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MG MGB Technical - OD switch

Paul gave helpful advice recently on wiring in a fuse to the OD switch. We followed his advice on a friends car and wired to the white wire, but on my cat, a 1975 BGT rubber bumper, the wires are red and green on a dash mounted switch. Which one do I connect the fuse to?
R E Merrall

Rod, as Paul has said in previous threads, the easiest place to fit an inline fuse is at the jumble of wires that are in the engine bay by the pedal box on RHD cars. Take a look at this site for the wiring diagram for your car.

http://www.advanceautowire.com/mgb.pdf


Looking at the diagrams I think yours would be similar to my 76 roadster and I have fitted an inline (35A) standard fuse between the yellow and yellow/red wires in the loom for my overdrive. I have also fitted (as recommended by Paul) an inline fuse in the boot for the fuel pump.

Andy
Andy Robinson

A couple of things. According to Clausager in "Original MGB" Quote" from the start of rubber-bumper production, in 1974, stalk operation was found on all four-cylinder models with overdrive." End of Quote.

The O/D switch on your car should be incorporated into the wiper stalk, not on the facia. The wiring diagrams show that right from the start of MGB production to the end, the O/D switch always had a white wire feeding the power, and a yellow wire taking the power to the O/D. Never red and green.

It looks as though a PO has altered the switch position,(maybe the stalk switch became faulty) and since the wires probably weren't long enough he has used red and green to extend them. You should fuse whichever of your wires carries the current to the switch. Easy enough to find out with a cheap multimeter.
Mike Howlett

Rod,You might have a B with a retro fitted O/D gearbox, or a unit which has been re-wired at some tome. Maybe after a short circuit!! Irrespective of the colour coding an in-line fuse can be connected on the live side of the switch, at the switch. Live side is easy to determine either with a meter or test light. Switch will be live with ignition on.
Allan Reeling

Interesting, the stalk does have OD marked on it so clearly changed at some time.
R E Merrall

Many people who swapped out the stalk switch installed the gear lever switch, probably the most obvious and ergonomic place to control the O/D. Often using the stalk switch to operate something else; fan override being one.
Allan Reeling

"Switch will be live with ignition on."

And the OD manual switch off, otherwise both sides will be live and you are no further forwards.

Ideally the fuse should be as close to the supply as possible i.e. in the white, but when adding one to a dash switch like this there is only an inch of difference, and the only extra protection of having it in the white is in case the switch itself should go internally short-circuit to the dash.

All MGBs of that era had the column switch and main harness wires, regardless of whether they had OD or not. OD was pretty rare in North America, and many have used that switch for a fuel pump cut-off as it is a moments work to move the white from the rear harness (i.e. for the pump) from the white in the main harness to the yellow.

Personally I find the column switch very convenient for flicking OD in and out, and it anticipates paddle-changes of course. That's on the V8 which doesn't have OD 3rd, so there is no need to flick OD out when changing down anyway. I can reach the dash switch on the roadster with a finger-tip so can flick that out (or in) when changing gear.
Paul Hunt

My '72 4-speed roaster has the wiper-stalk OD switch (no idea if it's original but believe it is) that I love. I rarely use it in 3rd but in fourth in the hills and mountains it's wonderful. Jud
J. K. Chapin

Forgot to ask what rating for the fuse on the OD circuit. I am currently using a 15amp which seems to work OK
R E Merrall

The LH overdrive requires less that 1 amp to operate. A 5 amp fuse would be more than sufficient for your application. RAY
rjm RAY

I've used standard glass 17 amp rated, 35 amp blow. The fuse is there to protect the wiring which is the same as other fused circuits, you have two spares in the fusebox, and it avoids a proliferation of values and types.

The D-type takes 17 amps to pull the solenoid in, then 2 amps to hold it.
Paul Hunt

Paul, I believe that the OP listed his car as being a '75. It would have had a LH OD installed and would only require a fuse rated at 1 amp to protect the circuit. RAY
rjm RAY

I know, I just mentioned it in case someone considered using a 1 amp for a D-type ... which needs two fuses anyway, or the wiring altered.
Paul Hunt

The D type is a whole different animal. It requires a lot more power to engage OD and almost as much power to keep it engaged. It never hurts to cover both types of OD models used on the B. RAY
rjm RAY

"The D-type takes 17 amps to pull the solenoid in, then 2 amps to hold it."
Paul Hunt

Only two amps? Interesting. For some reason, I thought it was more. RAY
rjm RAY

Paul-
If the D-type Overdrive solenoid requires 17 Amps to energize it and 2 amps to hold it in, then what does the solenoid of the LH Overdrive unit require?
Stephen Strange

1 amp, operate and hold, there is no dual winding or switch as on the D-type.
Paul Hunt

Paul-
Thanks for that. Duly recorded.
Stephen Strange

The LH type uses a simple solenoid that draws its internal plunger away from, or against, a small ball bearing to open or close an oil passage that allows the OD to engage or disengage. A much simpler design that requires minimal amperage to operate than the D type OD. RAY
rjm RAY

The LH solenoid plunger presses a ball against a seat to block an oil bleed, which raises the pressure to engage OD. When you de-energise the solenoid it is simply that oil pressure that pushes the ball away from the seat, and hence the solenoid plunger, the solenoid doesn't retract the plunger as such and there is no return spring. That's why you get no clicking (engine stopped) with the LH (except perhaps quietly the first time it is powered) unlike the D-type which clicks every time, and quite loudly.
Paul Hunt

This thread was discussed between 24/10/2015 and 04/11/2015

MG MGB Technical index

This thread is from the archive. The Live MG MGB Technical BBS is active now.