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MG MGB Technical - '66 GT Wiper Switch

I need some help understanding the wind-screen wiper switch on a '66 MGB-GT. What I think should be: With the switch out of the car, both black/green (BG) and a red/green (RG) [--The wiring diagram shows light-green--] wires are energized when the wiper is “at rest.” When only the RG wire is grounded the wiper runs and the (BG) wire is no longer “hot.”

Here's my guess; RG is grounded when the switch is in the “on” position and BG is not grounded and only “hot” when contact is made as the wiper motor passes through the “at rest” position, but as soon as the switch is flipped to “off” the RG is no longer grounded and BG becomes grounded to stop the wiper motor at the “at rest” position. If this is the case, I can't think of another way of getting around a bad switch but to replace it, and they're a bit pricy.

Thanks in advance for any help.
David Werblow


Have you been to the Pages of Bee & Vee web site? It has been put together by Paul H, who posts here. It should be a good place to get the understanding of your system. The home page address is below. The Hammer & Spanner section has the tech info including an explanation of the various wiper systems.


C R Huff

Paul Hunt is the resident expert on wipers. He will probably join in here but his web site (the one charley listed) has all the various wiper wiring diagrams. The 67 (and potentially the 66 depending on when it was manufactured) GT had a wiper different from the others.
Bruce Cunha


As said above see Paul's webpage.

IIRC the switch is simply an ON - OFF one, that shorts out the park function. This function basically opens the ground leg of the motor, when it approaches the park position. By shorting it out the motor continues running. If your switch is dicky, maybe a relay in the circuit will allow you to retain the existing switch and give reliable working.

Herb Adler

If you have square wiper motor it is simple, but the motor may run on a little after the park switch turns it off, so the blades start to creep back up the screen. If you have a round wiper motor, it's more cunning than that, and the park switch kills the motor dead by shorting out the motor in the park position. As others have said Paul Hunt's your man - read about it here
Mike Howlett

The 67 GT is a real odd-ball and can have one or two if not three switches, motors and wiring arrangement, one or more of which is unique to the GT! Several times I've thought I've got to the bottom of the variations, but you seem to have come up with another with your red/darker green wire. Do you have the sub-harness that has a multi-way plug with three short tails for spade terminals at the motor, plus a long red/light-green (or red/green) tail that comes all the way from the manual switch?

The 12v you can see on the black/green and red/green when parked is from the park switch and is connected to the motor, which is backed by 12v all the time, when it reaches the park position to stop the motor very quickly and consistently, rather than the earlier 'just disconnect 12v and let the motor spin down' system, where the actual parked position of the wipers varied according to how wet the screen was.

What problem are you trying to solve? The early toggle switches were pretty generic in that they could have several spades and you used which ones you needed, and hence were not restricted to specific switches for particular functions. However if you have the early rocker switches (the UK didn't get out type until 73) there is less flexibility, especially given the logos. It's basically a single-pole double-throw switch, where to run the wipers black/green is connected to earth, and to park the black/green is connected to the red/light-green.
PaulH Solihull

This thread was discussed between 17/01/2013 and 18/01/2013

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