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TR6 J-Type or A-Type Overdrive for 1972 TR6

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R_Lyle

R_Lyle

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I've got a late 1972 TR6 (Aug '72) and I have the opportunity to buy an overdrive transmission (the transmission and the overdrive unit) for it from another British car enthusiast. What I'm wondering is which transmission should I buy: the one with the A-Type or J-Type (he has one of each). The seller tells me that he thinks I need to use the one with the A-Type since there is some difference with the spline (the 1969-1972 had the A-type while the 1973-1976 used the J-type). What will work with my car? Is the J-type any better or mor reliable then the A-type?

1972 TR6 cc84566
 
The splines aren't so bad to deal with (you just have to find a flange from a later TR6 w/J-type) but the rear mount looks like a pain to deal with. The J-type goes right through where the early rear crossmember went, and it's mounting point is maybe 5" farther back than with the A-type. There's a good article at https://www.buckeyetriumphs.org/technical/jod/AdaptingJOD/AdaptingJOD.htm that outlines one good method of dealing with it.

IMO the J-type is an improved design and will likely last longer. There are also more ways it can be serviced in the car, and parts are more readily available (I picked up an entire unit for spares, for about $130 on eBay). But it shifts slower than an A-type, which IMO is less 'sporty'; and because the shifts are even slower at low speed, it's normally not setup to work in 2nd gear. Given the choice, I'd go with the A.
 
It was my understanding that the J-Type had a different mounting point on the rear of the transmission so you had to build (or buy) a bracket. I found some directions on the internet on how to build the metal bracket so that is not a problem. What I worry about with the A-type is the solonoid switch which is so expensive to replace. I was not aware of the different flange situation.
 
<span style="font-size: 26pt"> <span style="color: #FF0000">A</span></span>
 
R_Lyle said:
What I worry about with the A-type is the solonoid switch which is so expensive to replace.
Click to expand...
Huh? You lost me there; the J-type solenoid is more than twice the price of the A-type! See for example https://www.zeni.net/trf/TR6-250GC/158.php

That's reasonable of course, as the J-type soleoid is more complicated (it includes a hydraulic valve while the A-type is just a solenoid).

BTW, I feel it's a worthwhile modification to add a fuse to the circuit for the A-type, to protect the solenoid in case it hangs up for some reason and the contacts don't open.

A-type solenoids have two coils internally, the holding coil only draws about an amp and can run forever; but the pull-in coil draws about 20 amps and normally is only energized for an instant. When the plunger reaches it's final position, it pushes against some internal contacts that open the circuit to the pull-in coil. If the plunger hangs up for some reason, or the little plastic pushrod for the contacts breaks or falls out; the pull-in coil will quickly overheat. By using a fuse smaller than 20 amps, the fuse will blow first and offer at least the potential for fixing the problem with the solenoid.

Stags actually came with such a fuse, but TRs did not. On my 3A, I used an 8 amp "slo-blo" just because that's what I had on hand, but I believe an ordinary 5 or 10 amp fuse would do fine.
 
Yes I see that you are correct on the solenoid cost (A-type is much cheaper). I also checked on Quantumechanics web site to see what the cost of the wiring kit and other parts needed to add the A-type overdrive (The Roadster Factory also lists a bunch of parts). Not sure if I have the complete list here but it appears you need the longer speedometer cable, right angle drive, plastic trim on the steering column, two wiring harnesses, and a relay. Any instructions out there on how to do the wiring? In the final analysis I think the A's have it.
 
R_Lyle said:
...it appears you need the longer speedometer cable, right angle drive, plastic trim on the steering column, two wiring harnesses, and a relay...
Click to expand...

On a TR4 you need either the longer cable or an angle drive (not both) perhaps the 6 is similar? You will also need a switch. Again I only know a TR4 which may differ but I modified the existing non-OD nacelle to take the OD switch (had to make a D-shaped hole).

I also added a few holes in the tunnel to simplify service and maintenance. The oil change hole takes a standard rubber plug, the access for the solenoid and on the opposite side for the angle drive and adjustment (not shown) are covered with sheet metal plates.

Tunnel%20Holes.JPG
 
I asked the guys at Quantum Mechanics to see what they thought and their opinion was the J-type uses 1970 technology while the A-type was 1948 technology. Their feeling is the J-type is a better design and that I just needed to build the rear mouning bracket to make it work in my 1972 car.
 
Basically what I said. But you'll never know what it feels like to flip the switch at about 4500 rpm in 2nd and have the front suspension pop up again while an unseen giant boot kicks you an extra car length down the road.

And the technology ain't all that different, it's still a 2-speed planetary transmission behind a hydraulically operated cone clutch. Biggest difference is that the A-type carries pressure all the time for those instant shifts; while a J-type has to 'think' about it while the 1-lung pump (running at driveshaft rpm) builds up enough pressure/volume to move the clutches.
 
You should be able to put either one in your car. I have a 73 TR6 and was able to get an A type and put it in my car. Do I use it in 2nd gear? Hardly ever. I mostly use it on the highway and it sure is nice.
 
<span style="font-weight: bold"><span style="font-size: 26pt"><span style="color: #FF0000">J</span></span></span>
 
TR6BILL said:
Why does one have to edit a "<span style="color: #FF0000">J</span>?"
Click to expand...
To make it big and red?
 
<span style="font-size: 26pt"><span style="color: #FF0000">HVDA</span></span>
 
<span style="font-family: 'Arial Black'"><span style="font-style: italic"><span style="text-decoration: underline"><span style="font-weight: bold">lol'</span></span></span></span> <span style="font-size: 26pt"> </span> <span style="color: #6600CC"> </span>
 
What you need to check since your 72 was built near the break from A to J is weather you have the tabs welded to the lower frame flange to bolt on the J style
crossmember.They may be there or not.If there, get the J type.IMHO the sweeter of the two.
MD(mad dog)
 
Bought the J-Type transmission. Now I need to install it. Does anyone happen to know if there is a wiring diagram out there? I can get the parts from TRF or Quantum Mechanics but I don't think I know how to do the wiring.
 
R_Lyle said:
Does anyone happen to know if there is a wiring diagram out there?
Click to expand...
It's pretty basic, as the J-type doesn't require a relay and has only one lockout switch. One source is
https://www.advanceautowire.com/tr2506.pdf
Scroll down to the page for a 72 TR6 (page 4), and look for the box with the OD wiring at the upper RH corner.
 
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