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Using overdrive.

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robert_ellison

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What is the correct procedure for using the overdrive. I have a BJ-7.

Is there a certain speed in which to engage it? It there a specific procedure? At what speed should the overdrive be disengaged?
 
I think you have some leeway with its use. While my car doesn't currently have overdrive fitted, it used to (and I've had several MGBs with ovd).

I treat it just like another gear, let the RPMs drop on the upshift, and goose the throttle to raise the RPMs on downshifts.

I didn't use the overdrive until I was upwards of 50 MPH (and expecting sustained speeds), but would occaisionally run 3rd/3rd ovd on curvy mountain roads, instead of shifting 3rd/4th (I didn't always live in flat Ohio/flat Louisiana, I bought the car when I lived in San Francisco).

I suppose the most important thing, is not to disengage overdrive at a speed that exceeds the RPM limit in the direct gear (be it either 3rd or 4th), and to let up on the throttle slightly upon overdrive engagement.

While they're robust, for the most part, you shouldn't put undue strain on the unit.

Above all else, DO NOT EVER attempt to drive in reverse if there's any doubt that the overdrive has not disengaged. Make certain that the gear-selector switch that only allows overdrive use on 3rd/4th is operational.

Little known fact: if the 3rd/4th selector switch is getting worn out, you can put some pressure on the shift lever (pulling it towards the driver in a LHD car) and get a "temporary" kickdown, or passing gear.

Sometime in the near future, I'll be rebuilding my side-shift/ovd xmsn (all the gearbox internals are already prepared with polymer dry-film lubricants & thermal barrier/thermal dispersant coatings), so expect a pictorial essay on the subject.
 
Don't use it in reverse.... absolutely.'
Spot on !!!
 
I agree with everything Randy said. But I do use slightly different speeds. With all the work it took me to get my overdrive operational I tend to baby it. I tend to engage and disengage at around 40 to 45 mph. I let up on the throttle just before I flip the switch and then back down when it is engaged. I also will frequently remain in OD at speeds well below highway speeds if I can sustain at least 30 to 35 (I'm in flatland mostly). I will come out of OD if I am going to stop. If I forget, I'll usually just blip the throttle as I'm sitting in 2nd gear with the clutch in or neutral if Im stopped for more than a few seconds,before I resume moving to get it to kick out of OD. If I'm using it to down shift I'll come off the accelerator, flip the switch off in 3rd or 4th at around 45, and then just blip the throttle when I want it to down shift. Lots of fun and you get to use the word "blip", one of my favorites. I kind of wish I was doing this in the days when parts were plentiful and inexpensive, and my labor didn't require the use of Ibuprophen. I'm sure the folks who rally or race have a different outlook on using the overdrive.
 
I was always mystified why they chose not to have a mechanical reverse disconect like other manufacturers did.
It's not like there weren't other o/d units out there to look at.

I've got o/d in one of my other marques, leave it in o/d, below 26-28 the governor drops, relay releases the solenoid, it will stay in o/d until you let off the throttle, and it just free wheels.
Put it in reverse, there is a peg in the rear cover that is pressed forward when in o/d, reverse slider contacts this and manually removes the o/d engagement.

Just always surprised me, that's all.
 
Bob, You should learn at apprx what road speed and engine speed your car will do by just some leasurely driving around. Then know that the overdrive will decrease your engine rpms by about 400 to 600 rpms in 4th gear. I give you that range because it has been awhile since I've driven mine since it is apart right now.
So here is the gist of it. If your are running along in high gear at about 3000rpms and you upshift into overdrive, your engine rpms will drop by about 500rpms if you maintain your same road speed.
Now if you are running along in o/d and you slow to say 50mph and your rpms have dropped to the point that your might lug your engine to accelerate again, just drop it out of o/d and you have increased your rpms by about 500. Just don't get near your redline, unless you really like to push your engine.
One other point, never shift your overdrive without some acceleration on the engine. Your o/d can be damaged by allowing it to shift when in a deccelleration mode. Your throttle switch is suppose to preclude this happening. But if a throttle switch is maladjusted this situation could occur. You will feel a sever shock to the driveline if this happens. I have eliminated the throttle switch on my car. When upshifting, you are always in an acceleration mode so no problem there. And when I downshift out of o/d, I just apply alittle gas to the throttle as I hit the toggle switch. Makes a cool effect as I blip the throttle and knock it out of o/d at the same time. OOh that sound.
 
I believe that the overdrive will not change without pressing the pedal 20% if it is properly adjusted. This is designed to prevent damage. You should be able to shut it off and not get any shift UNTIL you do give it some pedal.
 
The purpose of the overdrive throttle switch is to keep the overdrive engaged if you are operating at less than about 1/5th throttle. Your overdrive should engage normally if you are completely off the throttle. What the designers wanted to avoid is coming out of overdrive at 70 miles an hour with no torque coming out of the engine to avoid traumatizing the gearbox. When you flip the OD switch off, the throttle switch keeps the relay coil supplied with power until you open it by depressing the pedal. You can go into overdrive with or without the throttle opened, as you wish.
 
Roscoe, I concur. The 3-synhcro MGB gearbox with a nearly identical, but slightly smaller Laycock overdrive, used a slightly different approach. A vacuum switch. The operating solenoid is nearly identical, if not interchangeable.

The later all-sync MGB xmsn with, I think, a type LH Laycock overdrive did not use any type of throttle switch, and I'm not entirely sure, without checking a schematic, if there was an isolation relay in the circuit. Engage at will, with only a 3rd/4th enable switch.

The last time an overdrive xmsn was fitted to my Healey, prior to restoration, I was using a T-handle (turn to lock) choke cable mounted directly on the shift lever to pull up the operating lever on the overdrive. It worked just fine. I caught lots of flack from the <span style="font-style: italic">Learned Elders</span> that I would easily engage reverse while the overdrive was called up, but the truth was, you couldn't put your hand on the lever without knowing-by-feel the position of the cable. At that time__mid 80s__I had already rebuilt dozens of overdrives (including all the overdrives brought to our shop from XKs Unlimited).
 
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