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MG MGB Technical - D type OD trouble shooting

The overdrive (D type) in my 67 GT started acting up a few months back. It would periodically not shift into OD.

In diagnosing it, I started by checking power and found I had power all the way to the solenoid at the transmission. Used a charged battery and put power to the solenoid wire and got nothing.

Pulled the solenoid and put power to it again and again nothing. Purchased a Victoria British solenoid. Installed it (with the gasket). Had the wife activate the OD while I was under and heard it click.

Went for a test drive and no OD. Rechecking, I have power, but no click. Tried 12v to the solenoid wire underneath and it clicks. I checked the connections and they all are good. Wire harness from the trans to the engine is about 9 years old. Conductivity test is fine.

I am going to run a new wire from the relay to the solenoid to see if it is a wiring issue. If that does not work, U will try a different switch. I would appreciate any other diagnostic I can do with the transmission in the car.

The OD was professionally rebuild in 2008. I know the rebuilder and have been conversing with him, but thought I would see what the group thinks.
Bruce Cunha

Check the relay and vac switch if still fitted.
Chris at Octarine Services

Ditto. First check, does the relay click? The manual switch operates the relay, and the relay operates the solenoid via the gearbox switch which has to be closed. However the relay gets the 12v which it pass on to the solenoid from a different source to where the manual switch gets it from to operate the relay, so both sources must be good to operate the solenoid. However that wouldn't explain getting 12v all the way to the solenoid wire bullet connector but still no operate.

Also the solenoid has a pull-in winding via a normally closed switch, and a hold in winding for when the solenoid has operated and the switch has opened. This switch is adjustable and has to be set-up correctly. The pull-in winding is 0.7 ohms which takes 17 amps, and the hold-in is 6 ohms which takes 2 amps, so that is another test to make.

Ordinarily it makes more sense to measure the current in the solenoid as that will tell you exactly what is happening, electrically speaking. Easy enough on the LH which only has a single winding taking about 1 amp, less so for the D-type unless you have an ammeter that is good for 17 amps, so you may have to settle for resistance. Not ideal as the gearbox switch and relay contacts can develop oxidisation which will give a high reading on an ohmmeter which only passes a microscopic current which can make you think they are faulty, but which burns off as soon as normal circuit conditions are applied. OTOH, the D-type should click every time you operate and release it as a further diagnostic, whereas the LH-type doesn't.
paulh4

Thanks.

Took the car out on the road today. Flipped the OD and nothing, but leaving the switch on, it eventually kicked in (car was under power and driving at about 65 mph.

OD disingauges immediately upon turning the switch off.

I can hear the relay on the firewall click when I turn the key on, put it in 4th and flip the switch. I did put the voltage meter on conductivity and hooked it to the vacuum switch. Applying a vacuum does cause the switch to work. I also checked the transmission switch and it is also working.


So it appears the OD is working, just not kicking in as quickly as it should.

Paul Can you expand on the adjusting of the switch? I know there is a rod with an adjustable nut behind a bolt on plate on the transmission. This appears to be the lever that actually engages the OD. My rebuilder thinks this may have loosened. It is hard to get to, but a couple thin wrenches should get to it. I have to get the car up to check this.
Bruce Cunha

I can only give what is in the WSM as I have no personal experience with this system, and you have the gist of that. Basically the solenoid pulls a lever towards it, and that lever has a hole in it, which should line up with a hole in the casting behind it. A 3/16" rod should pass through the lever hole and into the casting. If it doesn't, with the solenoid spindle pushed into the solenoid as far as it will go, the solenoid spindle must be prevented from rotating with a spanner on two flats on the spindle, while another spanner turns the self-locking nut to move the lever either forwards or back, until the two holes line up. The operate it electrically several times making sure the rod will pass through the lever into the casting hole, and that the current does not exceed 2 amps. If the current remains at 17 amps the switch is not opening, which could damage the solenoid from excess heating.

There is a way of checking the pull-in winding using a 21W indicator bulb in series with the solenoid. This has a working resistance of about 7 ohms i.e. very similar to the pull-in winding, so with the two in series you should have about 6v across each. If you have most of the voltage across the solenoid, then the pull-in winding is not in circuit, i.e. the switch may not be closing or the winding is faulty. Note that with the bulb in series the solenoid probably won't operate. Also not that any bad connections back towards the supply will also reduce the current, maybe to below what will pull the solenoid in. But if you see 12v at the solenoid wire that aspect should be OK.

Really you need to find some way of determining whether the solenoid has operated or not when you are driving, so see whether the problem is electrical or hydraulic/mechanical. The only way I can think of doing that, is to monitor the position of the lever somehow.


paulh4

Correction! A 21W indicator bulb is about the same as the HOLD-in winding, and that will result in about 6v across each. If you see about 1v across the solenoid and the rest across the bulb, that indicates the pull-in winding is in circuit, as it should be, as there is not enough voltage across the solenoid to operate it.
paulh4

Thanks Paul. I will check it out.
Bruce Cunha

Sounds more like a low oil pressure issue than electrical.

If the solenoid operating arm is correctly adjusted and the OD still doesn't work then I would recommend draining out the gearbox oil and replacing it with auto transmission fluid - this is highly detergent and will flush out any dirt in the unit and dissolve any gum.

The OD & gearbox can run perfectly well on the ATF but it WILL seek out any leaks!
Chris at Octarine Services

X2 with Cris; sounds like low hydraulic oil pressure. Suggest locating someone with access to a pressure gauge to find out what is the actual hydraulic pressure reading. As per John Esposito; Quantumechanics Ltd.; the D Type s/b set to 500+ psi. I would also call him to discuss your issues.
Rich Boris

Great point. I changed the oil back in February along with checking the filter. All looked good. Trans has about 25,000 miles since a full rebuild.

One of our club members has a lift and a OD pressure gauge, so I will arrange to get that checked.
Bruce Cunha

This thread was discussed between 19/07/2017 and 23/07/2017

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