View Full Version : 2nd Gear OD or Not

08-01-2011, 01:27 PM
The 74 TR6 I'm building had a new motor, trans. and overdrive (type J) when I got it. Problem is, the Top Cover (shifter) is from an earlier model, has switches on top for 2nd and 3/4th overdrive. It does not have switches on the sides for reverse lights or seat belts. It does have the wiring connectors for all.
Can I use the 2nd gear OD or would it be a problem with the type J?
Where and/or how can I hook up a reverse light switch?


08-01-2011, 01:47 PM
I don't think that the "J" type was designed for the "A" type 2-3-4 O/D operation. I could be wrong, but Randall is pretty familiar with these and should be along shortly to answer your question in greater depth.

08-01-2011, 03:12 PM
Larry , are you saying this as you have compared it to the top cover pics?
And Paul is correct . Yours didn't have OD in second if it is a stock OD


Geo Hahn
08-01-2011, 03:52 PM
Not an expert but I'm thinking the smaller piston in the J-Type means that your coffee could cool in the time it would take for the OD to engage if you could select it in 2nd.

08-01-2011, 04:31 PM
I thought the issue was torque. The J-type was felt not strong enough to handle the extra torque of the lower gear so they made it 3-4 only.

08-01-2011, 04:56 PM
The switches are all on the top

08-01-2011, 05:23 PM
No, I am not comparing it to the pictures.
Yes, I know my model did not have 2nd OD.
The top cover is from a model (likely not a TR6) that had 2nd OD and for some reason the wiring harness has connectors for a 2nd OD. Again, can I connect the 2nd OD with a Type J or should I bypass it? Probably a torque issue but just checking. Also, how can I connect a reverse light switch when there is not a hole for it in the cover?

08-01-2011, 07:28 PM
I honestly don't know the answer, but I would ASSUME that if it were this simple, it would be a well known fact (or TR6 urban legend) by now. The "A" and "J" were completely different units.

08-02-2011, 06:40 AM
you should have a reverse switch, when you put the car in reverse gear shift to the right and back the rail moves left and forward so the hole front left is where the reverse switch goes


08-02-2011, 09:45 AM
The A-type carries full hydraulic pressure all the time, which gets dumped more or less instantly to the pistons when the solenoid pulls in. The J-type does not carry full pressure all the time, so after the solenoid moves, the little one-lung hydraulic pump has to build pressure to operate the pistons. And since it gets only one stroke per driveshaft revolution, it can take quite some time to build pressure at 2nd gear speeds. The OD clutch will slip from the time the pressure overcomes the return springs, until it is high enough to lock the OD clutch; which could lead to rapid wear if you ever shift under power (either intentionally or accidentally).

IMO it would handle the torque OK (of a relatively stock TR6 motor), once full pressure was built up. If not, it's a fairly easy matter to shim up the pressure without removing the unit from the car. But you would need to be careful to always wait for it to build pressure before applying throttle, which to my mind would more or less defeat the main advantage of having OD in 2nd.

If your cover is not drilled for a reverse switch, then you'll have to add one. Check the reverse fork, but I believe most if not all of them had the ramp on the back to operate the switch. Check the hole location carefully, make sure it is centered over the shaft and the correct position fore/aft so the ramp will pass the switch as you shift from neutral to reverse. Then it's a simple matter of drilling and tapping to the rather unique thread involved (might be M16x1, but I don't recall for certain & don't have my materials handy to check).

08-02-2011, 12:55 PM
Thanks guys,
Sounds like I need to leave the 2nd OD disconnected. I drilled the plugs on the side to find out if there is a ramp for the backup lights. Well, there is not so I'm thinking I'll mount a switch of some type at the reverse rod that will actuate when it's put in reverse.

Randall - Great description of the operation. I like it when someone knows and takes the time to explain.

08-03-2011, 12:43 AM
Just for clarity, I was talking about a switch on top, similar to the OD switches. Those plugs on the sides are the blanking plugs from drilling the passage for the shift interlock; not where the reverse switch mounts.

On the later cars with the reverse switch on the side, the reverse shift rail was also modified to operate the switch.

08-03-2011, 01:57 AM
I had according to a miss wiring the J-OD from 1st gear. I found no trouble with the OD and my TR has about 170 HP (which I rarely use at road traffic).
I think there is no problem with OD from 2nd on.


08-03-2011, 11:38 AM
Larry, I have my 75TR6 with J type wired for 2nd 3rd 4th.I did this back in 1990 when I restored this car and have had no problems with the unit to date.The main reason I did this is because when I built the engine I moded it considerably with higher compression ,performance cam etc. I use the 2nd gear od all the time in city driving to stay above the cam lope.
It shifts into 2nd od no different than any other gear under normal throttle application, however under wot it will chirp the tires when engaged.I don't believe this hurts the od unit but I do believe that in time you will tear up the diff. mounts if they have not been re-enforced. This I believe is the reason that Triumph dropped the 2nd gear switch back in the day.It may have been cheaper to blank the hole in the shift cover than to re-enforce the frame.
the j type was introduced in the early 70's and used in Volvo,Triumph,Vauxhall/Opel, American Motors,Chrysler and Ford Transit vans
The P type marked the last updates and included both a Gear Vendors US version and a Volvo version.Over a period of 40 years Laycock manufatured over 3 1/2 million units,over one million went to Volvo.
In 2008 Gear Vendors in El Cajon Ca. purchased all the assests of GKN to continue production of the US version and all spares for J and P types world wide(more info available at gearvendors.com)
Over Drive spares in the UK was founded in 1986 by Ernie and Stella Garbutt. Ernie was an Ex-Laycock engineer with Laycock DeNormanville .When Laycock went into liquidation,Ernie was able to purchase all the company's obsolete stock ensuring that most of the parts supplied nowadays are still original Laycock parts!
Sadly Ernie passed in early 2007,but his wife and son in law are still very much involved with O/D SPARES(google this for more info)

08-03-2011, 11:58 AM
forgot to add, the only change I made is to the original OD switch. I mounted a switch in the dash plinth as per pics.

08-03-2011, 12:17 PM
Here's some pics of a spare box i had in the back of the garage, which i removed from a wrecked 75 tr6 a long,long time ago. i believe the top to be original to the box with a plug installed for 2nd gear switch, even the harnes has wiring connections for the 2nd gear switch!
pic 1 with plug removed,pic2 indent in shift rod vaguely visible for 2nd gear switch,pic3 od plate numbers pic4 casting date

08-03-2011, 12:34 PM
Years ago I purchased a 75 TR6 with overdrive. The guy I purchased it from said it had overdrive in 2nd, 3rd and 4th gears. He called it a 7 speed. I thought he didn't know what he was talking about until I drove the car. The 2nd gear OD actually worked pretty good. He also added a small led light on the dash which was wired between the OD switch on the column and the transmission so you knew for sure when the OD was turned on. The car is now my parts car and I'm thinking about putting the trany in my 76 if the heat ever breaks here.

08-07-2011, 08:38 PM
Thanks all. I think I'm going to keep the 2nd OD and try it. I can always disconnect it. As for the need for a reverse light switch, I believe trying to drill and tap a hole on the top is more than I want to tackle. I've decided to just mount a limit or micro switch on the reverse slide below the shifter.
The light on the dash is a good idea. Especially for someone forgetful like me.


08-07-2011, 08:46 PM
I want o/d on reverse. I'd like to be able to get out of trouble as quickly as I got into it. :laugh:

08-08-2011, 12:33 PM

You will need to drill a small hole in the front of the early topcover if you are going to use it with your late TR6 J-type transmission, otherwise you'll blow oil out of the seals. The hole is a vent hole and you can see it in 2nd picture of the transmission above. It is the small hole on the top front left of the topcover near the bolt head. Early non overdrive transmissions were vented in the tail piece and the A-type overdrive has a vent on the top. When they switched to the J-type overdrive, which is not vented, they moved the vent hole to the topcover.