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Overdrive problem

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JulesG Julian Garrish
Pulborough, UK   GBR

I’m new to this forum, though I know there have been several posts regarding overdrive issues before. None seem to have the same problem I have, though. I have a BN2, not that I thinks that makes much difference, as I believe all Healeys have more of less the same unit. My overdrive works fine for a while, and then cuts out. Having done so, however, it cuts back in almost immediately. Both actions put quite a strain on the engine and driveline, so once it starts doing this I just switch out of overdrive and continue without it.

It seems to me that the relay and solenoid work okay to switch the overdrive in, but then either the secondary coil on the solenoid, or maybe something hydraulic in the overdrive unit itself, are not sufficient to keep it engaged.

Has anyone else experienced this? I’m trying to pinpoint a likely answer, without starting from scratch testing every individual component.


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Jack T Avatar
Greensburg, PA, USA   USA
I would suspect the electrical switch on the gearbox. See if keeping a bit of side pressure on the gear lever has any effect; to the right should keep OD engaged, left would open the circuit and disengage OD. That is assuming the switch is mounted similarly as on the later boxes.

JulesG Julian Garrish
Pulborough, UK   GBR
Thanks, Jack, I’ll try that and see what happens.

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San Jose, CA, USA   USA
Had similar symptoms on my BJ8 a few years ago. Took the relay out, opened it up and filed the points in a motel room and problem fixed (did replace the relay).

rusty1c Avatar
rusty1c Peter D
Antioch, CA, USA   USA
1961 Austin-Healey 3000 BT7 "RUBY"
1963 Chevrolet C10 "Rusty"
Check out this posting a few weeks ago.

JulesG Julian Garrish
Pulborough, UK   GBR
Thank you all very much, at least that’s given me some leads to follow. The thing with my issue is that it occurs when just cruising along - not accelerating, decelerating, or touching anything (gear lever, O/D switch, nor anything else). I find the strangest part is that it cuts straight back in, after only a second or two (of course, it then happens all over again after ten or twenty more seconds).

Jack T Avatar
Greensburg, PA, USA   USA
Yeah, the switch can be very touchy. Mine started cutting out intermittently then finally would not allow od to engage at all. Removing the fibre washer and replacing it with a smear of sealant solved the problem.

To properly diagnose the issue you will need to remove the gearbox tunnel so you can see what's happening. Between the workshop manual and online discussions it's not difficult to diagnose.

Impala Avatar
Impala Frans Dusseldorp
Oud Gastel, NB, Netherlands   NLD
Hi Julian,
I had the same problem with my 3000 MK1. Detecting the problem with extra pressure on the stick to the right will keep the overdrive in place. As earlier mentioned by Jack T.
After replacing a new switch the problem was solved. It's difficult to change the switch from underneath the car but with some patience it is possible.

Good luck,

Leyburn, North Yorkshire, UK   GBR

My money would be on the gearbox switch operated by the gear lever. To save having everything live etc, just disconnect the wires on the switch, and put a bulb and battery onto the switch, then waggle the gear lever to your hearts content, if the light keeps going on and off, there's your problem. If as I think is, a washer under the switch, find a thinner one, or make it thinner, if there's no washer file little from the mating face of the switch, this should improve things. If thats not the problem, here is a couple of good reads !

Good luck Stan,

BTW used to live near you until 2005, when I retired to the Yorkshire Dales, great Healey country smiling smiley

Dave2526 Dave Deamer
Seal Beach, CA, USA   USA
I had a similar problem with my BN1. The culprit turned out to be the centrifugal switch. The problem can be diagnosed by adding a bypass wire from the O/D dash switch to the relay bypassing the centrifugal switch. Symptom was the O/D working fine at low speeds and then intermittently shutting down above about 3200 rpm in third ( with an alarming rise in the engine speed).

Albuquerque, NM, USA   USA
I can think of two other items that can cause these symptoms with a Laycock DeNormanville overdrive.

First, the overdrive solenoid has two circuits, one of which pulls the overdrive into operation and draws a momentary amperage of about 20A. The other is a hold-in circuit that draws about 2A. When the overdrive is energized the pull-in circuit operates and a switch in the solenoid that turns the pull-in circuit off. If the hold-in circuit is weak or if the switch is failing the OD will drop out and the pull-in circuit may again be energized. This can be solved by replacing the solenoid.

Second, the hydraulic accumulator in the Austin Healey spec overdrives has a reduced bore size from the standard size to moderate the force of engagement. This was accomplished by inserting a bore liner with a smaller piston. The liner is sealed to the bore by a pair of rubber o-rings. As these o-rings age and harden they tend to leak, which reduces the pressure in the hydraulic operating circuit. When the pressure is reduced beyond a certain point the overdrive will drop out. This symptom usually occurs when the transmission/overdrive oil warms up, thus reducing its viscosity and allowing greater pressure drop in the hydraulic circuit. The solution is to remove the OD side cover, remove the bore liner and replace the o-rings.

Bill Lawrence
BN1 #554

P.S. No centrifugal switch on a BN2...

PAN Avatar
PAN Alwyn Keepence
Ipswich, Queensland, Australia   AUS
G'day Bill,
I have had this exact problem with my overdrive but I did not suggest this to Jules because it was not intermittent. Once the oil warmed and the o/d disengaged it would not re-engage until the oil had cooled. A pressure test on my overdrive showed that once the oil was warm the reading dropped to 150psi. Replacing the accumulator piston solved the problem, but not straight away.
You mention that there are two o-rings on the piston. The replacement that I received from my supplier had only one o-ring groove. It wasn't until my repairer machined a groove in the piston for the second O-ring did the unit operate properly.
When I reported this issue to my supplier, the response was that only one O-ring was ever used by Laycock De Normanville. Yet the 100 workshop manual clearly shows two O-rings. I presume that later model o/ds require only one O-ring.
Dave Deamer's suggestion to bypass the centrifugal switch has merit. I have read that this item did cause trouble during production and was eliminated on the BN2.

Albuquerque, NM, USA   USA

I suppose the severity of the problem varies a little bit from unit to unit or with the state of deterioration of the O-rings.

I can't remember if mine has two O-rings or one, but my service manual shows two for whatever that's worth.

The centrifugal switch works on mine, but it may not be engaging/disengaging at the right speed because I've changed the final drive ratio. I think it was included originally to act as a kind f automatic gear change between cruising and deceleration. It would be nice to know how it's supposed to be adjusted, but I'd probably just screw it up.

Bill Lawrence
BN1 #554

JulesG Julian Garrish
Pulborough, UK   GBR
Hello everyone,

Thank you all for your various advice. Having diagnosed the problem, it seems it was excessive movement in the gear leaver assembly / linkage that was causing the gearbox switch to cut out, as several of you had suggested. Thanks for that, as this wasn’t something I would have thought of - my money would have been on failure of the solenoid’s secondary circuit, but this appears to be OK - at least for now!


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