very occasionally break up. The solenoid has a tiny spring inside which is known to weaken and prevent the valve from releasing pressure properly.


THERE ARE 2 CLUTCHES in an overdrive.
The inner clutch transmits drive when the OD is switched out and depends on the lockup of the UDC.
This inner clutch is engaged locked and held in place by 4 large springs and the helix angle of the planet gears.

The outer clutch is the one that moves against a special cast iron brake ring, disengages the inner clutch and transmits drive via the sunwheel and planet gears.
On over-run the UDC transmits torque back into the OD unit.

[trunt]

This is a unidirectional clutch problem, maybe mixed up with the relief valve dashpot playing up.
Most people have no idea an overdrive is always engaged whether it's in or out of overdrive.

Laycock made stacks of overdrive units with defective Unidirectional clutches, and the relief valve is known to stick or very occasionally break up. The solenoid has a tiny spring inside which is known to weaken and prevent the valve from releasing pressure properly.


THERE ARE 2 CLUTCHES in an overdrive.
The inner clutch transmits drive when the OD is switched out and depends on the lockup of the UDC.
This inner clutch is engaged locked and held in place by 4 large springs and the helix angle of the planet gears.

The outer clutch is the one that moves against a special cast iron brake ring, disengages the inner clutch and transmits drive via the sunwheel and planet gears.
On over-run the UDC transmits torque back into the OD unit.

Lots of good points here and all should be considered but my question is, why does it not happen in 1/2 gear as well?
OK, the speed and torques are varying between gears and it may be sensitive to that but everything spinning in the OD, even the unidirectional clutch, spins according to car speed, not engine rpm or which gear is selected.

Terry

[Andrew S]

Hi Terry
I presume this comes back to the fact the power goes through the selector switch on 3rd and 4th only and cuts out 2/3rd

[trunt]

Hi Terry
I presume this comes back to the fact the power goes through the selector switch on 3rd and 4th only and cuts out 2/3rd

Im sitting here in the dark after an eyeball injection just thinking about Overdrives!!

If the circuit I linked to - http://tvrna.tvrccna.org/tech-m06.html is correct then 12vpower is supplied to the relay coil, then both the OD switch AND either of the 3/4 inhibit switches give the relay coil a ground and it energises the relay. Separate power is fed to the relay contact which when energised closes the contact which engages the solenoid.

Initially I liked the theory of the relay contacts bouncing but that cannot be as it would happen in any gear, now I prefer the od switch or its wiring shorting, not to ground but to each other. One hell of a long shot!! but for me the only thing that explains the rpm sensitivity and it only happening in 3/4 gear. Now why only when cold?? perhaps the engine shakes more when cold?? I'm clutching at straws here

The other thing brought up by down the plughole is the solenoid itself. But that should happen in 2nd gear as well, (unless there is not enough pressure yet because of the low road speed to cause a problem)

Bottom line is that by removing all power (eg pull the relay) you can determine weather its electrical or mechanical and go from there.

[trunt]

Im sitting here in the dark after an eyeball injection just thinking about Overdrives!!

If the circuit I linked to - http://tvrna.tvrccna.org/tech-m06.html is correct ...

Hah maybe its not, see these diagrams http://www.advanceautowire.com/tr2506.pdf The circuit got simpler in the later years, still could be an iffy OD switch though!