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Overdrive Problems

Healey Boys

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966 Auston Healey BJ8.  With the overdrive engaged I downshift from 4th to 3rd and as I apply the brake the overdrive  disengages and then as I accelerate (still in 3rd) the overdrive engages again.  Otherwise The overdrive is functioning properly. I am using 30W/ND. Any suggestions as to why this i happening and what the fix might be.
 

Bob_Spidell

Yoda
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Healey--actually, Laycock De Normaville--overdrives are electro-hydraulic. When disengaged, 8 (IIRC) very strong springs hold a wet clutch engaged with a cone housing so that the drive is essentially one piece. When you throw the switch, the electrics open a valve through which highly pressurized oil is directed to two pistons that simultaneously disengage the cone clutch and lock a planetary gearset's sun wheel, causing the planetary gears to rotate about the sun wheel and drive an outer 'annular' gear which overruns the transmission output, hence 'overdrive.' As mentioned, very high hydraulic pressure, usually 400psi or higher, is achieved by a piston pump driven by an eccentric cam on the gearbox output shaft.

First, I wrote this:

My best guess, since this happens when the engine slows--and shifting from 4th to 3th will not interrupt the electrical flow which operates the solenoid that switches the HP oil circuit and prevents engagement in 1st, 2nd or reverse--is that the pressure from the pump is marginal. Theoretically, since the gearbox and OD are bathed in relatively clean oil the pump should last forever (I just went through my BJ8's OD and the pump was in good shape after over 200K miles). You can try a thicker oil--20W-50 works well--but the best diagnostic is to put a pressure gauge on the OD to make sure you're getting adequate pressure.

Then, after thinking about it, I wrote:

This may be normal. Since the drivetrain is slowing, your pump may not be able to maintain the necessary pressure below a certain RPM (even if it's in top nick). I can't say what this RPM is, because I don't think I've ever intentionally operated my ODs this way; when slowing, I throw the OD switch then blip the throttle to disengage the secondary circuit--which is there to prevent the overdrive from disengaging; and applying potentially damaging reverse torque to the OD--and either run in straight 3rd or 4th or downshift, then re-engage the OD when accelerating. Straight fourth and 3rd OD are just a fraction of a ratio apart, I can't really think of a good reason to do this. There is a roller clutch in the OD that I believe is there to prevent reverse torque from damaging the unit, but shifting from 4th/OD to 3rd/OD is basically dropping two gears.

Then, I thought even more:

I think this is normal. Sometimes, when I coast in gear/OD to a stop, then accelerate, the OD will engage when I hit 3rd gear at a certain speed. I believe this is because there is a minimum RPM required to actuate the hydraulics; if I had to guess I'd say it's around 1,500RPM. Don't think I've ever seen a spec.
 

roscoe

Jedi Knight
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A L de N overdrive that is operating to specifications, as long as you move straight line from 4th to 3rd, should not disengage when downshifting from 4th to 3rd, unless you do some weird shifting where you hit neutral and allow the shift lever to move side to side. I suspect that Bob's first thought is correct. Your OD is operating at the edge of the oil pressure it requires so when RPM drops a bit it disengages and the reengages as you accelerate.

I have to disagree about the OD oil pump lasting forever. There is an oil "non-return" valve that consists of a seat with a ball bearing backed by a spring in the OD pump assembly. This valve allows pressure to build in the accumulator. Years ago when I was troubleshooting a non operating OD, after determining I had low pump pressure, I found the machined seat where the ball bearing was has badly pitted and thus would never allow the oil pressure to build enough to work. After lapping the seat for several hours I was back to the desired 400+ psi and havent had a problem since. There are also o rings on the accumulator and on the operating pistons which can and do get worn/ hardened and otherwise ineffective.

There is a lovley article to be found at https://www.buckeyetriumphs.org/technical/AOD/AOD1/AOD1.htm
which covers all of this in excellent detail and should be required reading for anyone head scratching over a poorly behaving OD.

Making a tool to check OD pump pressure is fairly easy. You do need a spare cap that goes on top of the operating valve on the side of the gb opposite to the solenoid. All you do is drill a small hole in it and weld a pipe fitting to which you can attach a 0 to 500 psi pressure gage. When installed you can drive the car with the transmission cover off ( mind your neck tie in the driveshaft) and watch the gage to see the operating pressure.
 
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When I had a similar problem Dave Nock directed me to the relay and told me to op0en it and clean the points. Worked perfectly.
I am not sure there is any advantage to multi grade oil in the gear box. If you are in real cold climate, a thinner oil is helpful (in the engine). I am not aware of a multi grade without detergent. The detergent holds debris in suspension. You want the particles to drop to the bottom and out of harms way. The suspended metal particles act like an abrasive to the gears. 30wt non detergent (BMC recommendation) or Redline is what I have always used since the 70's with never any problems.
 

Bob_Spidell

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Good point, Jon. Re-seating the check valve balls is SOP when overhauling an OD. A new ball, if necessary, and a good smack on it with a drift whilst in the seat usually does the trick.

I found out recently that one size chromed ball bearing--I think it's 5/16"--is used in many places in a Healey, from the seat rails to the OD. I bought a bagful from a specialist ball bearing vendor--ain't the 'web' great?--for a few bucks so I always have some on hand.
 
OP
H

Healey Boys

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Thank you all for your suggestions. I think I found the problem thanks to another Austin Healey owner. It appears the nylon bushing under the gear shift lever is warn. The play in lever is causing the overdrive switch on the gearbox to turn the o/d on and off. I road tested the car as follows: in 4th gear with the o/d engaged; I down shifter in 3rd but kept my hand on the gear lever and held it in a forward/to the right position. The o/d stayed engaged as long as I held the lever; when i removed my hand and allowed the lever to move the o/d disengaged. I could literally turn the o/d on and off using the gear lever. Will not know for sure until I install a new bushing but I'm pretty certain that is the problem.
 

roscoe

Jedi Knight
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Not sure why it would reengage when you accelerate but that certainly would be the simplest fix. Of course it could be the 3rd/4th switch out of adjustment or wear on the tab that engages it. Perhaps a bungee from the shift lever to something solid to hold it in place ��
 

vette

Darth Vader
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I was thinking it is the switch on the trans for the 3/4 gate. His explaination about the bushing would cause such an issue. I suspect that it is his habit of making the shift that makes it appear that it is the acceleration that is reengageing the OD. I think he is losing electric power at the switch as he shifts but is clutching and accerating so quickly that he thinks it is the acceleration that is reengaging the trans when really it is the fact that he is getting the shifter stabilized in the 3rd gear position enough to reengage the OD.
 

RAC68

Darth Vader
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Hi All,

Having taken the diagnostic ride with Frank (Healey Boys), I can say that his reference to acceleration was made prior to testing and with reference to normal OD activation/deactivation. As Frank indicated, what was determined during the test run was that either a loose OD switch or a badly warn lower bush was allowing the lower portion of the shifter to misalign and not maintain OD switch activation. Testing showed the switch and electricals to maintain OD activation through the downshift when the shifter was pressed forward and to the right. Allowing the shifter to move toward center and free of this restricted positioning resulted in the deactivation of the OD (release of the OD switch mounted on the left of the shifter housing) but moving it back to the restricted positioning (forward and to the right) immediately reengaged the OD.

My bet is on the lower shifter bush to be warn and excess movement is allowing the switch the freedom to release. However, a warn and/or loose switch could also be the cause or a contributing factor and will only be determined when the shifter is disassembled sometime next week. Since the OD does perform properly in all operations, including when downshifted form 4th OD to 3rd OD when movement is restricted, I don't believe any internal issues or further electrical issues are present.

Ray(64BJ8P1)
 
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Healey Boys

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Thanks Ray...you described the diagnosis perfectly. I do plan on purchasing a new switch just in case the bushing is not the real problem but I/we are pretty confident the new bushing will do the trick.
Again, thanks to all of you for your comments and suggestions.
 
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