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MG MGB Technical - Testing overdrive solenoid

While the body is away being painted I am looking at the engine and gearbox from my 1971 B roadster. This afternoon I thought I would check the overdrive (LH type) and applied 12v to the yellow/red wire with the negative from the battery connected to the gearbox case.

When I moved the gear lever from the 1-2 plane to the 3-4 plane I expected to hear the solenoid click, but it didn't. I checked the inhibitor switch for function with my meter and it works properly, passing current in the 3-4 plane but not in the 1-2 plane. All the wiring seems to be intact and working.

Obviously the gearbox has no oil in it - would this prevent the solenoid from working? Am I correct in thinking that the solenoid should click audibly? Will I have to remove the solenoid and clean it?

Overdrive is a mystery to me and I am loath to disturb it. As far as I know the O/D worked the last time the car was used, but that was probably 5 years ago.
Mike Howlett

Unlike the D-type the The LH solenoid usually doesn't click with the engine off unless perhaps it has new O-rings. The LH plunger pushes a ball against a seat to stop oil flow which engages OD. When driving and the solenoid is de-energised oil pressure pushes the ball and plunger back, and when it is energised again they move forwards. That would make a click, but you won't hear it with the engine running of course. If you drove the car and engaged OD, switched off with OD disengaged, then powered the solenoid, you may hear a click, but just the once until it is run again.

The best way of testing the circuit is with an ammeter - it should show about an amp when energised, a bit less when the alternator isn't charging. That only tests it electrically of course, the plunger could be jammed or the O-rings faulty which would prevent OD engagement. Both easily sorted from underneath when everything is back in the car.

BTW ohms is not a good test of the circuit as the switches can appear to be high-resistance until powered in the normal way, but the solenoid itself measures about 15 ohms.
paulh4

Thanks Paul, I was hoping you would be able to help. I'll try measuring the current draw. I'm led to believe from the PO that the O/D worked when last used. The only other O/D I have in my garage is on my Triumph Dolomite. There the solenoid is external and definitely clicks.

You are right of course about using the meter to test the switch, but I also tested it with a battery connected to make sure full voltage was reaching the wire entering the solenoid housing.
Mike Howlett

The Dolomite sounds similar to the D-type then, they are external. They take about 20 amps for the pull-in winding when initially powered, which is beyond most ammeters, but then they should always click.
paulh4

The Dolomite 1850 has a J-type O/D. And you are right about the pull-in current - that's why I fitted a relay. The little switch on the gear knob was never designed for such a load.
Mike Howlett

MGB D-type has a relay and a vacuum switch, the switch keeps the relay operated and OD engaged if the manual switch should be turned off on the overrun. The switch is NLA but it's worth keeping the relay for the reason you say, I wouldn't like to put that current through the MGB dash switch let alone a gear-lever switch.

Do you have the circuit fused? On MGBs at least the harness up the lever can short out and do quite a lot of damage as it's unfused from the factory. Even without that gear-lever switch I have fused both mine.
paulh4

Fuses are a good idea Paul. I shall attend to that. The '71 MGB has the dash switch so is not such an issue, but the Dolly has the gear lever switch.
Mike Howlett

This thread was discussed between 07/11/2021 and 08/11/2021

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