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overdrive lession required

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H

Hoghead

Jedi Trainee
Offline
What is proper overdrive operation procedure

The owners manual is of no help and 40 years ago I was taught to back off on the throttle when engaging and kick the throttle when disengaging - is this the recommended method of operation?

Any risk if/when you forget to disengage when downshifting to second?
 
Hoghead said:
What is proper overdrive operation procedure

The owners manual is of no help and 40 years ago I was taught to back off on the throttle when engaging and kick the throttle when disengaging - is this the recommended method of operation?
Well this will keep you out of trouble.

Any risk if/when you forget to disengage when downshifting to second?
Click to expand...
 
What works for me:

- to engage O/D, throw the switch on the panel
- to disengage O/D, slip the clutch just a teensy bit, and simultaneously blip the throttle (it is a downshift, after all). Done just right, there is no lurch whatsoever in the drive train (also, the 'kickdown' switch will not let the O/D disengage if the throttle isn't opened a certain amount; reverse torque is to be avoided).

Can't say this is 'correct,' but my O/D has 180K miles and is going strong.
 
Hoghead said:
...

Any risk if/when you forget to disengage when downshifting to second?
Click to expand...

There is a position switch in the gearbox that won't allow the O/D in second or first. IIRC, if you leave the O/D engaged and downshift into second, the O/D will remain engaged until you open the throttle the aforementioned amount. Since you're downshifting you'll be goosing the throttle and the O/D will disengage, even if the panel switch is still in the 'O/D' position. If you don't goose the throttle on the downshift the O/D will disengage next time you do.

I'd have to study the schematic to remember how it works, but IIRC the panel switch and the gearbox 3-4 switch are in series, so both have to be closed to engage. The 'kickdown' switch offers an alternate circuit to the O/D solenoid AFTER it's engaged, until the throttle is opened the required amount. It's all done to prevent large changes in torque amount or direction from damaging the O/D, specifically the roller unidirectional(?) clutch.
 
Bob, The 1-2 switch disengages the OD without the need to blip the throttle. It cuts the power to the solenoid and the throttle switch -- effectively bypassing it. The throttle switch only comes into play if the gear shift is in 3rd or 4th. There is no need to disengage OD (with the dash switch or throttle switch) when shifting down to 2nd or 1st -- the gear shift switch does it for you.
 
If you intend to go out of over-drive, turn the switch to "normal" position (off) and you can drive for a while in over-drive until you push the accelerator (gas pedal) about 20%. Should be not bucking or surging which will shorten the life of the gear box. This is great for getting off the freeway. You should not be feeling anything but lower RPMs when engaging the overdrive. If you notice anything else, you have to adjust the over drive unit on the fire wall. This is a real slick system for its age.
 
At this point, I agree with almost all that has been said.

Coming out of OD requires the flipping of the OD switch and an increase in engine RPMs to approach, or increase, the speed of the car when the OD is disengaged. In short, the OD should be disengaged on an increasing RPM and not when it is falling.

Hope this helps,
Ray (64BJ8P1)
 
TimK said:
Bob, The 1-2 switch disengages the OD without the need to blip the throttle. It cuts the power to the solenoid and the throttle switch -- effectively bypassing it. The throttle switch only comes into play if the gear shift is in 3rd or 4th. There is no need to disengage OD (with the dash switch or throttle switch) when shifting down to 2nd or 1st -- the gear shift switch does it for you.
Click to expand...


Thanks for the correction ... the schematic gives me headaches.
 
tahoe healey said:
If you intend to go out of over-drive, turn the switch to "normal" position (off) and you can drive for a while in over-drive until you push the accelerator (gas pedal) about 20%. Should be not bucking or surging which will shorten the life of the gear box. This is great for getting off the freeway. You should not be feeling anything but lower RPMs when engaging the overdrive. If you notice anything else, you have to adjust the over drive unit on the fire wall. This is a real slick system for its age.
Click to expand...

Or, if you're stuck behind a slow-arsed Ferrari or Lambo you can throw the switch and when you get a chance to pass just stomp on the throttle and bingo! ... automatic downshift to passing gear (aka 'kickdown').
 
The nuts and bolts of what is going on in the OD:

there is a double sided clutch, one side for OD is engaged by 390psi oil pressure and has no problem overcoming the engine torque. The other side when returning to normal drive is re-engaged by the springs that oppose the oil pressure and they are not strong enough to hold the engine back-torque on overrun.

The throttle switch was added so that the driver can't ever end up in a situation where high engine braking is required when the OD switch is turned off as this overloads the direct drive clutch. Once the clutch has engaged however, the static friction is enough to hold any loads from the engine.

If OD is de-selected when the throttle is open (kickdown) then there is a sprag/one way clutch to back up the direct drive clutch and this takes over if it can't handle the torque.

So in summary there are four ways the OD works:
upshift open throttle- 390psi oil pressure is stronger than the engine torque
upshift closed throttle- same as above
downshift open throttle- sprag clutch backs up the main clutch so it can't slip
downshift closed throttle- no mechanical solution so have to rely on the OD relay to disallow engine braking while disengaging OD.

Andy.
 
I've always thought the high oil pressure is used to force two pistons to overcome some rather strong springs to force the main, cone clutch to disengage, locking the sun gear and allowing the planetary gears to be rotated in their carrier around the (now) stationary sun gear, causing the annular gear to overrun the planetary gears and their carrier--hence 'overdrive'--and increase output RPM.
 
Hi Bob,

I guess 'strong' is a bit of a vague term. It takes about 120psi to unseat the direct drive clutch against the springs. You can test this by fitting a pressure gauge and turning the output shaft backwards by hand (with OD lever engaged) until it locks up. this is the point where the OD clutch is just engaging and begins to fight the sprag. So that means the direct drive clutch is effectively engaged by 120psi and the OD clutch is effectively engaged by 390-120 = 270psi.

A better way of saying that is the force keeping the OD engaged is 2 1/4 times the force keeping direct drive engaged.

Andy.
 
The parts manual shows a (cone-shaped) clutch and a unidirectional clutch (essentially roller bearings on a ramped inner race). It takes oil pressure to operate both? I don't see both an O/D clutch and a direct drive clutch--what am I missing here?
 
The cone clutch has lining material on both sides, these are the OD and direct clutches depending on whether the clutch is pushed forward by the oil pressure or backward by the springs. The sprag/unidirectional clutch works in tandem with the direct drive clutch to hold the engine power but it can't work in the back-drive direction for obvious reasons so the direct clutch and springs have to go it alone for engine braking.

The earlier style of OD was by Borg Warner and it has no clutches at all, only a solenoid operated peg that jams in a locking ring. there is no engine braking when in direct drive as the unidirectional clutch just freewheels and you also have to back off the throttle to shift into OD. There is also no switch to engage OD, it is done automatically by a flyweight governor at about 30mph. I have one of these on my Ford.

Andy.
(procrastinating on writing up a whole heap of paperwork)
 
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