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MG MGB Technical - Other OD recommendations

I have been dealing with an intermittent OD on my 67 GT. (D type) for sime time. I have checked the electrical system multiple times and it is working correctly (measuring the voltage at the solenoid). I have replaced the solenoid and also checked the adjustment on the internal lever that the solenoid activates.

OD usually works when I first start driving, but then will not work. Flicking the dash switch on/off sometimes gets it to ingauge, but it does not work every time I do it. I tried moving the shifter around while in 4th and that has no affect on it.

Oil is full, screen is clean.

Someone recommended running ATF through the transmission to clean it. Is that a good idea? If so, I will give it a try.

This unit has about 5000 miles on it since a rebuild and I have changed the oil twice since the rebuild.

My rebuilder lives quite a ways away. I would rather not have to pull it and send it back if possible.
Bruce Cunha

In the past people have said that ATF has flushed their OD and solved a problem, others have said they had to keep using it or the problem comes back.

You can check voltage as much as you like but unless you check for current you still don't know if it is being energised or not. The problem with the D-type is that it takes about 17 amps to pull in and 2 amps to hold, the first of those are beyond most home multi-meters.

The other thing is to connect a pressure gauge in place of the relief valve plug, and that will tell you exactly what is and isn't happening - 400 to 420 psi with OD engaged, nothing disengaged.
paulh4

You can safely run the OD & gearbox on ATF Dexron 2 if you want.

A flush through using ATF for a short while will dissolve any deposits that are causing intermittent operation.

Before doing that there are two ball valves that need to be checked - the operating ball valve accessed from the top after removing the gearlever turret - once the plug is removed the ball can be lifted out with a magnetised rod and the seat examined for dirt - once cleaned the ball can be reseated using a soft metal drift.

The second ball valve lives under the centre plug on the bottom of the od unit and should be cleaned & reseated - additionally the spring may be weakened, it can be replaced or stretched slightly by twisting a small diameter drill or screwdriver shaft down the coil.

Oil pressure on a D type should be 540 to 560 psi and is measured from the operation valve port - the figures Paul quotes are for the LH type OD.

See the manual for all the details - I have the manual online at https://www.octarine-services.uk/mgb.pdf - pages 142 & 143.
Chris at Octarine Services

What would be the thinnest fluid you could use as a cleaning-flush for say just one road run of the car (30-50 miles say) to get it fully around all the parts when interacting with each other, engaging as many gears as often as practical/possible?

Then warm/hot drain of cleaning-flush fluid for as long as possible and refill with the recommend oil of user's choice for another 2 years (or 24k-miles) use.
Nigel Atkins

"Oil pressure on a D type should be 540 to 560 psi and is measured from the operation valve port - the figures Paul quotes are for the LH type OD"

Thanks, I'd not noticed the different figures for the D-type before.
paulh4

Ok, good advice. Thanks Chris for the link.

Paul. As for the voltage. I hooked up a long wire from the dash switch that I could activate under the car. With the side plate off, I can see the solenoid pulling on the activation rod. Would that indicate the solenoid is working correctly?
Bruce Cunha

It should do, Bruce, assuming it is operating through its full travel. But then you need to be able to do that when the OD won't engage, which ordinarily would need you to be driving the car. If you can get the car safely supported and run the rear wheels in 3rd or 4th, switching OD in and out, and still see the solenoid operating in exactly the same was when OD is engaging to when it is not, then you need to do the pressure test.
paulh4

This thread was discussed between 07/05/2019 and 08/05/2019

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