OK, I have always heard, and always repeated, that a J or D type overdrive
should NEVER EVER be driven engaged in reverse, even for a short distance.
The story was always told to me that even a few feet could result in a
catastrophic failure. And I believed it, emphatically. It had something to
do with the planetary gears rotating the wrong way or something, and
splitting the case. At least, that is how it was explained to me.
NowI aint so sure. I hope someone can show me were I am wrong here, but I
see no reason for the OD to be damaged if the solenoid is somehow left
engaged and the car driven in reverse. Not that I think this would be a
good thing, mind you, I just dont think the OD world would end immediately.
Why do I say this?
Recently I had an issue with a J type OD that would not come out of OD. The
solenoid worked fine, but the thing would not drop out of OD until my speed
dropped below about 25 MPH. A fluid change and a filter cleaning didnt
help. So I was forced to actually think about the problem a bit, and try
and figure out how the OD unit works.
Both types (J and D) operate the same basic ways. There are differences, to
be sure, but they are similar. The cone clutch is driven forward against
the clutch springs by hydraulic pressure. This pressure is generated by a
pump piston, which is driven by a cam on the output shaft of the gearbox.
It can be viewed that the solenoid is a valve that allows the pressure from
the pump to flow, and thus move the cone clutch. No pressure no cone clutch
movement, and thus no OD action, the OD unit is just a straight through
drive. Just like it is every time the solenoid is off, and you select 1st
gear or reverse (and 2nd gear also on some cars).
Now, pressure is the key. It takes a bit of pressure for the system to
work. Remember that 25 MPH figure above? It turns out (with a 3.89:1 rear
end) that to generate the required pressure (something over 350 lbs/sq in)
the output shaft of the gearbox (and so the input of the OD) must be moving
about 1500 RPM. Now remember this is not engine RPM, but instead it is
drive shaft (output shaft of the gearbox) RPM. Drive shaft RPM is solely
dependent on the speed of the vehicle (for any given drive axle gear ratio),
not on engine RPM or gear selection.
Hmmmm.how many engine RPM would you need to pull to get 1500 drive shaft
RPM? In my 77 Spit (the car that started this inane prattle) reverse has a
ratio of 3.99:1. So to develop enough pump pressure to engage the OD I
would have to pull about 6000 RPM in reverse. Actually more, since my rear
axle ratio is not the stock 3.89:1, I have swapped in the Mk IV 4.11:1 unit.
So even if your OD is stuck engaged, how often would you pull the 6000
engine RPM needed in reverse to make enough pressure to activate the cone
clutch?
Or have I got this all wrong?
Darrell, in the High Desert of California
62 TR-4 CT 5368 LO (daily driver)
62 TR-4 CT 10440 L
62 TR-4 CT 13108 L (long term project)
65 Spit FC 51603 L
67 Spit FD 2890 L (daughters car)
77 Spit FM 62888 UC (other daily driver)
79 Spit FM 98233 UC (wifes car)
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