Overdrive instructions

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tannyo
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Overdrive instructions

Post by tannyo » Thu Apr 15, 2010 11:36 am

1971 MGB Tourer and GT Handbook wrote:The overdrive unit, controlled by a switch on the steering-column, provides a higher driving ratio for use with third or fourth gear. To engage overdrive move the switch lever towards the steering-wheel; to disengage move the lever away from the steering-wheel. Accelerator pedal pressure should be maintained and it is not necessary to depress the clutch pedal during engagement or disengagement.

Overdrive can be engaged at any throttle opening when in third or top gear. If increased acceleration is required the overdrive can be 'switched out' without alteration to the throttle setting. Do not 'switch out' the overdrive when traveling at speeds exceeding normal third or top gear road speeds.

In certain driving conditions while traveling in third gear, the overdrive can be switched in to provide a top gear ratio or out to provide third gear acceleration without the necessity of manually moving the gear-change lever.

If for any reason the overdrive does not disengage, do not reverse the car otherwise extensive damage may result.
Any thoughts on the throttle position the handbook espouses?

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Re: Overdrive instructions

Post by Steve Simmons » Thu Apr 15, 2010 1:05 pm

The overdrive works on a clutch cone. You can engage or disengage at any throttle just like you could engage or disengage the clutch under any load. Just like the clutch however, the overdrive's clutch can suffer damage or premature wear from continuous abuse. It's generally good practice to adjust the throttle when shifting in and out of overdrive the same way you would if shifting through any other gears - lighten up the throttle to up shift (engage overdrive) and give it a bit more gas to downshift (disengage overdrive).

When downshifting with the stick, you give it more gas to match the RPMs and avoid "lurching" the car. The same goes for overdrive, only instead of avoiding lurching, you're reducing the shock on the unit from slamming into a lower gear ratio. This is especially important on D-Type overdrive units as found on Mark I MGB. In fact, these model cars came with a vacuum inhibitor switch which prevents the overdrive from disengaging while the engine is in overrun, or engine braking. The instructions you quoted above are obviously for a later car since A) the OD switch is on the steering column and B) there is no concern for the safety of the OD clutch by shifting down during overrun. These later "LH" model overdrive units are larger and more robust than the earlier ones so they can stand a bit more abuse.

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Re: Overdrive instructions

Post by tannyo » Thu Apr 15, 2010 2:10 pm

Yes the instructions are for the later overdrive unit that is in my 1971 MGB GT.

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Re: Overdrive instructions

Post by Jimmy » Fri Apr 16, 2010 7:06 am

Since the MG overdrives are basically what became GearVendors, they should be pretty stout and not require any babying.
However, Gearvendors recommends against downshifting its 30,000-lb. GVWR rated unit under deceleration.
I do like Steve and shift it like a regular gear, minus the clutch use, since that has to be easier on the friction material.
In baseball, running into someone is apparently a "collision".
But doing the same thing in a car somehow makes it an "accident".

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Re: Overdrive instructions

Post by tannyo » Fri Apr 16, 2010 9:34 am

So my question is does the overdrive need pressure (non-clutch use) to shift properly?

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Re: Overdrive instructions

Post by Steve Simmons » Fri Apr 16, 2010 10:02 am

No, it will operate whether or not the clutch is depressed. When you flip the switch, or in your case press the button, a solenoid activates the flow of oil through the OD unit. The pressure of the oil is what engages and disengages the OD. There are planetary gears and other things involved but the bottom line is that it will work just the same with or without depressing the clutch. Just for the record, some overdrive instructions, which I believe include those from Volvos which use J-Type OD units, specifically state to NOT use the clutch to engage or disengage overdrive. I don't know why they have this warning on those models.

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Re: Overdrive instructions

Post by Jimmy » Tue Apr 20, 2010 6:52 pm

Because if "nothing" is turning, there's no real oil pressure?
In baseball, running into someone is apparently a "collision".
But doing the same thing in a car somehow makes it an "accident".

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Re: Leycock Hydraulic Overdrive

Post by VWNate1 » Mon May 17, 2010 7:08 am

Often these units are run for many years & miles without any routine service then when it begins to stick in or out , the entireunit is blamed as faulty and sent out for unecessary expen$ive rebuilding when in fact all it needs is a routine service consisting of cleaning the magnet and screen and HOT draining the oil then re-filling with ATF , that'll dissolve the sticky deposits that hinder smooth & efficient overdrive operation .

Using this simple method , I was able to retro-fit my '67 MGB GT with a " junk " overdrive that in the end , shifted *perfectly* , didn't leak and (best of all) was FREE ! :bow: .

Shifting in or out , as mentioned , light throttle application is best as it makes for smoother transtion and doesn't beat up on it .
-Nate

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Re: Overdrive instructions

Post by Jimmy » Mon May 17, 2010 9:55 am

Hey, Nate, since regular oil seems to be the preferred lube, how about a 500-or-so mile cleansing with ATF, then reverting to engine oil?
In baseball, running into someone is apparently a "collision".
But doing the same thing in a car somehow makes it an "accident".

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Re: Overdrive instructions

Post by VWNate1 » Mon May 17, 2010 3:11 pm

As you wish .

ATF is the prefeered lube by factory trained BMC mechanics .

They suggested it and I tried it, *presto* I never had another problem out of it .

YMMV .

Try synthetic engine oils, they're very detergent too .
-Nate

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Re: Overdrive instructions

Post by Jimmy » Mon May 17, 2010 4:18 pm

How about some detergent mixed in with regular oil?
In baseball, running into someone is apparently a "collision".
But doing the same thing in a car somehow makes it an "accident".

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Re: Overdrive instructions

Post by Steve Simmons » Mon May 17, 2010 4:25 pm

Quantum Mechanics did a thorough test on fluids for Laycock overdrives. 30W ND came out on top. Here is a copy of their article:
I want to express our experience and opinion on the topic of the proper oil for use in the Laycock de Normanville overdrive units. We constantly get into discussions( and sometimes heated debates!) with British car owners on this subject. This month’s discussion will invariably get some interesting responses as it seems to be a rather emotional (and not necessarily logical) discussion item for British car overdrive owners.

Let’s start out with history. Historically even the recommendations from various car manufacturers are confusing. Some say to use multi grade oil (MG manuals), some hypoid oil (Triumph manuals), some non detergent oil and some even recommend automatic transmission fluid! No wonder everyone is so confused!

Many years ago (in a land far, far away (OK it wasn’t that far away or even that long ago)) we ran an experiment on overdrive oil. We rebuilt an A type overdrive unit and initially ran it with 30 weight non detergent motor oil. When spun up on our test bench at 1,000 RPM, it reached a normal pressure of 400 PPSI. When shifting the pressure dropped to 300 PPSI and quickly recovered to 400 PPSI. All was right in the world of overdrives.

We then drained the oil and replaced it with 10W30 multi grade oil. When spun on the test bench, initially it tested fine. However, after a few minutes of running, the oil pressure dropped to 300 and when shifting, to 200. Upon observation of the internals of the operating overdrive we found bubbles developing in the oil pump body and oil pump output passage. We surmised that the detergents in the oil were causing the oil pump to cavitate, and develop air bubbles as it pumped.

We then drained the oil again and replaced it with 90 weight hypoid oil. This time the oil pressure jumped to 600 PPSI! When shifted, the pressure dropped to 450 PPSI, which made the shift immediate and harsh. After a few minutes of running the oil pressure actually began to climb even higher. (Which made no sense since we thought the oil would thin out and the pressure would drop). We finally shut it off at 750 PPSI as we did not want to damage the unit. Even though the overdrive unit was now in the non overdrive position (solenoid disengaged), the overdrive was now stuck in overdrive and would not come out. The higher pressure had driven the sliding clutch member so hard into the brake ring that the clutch return springs could not return it to the non overdrive position. A tap on the brake ring with a hammer (the universal overdrive release tool), shifted it back into the direct drive position. After running a number of these test with the same result we found what was happening. The oil holes in the accumulator sleeve are very small. The 90 weight oil was so heavy it could not escape from the accumulator chamber as fast as the oil pump could pump new oil into it. So even though the accumulator piston had passed the oil hole relief position, the pressure continued to build up because the oil could not leave the system as fast as it was being pumped in. The accumulator piston actually bottomed out in the sleeve (similar to coil bind on valve springs). When removed we found the accumulator spring had been compressed and was no longer useable.

After replacing the spring, we then tried automatic transmission fluid. We saw the same results as we did when we used the 30 weight non detergent oil.

We then tried synthetic oil and the unit also worked OK although it began to leak from all sorts of places it had not leaked from before.

Based on these tests we have since and continue to recommend the 30 weight non detergent motor oil as the best oil to use in the overdrives.

Some other experiences with customer overdrives over the years have reinforced this choice. For example, we found a Jaguar compact overdrive with a broken accumulator piston and bent spring when it had been used with 90 weight oil by the owner. When the piston and spring were replaced and the unit filled with 30 weight non detergent oil, it functioned normally. A customer LH overdrive unit that was filled with 90 weight oil "pulsed" between direct drive and overdrive without even being switched on electrically. When the oil was flushed and replaced with 30 weight non detergent oil the unit worked normally.

Other noted problems with overdrives filled with 90 weight are excessive wear on the oil pump plunger wheel and the eccentric cam, probably due to the higher pressures developed. Also the clutch lining seems to be more deteriorated in units with 90 weight oil than those with 30 weight oil.

The use of non detergent 30 weight oil does not seem to affect the transmission parts or function. Bearings, synchros, gears and hubs do not seem to function any less effectively with the non detergent 30 weight oil as with 90 weight gear oil.

So there it is gang, our scientifically based rationale for using and recommending 30 weight non detergent oil in Laycock overdrives. I welcome further discussion and feedback on this topic by anyone interested. As always, thanks for reading this article!

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Re: Overdrive instructions

Post by tannyo » Mon May 17, 2010 9:23 pm

After replacing the spring, we then tried automatic transmission fluid. We saw the same results as we did when we used the 30 weight non detergent oil.
It seems that using ATF produced the same results as 30 weight non-detergent motor oil. What's the difference between the two?

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Re: Overdrive instructions

Post by malcolmr18zoy » Mon May 17, 2010 10:54 pm

ATF is high detergent!
Malcolm

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Re: Overdrive instructions

Post by tannyo » Mon May 17, 2010 11:40 pm

malcolmr18zoy wrote:ATF is high detergent!
Malcolm
The article surmised that the detergent in the 10W30 caused the pump to cavitate, but not with ATF. That doesn't seem consistent. Will it harm the overdrive unit to use ATF?

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Re: Overdrive instructions

Post by malcolmr18zoy » Tue May 18, 2010 12:09 am

Hi Tanny,
I would recommend using the oil specified by the manufacturer. They specify the type of oil for their products based on extensive testing, and, usually, know what they're talking about. The use of ATF in engines or overdrives, is only for flushing out deposits and not for extended use. The high detergent in ATF will loosen and dissolve most deposits, and leave things nice and clean. Then re-fill with the specified oil.
Malcolm

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Re: Overdrive Lubricants

Post by VWNate1 » Tue May 18, 2010 5:36 am

Sort of agree with Malcom here .

ATF is used in hypoid drive units and manual trannies worldwide and never a problem because it doesn't cavitate and has very good shear strength , plus the added benifit of preventing buildup of crud and varnishy deposits .

For some odd reason , many hear ' Automatic Tranny ' and instantly close their minds to learning a new thing .

I don't .

YMMV but those who know me , also know I own mostly junk , literally . I run it hard far and wide and don't have the typical problems most do .

Using proper lubricants is part & parcel of that .
-Nate

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Re: Overdrive instructions

Post by malcolmr18zoy » Tue May 18, 2010 9:38 am

You can use ATF in manual transmissions, and you can use engine oil in automatic transmissions, but I would always rather use the manufacturer specified oils. ATF was designed for automatic transmissions where it needs to have certain qualities. It is a good lubricant, and Rover recommend it in their LT77, and R380 five speed transmissions. The Mini, and Austin America automatic transmissions run on engine oil, and work fine. I've never heard of a manufacturer using ATF in a hypoid gear, but I don't doubt it. I think that hypoid oil has more extreme pressure additive ( EP) than ATF.
Malcolm

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Re: Overdrive instructions

Post by Jimmy » Tue May 18, 2010 11:43 am

Not to cloud the issue further, but I'd think the MGB transmission would prefer regular oil, or perhaps even 80W/90 gear lube, while the overdrive would be happiest with ATF.
Too bad they share whatever you put in the tranny.

For what it's worth, mine all seem to be quite happy with Castrol 20W/50.
In baseball, running into someone is apparently a "collision".
But doing the same thing in a car somehow makes it an "accident".

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Re: Overdrive instructions

Post by Steve Simmons » Tue May 18, 2010 2:04 pm

If it was just a matter of gear teeth then it would be simple. In this case however we're talking about 300-500 PSI of pressure, small passageways and a clutch cone. The oil needs to not foam as it's passed through the overdrive unit at these pressures and it needs to have the right amount of friction for the clutch cone. ATF is very high detergent, and any time you add something, you take away something else. I would worry most about the friction modifiers in ATF (and other fluids) which are designed for a different purpose than pumping through an overdrive module.

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Re: My Point !

Post by VWNate1 » Tue May 18, 2010 2:51 pm

Thanx for making my point Steve :thumbs:

ATF is designed to be pumped through various small orofices and gears etc. under extreme pressure without foaming and still provinding good high shear lubrication .
-Nate

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Re: Overdrive instructions

Post by Steve Simmons » Tue May 18, 2010 2:54 pm

No idea, really. It makes sense on paper but there are far too many variables to be able to determine the usefulness of ATF or any other fluid in a Laycock overdrive without controlled testing.

All I know is that I won't be the one to do that testing! Especially not in MY cars :D

For the record, I use plain old Castrol GTX 20W-50 in my MGA, MGB and MGC gearboxes. I do plan to run 30W ND in my 3-sync MGB OD if I ever get it back on the road.

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Re: It Has Oft Been Said :

Post by VWNate1 » Sun Jun 27, 2010 9:13 pm

' You can put a horse in the river but ,

Only a pencil has lead ' .

:P

:lol:

:hammer:
-Nate

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Re: Overdrive lubrication

Post by tannyo » Fri Jun 05, 2015 10:34 am

I'm going to revive this thread. Since the MGB transmission shares its oil with the overdrive, what oil should I use?

BTW, the manual recommends 20W/50.

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Re: Overdrive instructions

Post by Steve Simmons » Fri Jun 05, 2015 10:39 am

As outlined in the article I re-printed above, Quantum recommends 30W non-detergent. I've used 20W-50 Castrol GTX with no issues also, but these days I go with their recommendation since they rebuilt the unit I'm currently running.

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