Overdrive Throttle Switch

[GregW]

I’m starting to think about the linkage for my new carburetors. One of the things I don’t know is how the throttle switch for the overdrive works (my carbs were off when I got the car). I have read the shop manual and I can adjust the switch once I’ve fabricated everything. My question is: with the throttle closed, is the arm of the overdrive switch down and gets pushed up as the gas pedal is depressed? It may be tricky figuring out what 1/5 throttle is on the Mikunis.

 

[Keoke]

No Greg, the arm is up and gets pulled down when the gas pedal is depressed.---Fwiw---Keoke

 

[Dave Russell]

Hi Greg,
See if the attached sketch helps.
D

 

[GregW]

Thank you both,
That’s good news! That actually makes things easier for the design. I have to make a longer lever for the pedal (3.25” radius). I should be able to attach the overdrive switch somewhere between the pedal shaft and the carb cable on the lever. One last question I should have asked earlier. Any idea how may degrees of rotation the overdrive switch is supposed to have? Is it less than it’s unconnected full range of travel? Given that figure, I can sort out where on the pedal lever I need to locate the linkage for the overdrive switch.

Thanks again,
Greg

 

[Keoke]

Hi greg, the arm on the switch with the pedal at rest its centerline just about lines up with the top edge of the switch cover.When the pedal is fully depressed the centerline on the arm just about lines up with the top edge of the switches upper exposed terminal. I think the total travel of the arm in eyeball degrees is about 90, which is less than its unconnected full range of travel.---Fwiw---Keoke

I think this original setup in the BJ8 car was established to insure that the interconnecting linkage did not bind.

 

[Dave Russell]

Greg,
As you can see from the drawing, the first part of the travel to open the switch contacts is the only important part. I think the cam normally rotates about 55 degrees from idle to full throttle. It is actually free to rotate 90 degrees or more. The travel isn't critical as long as the switch opens at light throttle.

In reality, this switch is only to prevent drive train jerking if you shift down, out of OD, with the throttle closed. If you just applied a little throttle when shifting out of OD there would be no jerk in the first place. Even if you don't, the jerk is not severe.

What I'm saying is that you don't really need this switch in the first place. You could leave it out altogether & not have any real problems. I have done this on a V8 engine conversion (with the original OD transmission) with no ill effects at all. This would simplify things if you are not going for concours correctness. I think this switch is a bit of unneeded fluff with a reasonably knowledgeable driver.
D

 

[vette]

Hi Greg, Along with the thought as Dave Russel was saying, My throttle switch is not hooked up. I studied it when I firs bought the car and decided just not to use it for now. As far as the range of motion is concerned for your throttles on those wild carbs you've got, the intent is that the od not be shifted in a decelerating mode. When you do use the switch just adjust it so that there is just a small amount of throttle (acceleration) on the drive train. this avoids a harsch jerk to the trans and driveline. I understand that the configuration of the od doesn't take well to harsh jerks. With my car, I just control the od totally manually with the dash switch. During up-shift manuvers I am aways in acceleration, and during down-shifts of the od I just do a small soft Blip of the throttle. Coming off the freeways onto off ramps with stop signs sometimes causes alittle heal/toe manuver to bring it out of od, but that's why we drive Healeys.

/ubbthreads/images/graemlins/cheers.gif

 

[GregW]

Hi Keoke,
You’ll be happy to here that your inner protractor is doing just fine. Using your reference marks, I came up with about 85° of rotation, which I calculate to be about 3” out from the pedal shaft. If I was an Optometrist, I’d say “Have fun, I’ll see you next year” /ubbthreads/images/graemlins/grin.gif

 

[GregW]

Hi Dave and Vette,
Thanks, that’s good to know. I’m not familiar with the finer nuances of the overdrive. My previous LBCs didn’t have one. It sounds similar to shifting my truck into neutral without the clutch. A little throttle (just enough to unload it) and the tranny pops easily out of gear.

 

[Ed_K]

Dave,
I put your BN2 overdrive wiring diagram on the knowledge
base.

 

[John Loftus]

Greg,

Just don't do what I did when I first got my Healey which had the throttle switch disconnected. I thought, "this electronic overdrive is cool .. just flip it off and it's like downshifting"! Luckily, a mechanic told me that this was not a good thing to do and stopped this practice before I broke anything.

Cheers,
John

 

[Dave Russell]

[ QUOTE ]
Dave,
I put your BN2 overdrive wiring diagram on the knowledge
base.

[/ QUOTE ]
Hi Ed,
Thank you very much. The later models are very similar with only a minor wiring change for the 12 volt source.
D

 

[Editor_Reid]

[ QUOTE ]
I think this switch is a bit of unneeded fluff with a reasonably knowledgeable driver.

[/ QUOTE ]

Adding a layman's perspective, I've always thought that it was kind of complicated for what it did. I mean, there's nothing stopping you from snatching the gearshift lever out of fourth gear and jamming it into second gear, jolting the drivetrain and possibly overrevving the engine. There's also nothing to prevent you from trying to jam the gear lever into first gear while still moving. But we don't do those things because we know how to operate the machinery and we don't want the repair bills that result from abusing it.

Same applies to the overdrive. If you know how to operate it, and you don't like repair bills, then you take it easy on the machinery. Why we need a switch to prevent us from abusing the overdrive, and don't need some mechanism to prevent abusing the gearbox, is not completely clear. Why no rev limiter on the engine?

Anyway just musing. I agree with DR.

 

[jollyroger]

The overdrive was always the weak link in the otherwise truckish system. That switch was there to keep save the OD under hard loading. I race with guys who use OD in all the forward gears. They just make sure that they do things sorta smoothly.
The OD was used on the LeMans sprites in the sixties. They failed alot in the endurance department.
I think that driving without the switch is fine for most folks. That is my goofy view.
Anyone who questions my intelligence is correct. The proof is, I do race a Sprite.

 

[Dave Russell]

[ QUOTE ]
The overdrive was always the weak link in the otherwise truckish system.

[/ QUOTE ]
Maybe so -- In my experience the Healey 4 speed & the type A overdrive are some of the strongest available. I used a BN2 transmission, OD, & rear axle behind a small block V8 in a BN2. No signs of a problem. I think the type J OD may not be quite as strong.

As far as down shifting the OD with the throttle closed, how many folks would repeatedly shift from fourth to third & let the clutch out with the engine at idle? Same thing. Maybe I shouldn't ask.
D

 

[GregW]

Well, I finished the pedal lever last week. Thanks to everyone’s help on dimensions, which direction it should move things, etc. Went into work an hour early for a few days (add that on top of a 12 hour work day) to use the heavy machinery. Next to do is cable from there to the carbs. Woo Hoo. Here is a top and bottom view (I only made one) Of the two small holes, the left is overdrive throttle switch, the right goes to the carbs.

 

[Keoke]

Now them are right pretty Greg and seem to be just about on the money.---Keoke- /ubbthreads/images/graemlins/thumbsup.gif

 

[BUNDYRUM]

The O/D throttle switch was very useful on the early BN1 cars as it was used in conjunction with the centrifugal switch mounted in the rear extension housing of the O/D. The centrifugal switch prevented O/D from being engaged at anything below about 35mph whereas the throttle switch prevents operation below 1/5 throttle opening. Personally i have never bothered with the throttle switch as it was easier to bypass it than to try and figure out how it worked and set it up especilly when the throttle system used to have all the wear in the joints and linkages. I've never had any O/D problems caused by a lack of the switch. If you're originality minded then it appears you have plenty of help in that department. When you move the arm of the switch you should be able to hear when it operates and that will tell you if you have it setup right. Have to agree with Reid also. (It's nothing to do with "The Rolling Stones") but it's also called "symathy for the devil", if you don't have any sympathy then it's a devil of a thing to have to fix. Regards, BUNDYRUM.