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Some Basic Overdrive Questions

Drone Dog

Jedi Warrior
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i have never had a car with OD before so i was not really sure what to expect. i have read how they should operate and i believe mine to be operating as it should. it is certainly easy to look down and see you are running 65mph without even thinking about it.

i have also read posts on putting the car in OD and taking it out. i feel good about that.

but i wondered about how i actually have been using it.

so far if i feel like i am going to be running over 50 for more than a mile, which takes me about 2500 rpm, i tend to want to flip it up in to OD. it is funny but the engine seems quieter at 2700rpm in OD as opposed to 2700 rpm not in OD. maybe the extra speed just carries the sound away more or maybe the little extra load makes it seem quieter.

i have also pulled up to stop signs and forgot to take the car out of OD. then when i get back in to 4th gear and think about putting it in OD, i see i am already there.

then there are times i am just cruising on a back road at say 50-55 and drop the car in because it just seems to run there so easily. almost like a high idle.

so is there a harm to shifting the car in to OD for a mile or so? or better to run the extra RPM's out of OD?

does it hurt to leave the car in OD when you don't really need it?

just curious if there are some do's and don'ts when using the OD system? or a consensus on when and how much to use the system?

Thanks and if this is covered in another thread, sorry... maybe just show me how to get to it.


Jedi Knight
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If your system is working correctly it will drop out of OD when you downshift to 2nd. It will go back into 3rd OD as soon as you put it in 3rd if you have not turned off the O D switch. This makes for a less than perfect upshift in my book. I'd rather be in regular 3rd when going up through the gears and go back into O D at about 40 mph. I don't think it does any real harm except it might be a little hard on the OD clutch lining to do it as you described. I like to come off the gas just as I move into OD for a smoother transition. For good reason, folks who race may do things very differently but that isn't me. The only "harm" to going in overdrive for short distance is normal wear and tear on the parts that are ablated would be accelerated. But hey, you should drive it how you like it and fix it when it breaks..


Darth Vader
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As you have probably read already, you can put the OD in or out by using the clutch or you can put it in or out while constant acceleration without the clutch. Both work and neither harm. But don't shift out of OD without the clutch while decelerating or coasting, it puts an outrageous jolt to the drive line. As long as the clutch is in you can shift the Od with no harm. That includes while sitting still at a traffic light. So if you forget to take it out of OD when coming to a stop and while at the stop you notice that it is still in OD, with the clutch pedal down just flip the switch off, no harm done. If using the clutch then the scenario is just like shifting a 5 speed tranny. Your just moving the little switch on the dash instead of pushing a shift lever in and out of 5th. I've found that if using the clutch then the mental process is just like you were looking to engage or disengage the next gear. Just like a 5 speed. There's no way you can hurt it that way. One firm rule to practice is to not put the OD in and at the same time select reverse gear. Although the safety feature of your 3/4 shifter selector switch will keep the OD from being energized, it is good practice to remember this rule. The Od doesn't care what rpm you are running so you can lug it or scream it, no harm done. You can decide how slow you want to run the od based on how your engine likes slow speed running. A good engine in good tune should have no issues with running it slow. The OD should operate for many, many, many, many thousands of miles with no breakdowns so operate it as often as you like.
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I recently overhauled the OD in my BJ8. Between learning how the OD works, and reading some articles by pros, I've changed my technique for engaging it after having driven it one way for over 130K miles. Previously, I would just engage the OD while accelerating or cruising and feel the seat-of-the pants rush; now, I use the following technique:


The OD is engaged by hydraulic pressure--switched by the solenoid--pushing a couple of pistons against some really strong springs to bring a cone clutch into contact with an iron brake ring, locking the sun gear and allowing the planetary gears to rotate about it and drive the annulus. If you engage the OD while in gear, the cone clutch has to fight engine torque, which causes it to slip a little before it's stopped by the brake ring, causing a bit of wear. My cone clutch still had plenty of 'meat' on it and the only replacement I could find was a rebuilt from DWM which was expensive and requires the core be shipped to them. If you disengage the engine clutch whilst engaging the OD clutch not only should wear be reduced, but the OD should engage more quickly. I installed the uprated accumulator from DWM, and the OD engages almost instantly even without disengaging the engine clutch, so the OD engages quicker than I can depress the clutch pedal, throw the switch and let the clutch pedal out.

To disengage the OD, I press the clutch pedal in about a third of the way and blip the throttle. Most of the time, I can do it without a noticeable lurch in the drive train.

I'm having big fun with this:



Jedi Warrior
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G'day Drone Dog,
If your 'top gear switch is functional, your overdrive will disengage when the shift lever is moved across the gate, away from the 3rd/4th position. At rest, the o/d should not engage no matter where the dash switch is. This protection is meant to avoid o/d being engaged at the same time as reverse, which can lead to disaster. (Don't ask me how I know that!)
If your throttle switch is functional the o/d will stay engaged after the dash switch is moved to 'normal' until the accelerator pedal is 'blipped'. This is designed to protect the drive line from the jerk that Vette described. It can work like kick-down in an automatic transmission.
I rarely use the clutch when engaging or disengaging o/d but there is no harm in doing that. As Roscoe says, feathering the gas assists in a smooth upshift.
When I had the standard diff ratio in my 100 (4.125:1) I often found myself using overdrive for short distances but since fitting a 3.667:1 ratio I don't feel the need.
Drone Dog

Drone Dog

Jedi Warrior
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Thanks for the info.
i have been engauging the clutch both in and out of OD. when going out, i usually also give a little rev to the engine to release the kick down switch.

i just was not sure if there are any other limitations or bad practices i should be aware of.

so basically treat it like it is a 5th gear.
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"Over drive" means the engine runs slower than the forth gear RPM. Forth gear should run the drive shaft at the same speed as the engine (in theory). (third gear is a power gear running the engine at higher RPMs than the drive shaft giving more power)The overdrive should allow the drive shaft to run faster than the engine but with less power. It will lug the engine and put strain on it when you try to accelerate with the engine RPMs too low. It is designed for cruising at higher speeds to improve fuel economy (and noise). Generally I don't put over drive in until over 60mph.
Okay, fire away but this is what I remember form high school auto shop (which I aced).
Drone Dog

Drone Dog

Jedi Warrior
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i am not arguing wiht what you say...
i think my car runs 2500 rpm in OD at 60 mph. so you try not to use the OD under 2500 RPM?

John Turney

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Because OD also works on 3rd, on windy roads where I would be frequently shifting between 3rd and 4th, I shift in and out of OD while in 3rd. (3rd to/from 3rd OD).

Healey engines have a lot of low-end torque, so one can cruise at 2000 RPM, but if you want to accelerate at any significant rate, one should downshift.
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