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MG MGB Technical - Intermittent OD

Hi All. Took a nice drive today in the 67 GT with OD. Third time that after an hour or so, the OD does not engage.

Thought it might be the dash switch. Pulled it and put a meter on it. turned it on and off multiple times, but it is working fine.

I checked the leads on the firewall relay and the vacuum switch. All lines are tight and the vacuum tube is intact.

Need to jack it up to get under and check the transmission switch and the wiring to the solenoid. I would think I can put the meter on the orange striped wire coming off the vacuum switch, put it in 3rd or 4th and check conductivity at the solenoid.

The fact this acts up intermittently and typically after a period of working fine has me scratching my head.
Bruce Cunha

Bruce,

If it is the transmission switch, wiggling the gearshift may make it go in and out.

Charley
C R Huff

Bruce,

Have you checked the oil level, and when was the last oil change?

Regards,

Larry C.
Lawrence Cordeiro

Eliminate the electrics first. Easy with the 4-synch LH-type, but a little more complicated with the 3-synch D-type which I presume you have as you talk about the relay and vacuum switch.

The first thing is to turn the manual switch on and off, with the ignition on, and you should hear the relay clicking in and out.

It's the relay that powers the solenoid in the first instance, and you should also be able to hear that clicking in and out.

If you can't hear the relay then you need to check you have 12v and earth on the relay winding terminals W1 and W2. If you do, then the relay is faulty. If not, then check at the manual switch.

If both are clicking, then the problem is probably mechanical or hydraulic, and you will need a pressure gauge to check.

If the relay is clicking but the solenoid isn't, then either the relay isn't extending 12v on the yellow/red wire to the gearbox harness (check at the bullets at the rear right-hand corner of the engine compartment), or the solenoid isn't responding.

If you have 12v at the bullet connector (check both sides) then the solenoid isn't responding. The D-type solenoid has two windings - a pull-in, and a hold-in. The pull-in takes about 17 amps which is beyond the limit of most home-use meters, so it's not easy to check the current. The hold-in takes about 2 amps, and as the solenoid moves it opens a normally closed contact which disconnects the pull-in winding, leaving just the hold-in. This contact is adjustable as described in the manuals.

However if you part those two yellow/reds and connect a 21w bulb between them, one of three things should happen, when the ignition is on, the manual switch is on, and the gearbox is in an OD gear.

If the bulb doesn't glow at all then there is a break in the circuit, so you will have to check for 12v at the gearbox lock-out switch and at the bullet connector where the solenoid wire joins the harness wire. You may well need to remove the tunnel carpet and the tunnel cover panel to access these. If all of these show 12v then both windings of the solenoid are effectively open-circuit.

If the bulb glows at what seems to be full brightness then that implies pull-in current is flowing, which you can check by measuring the voltage with respect to earth on the gearbox side of the bulb - you should see around 1v. If so, electrically the circuit seems to be working, but if you can't hear the solenoid clicking it may be physically jammed, so not opening the contact to reduce the current to the hold-in value.

If the bulb glows at about half brightness, and has about 6v or so on its gearbox side, then that is the current flowing through the hold-in winding. This may be because the pull-in winding or the normally-closed contact is faulty, but it could also be because the solenoid has moved, opened the contact, and reduced the current from the pull-in value to the hold-in value as it should. But in that case you should be able to hear the solenoid operating, so if you can't you need to check the contact and the pull-in winding.
paulh4

Thanks Paul. That should give me a good weekend of evaluations. I will post what I find.

Just did an oil change prior to this run, it did the same think on a longer run last fall. When this occurs, the unit will occasionally kick in and out, but primarily it will not engage. Sure hope it is not the transmission switch. That is so fun to swap out.
Bruce Cunha

"That is so fun to swap out"

I didn't think it was so bad on the 3-synch tunnel with the large removable panel. The 4-synch is probably impossible without removing the gearbox, or cutting an access hatch.
paulh4

This thread was discussed between 24/04/2017 and 25/04/2017

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