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MG MGB Technical - Laycock overdrive

I have a later Laycock OD out of a 76 roadster that stopped working recently. I am having trouble sorting it. The unit was non functional when I bought the car. The rear oil seal was leaking leaving the unit and transmission with little oil. I replaced the seal, cleaned the relief valve and pump body assemblies and checked the electrics. The OD worked fine for several years - engaged within several seconds, did not slip and disengaged quickly. One day it just would not engage. I bench tested the solenoid. It "actuates" but not very forcefully. I also reinspected the relief and pump body assemblies as per a John Twist video. Still did not work. Worrisome is the debris found in the OD sump. I found about 1/8 inch of a greyish-silver paste like material ( like a thick paint or toothpaste) as well as a sprinkling of silver flakes (which may have come from the transmission). I am seeking advice on my next steps here. I assume I might test the solenoid as per Paul's instructions for the D type unit. But any additional guidance/suggestions would be greatly appreciated. Brian
r. LEARY

The debris sounds like it is more likely from the gearbox rather than the OD unit.

The solenoids don't pull strongly.

While the sump is off pull and clean the pump one way ball valve.

Pull the gearbox drain plug and inspect it for the same sludge.

Refill the unit with ATF dexron and drive for a while then drain and refill with engine oil.


Chris at Octarine Services

The later LH-type solenoid takes about 1 amp with the engine running, about 800mA off just the battery. Current should always be measured with the circuit connected and operated as it would be in the car so you can detect/eliminate bad connections anywhere in the circuit. These can be anywhere in the connectors and switches, as well as where the solenoid picks up it's earth from the OD casing. Testing on the bench only checks the solenoid at best.
Paul Hunt

Thanks Chris and Paul. I have a better idea of my next steps.
r. LEARY

see also posts on the other o/d thread here

>>The OD worked fine for several years - engaged within several seconds<<
to be me, if the o/d is operated correctly, it should change almost instantly

the clutch pedal does not need to be dipped or accelerator pedal lifted off fully, there should be load
Nigel Atkins

The LH solenoid requires less than 10 lb to actuate the hydraulics. From memory, the normal load it exerts is around 13 lb.
If you find the solenoid is exerting sufficient load when activated, your next steps are:

1. Check there is power getting to the solenoid. A simple idea is to link a test light in at the bullet connector where the solenoid joins the gearbox loom. Have the light on a long enough bit of wire to be observed while the car is being driven if need be. Common sources of power-feed failure are failure/wrong adjustment of the gearbox inhibitor switch, and (in rubber bumper models) a break in the wire going to the gear lever switch. The latter occurs at the base of the gear lever, and should be investigated urgently as the feed is unfused and a break there has fried more than one wirning harness.

2. Measure the hydraulic pressure at the relief valve (refer BMC manual). That will give you a clear steer whether the fault is in the hydraulics (mostly accesible externally) or whether the overdive needs to be removed. You should see around 400 psi on the gauge when the switsch is activated.

It's easy to do - drill a small hole (~3mm) through the centre of the relief valve plug, then counterbore & tap part of the way to accept a fitting for the gauge. Have a hose long enough to link that to a gauge strapped to the 1/4 light so it can be read while the car is being driven.

Ive described things further at http://www.mgparts.co.nz/advice/technical-notes/overdrives/


Paul Walbran

I experienced a similar failure with my LH OD unit. It turned out be an intermittent open circuit in the solenoid. I checked it several times and it always passed muster, except for the last time. A new solenoid brought the OD back to like new operation. RAY
rjm RAY

Thanks Ray. I will check the solenoid more closely. Too expensive to just buy a new one and just plug it in. I will also try running ATF for a week or so per Chris's suggestion. I think a lot of debris from the gearbox may have gotten into the OD passages. However, the weather is too nice for that now...
r. LEARY

a drain down and refill wouldn't take long after a good run and then of course you can reward yourself with another run to "test for leaks"
Nigel Atkins

Thank you Paul and Nigel
r. LEARY

If it's the solenoid, be aware of the difference between the complete solenoid (coil+housing+plunger) and just the coil - which is the only electrical part and is available separately and is much cheaper (about 1/3 the price).
Paul Walbran

As Paul states, the solenoid is available as a separate item. The related components, included in the complete solenoid replacement kit, seldom need to be replaced. None of mine showed any signs of wear. RAY
rjm RAY

Thanks guys. I really appreciate the info. I will update when I have a resolution
r. LEARY

Postscript: I substituted Dextron for about 2 weeks and then drained the unit. The oil pump was stuck and full of gunk. It actually shot out when I tried to fish it out of the housing. I cleaned everything up and it all runs great now. So, for those researching problems with an overdrive unit, be careful of units that had been either run dry for a while due to leaks or had an uncertain maintenance history. I had a dry unit, repaired a leaky seal and added fresh oil. However, the gearbox and OD unit apparently had a lot of gunk in it which eventually clogged the oil pump. OD units are very sensitive to dirt!
r. LEARY

good news, well done

as with a lot of servicing/maintenance it boils down to clean and lube

and a lot of prevention is for owners to simply follow routine driver checks as detailed in the Driver's Handbook and to follow the servicing schedule and do a thorough oil change on the g/box (o/d) every 2 years or 24k miles whichever is the sooner

for a thorough change have the existing oil as warm as possible and leave to drain as long as possible to get as much existing oil and muck out of the g/box (o/d) as possible

at the same time the o/d filter would be cleaned

and you can't trust previous owners to have done so, possibly ever
Nigel Atkins

This thread was discussed between 17/02/2014 and 05/05/2014

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