|The MGA With An Attitude
TWO SPEED WIPERS, With Relays -- ET-219
MGA Two Speed Wiper Motor Conversion - by Barney Gaylord
Since I haven't installed this yet, I don't have pictures. The short of the mechanical installation is to modify or fabricate a bracket if necessary to mount the late model MGB two speed wiper motor in place of the MGA wiper motor. The late model MGB motor gives a longer stroke for the drive cable. The late model MGB wheelboxes have a larger diameter wheel to accommodate this, but apparently they cannot be used on the MGA due to a short spindle which will not reach through the angled mounting bezels on the MGA. Another solution is to install a "105 degree" crank gear in the late model MGB wiper gearbox. This part may be available from Brown and Gammons.
Then you need a special electrical circuit, with a few choices of approach. The original MGA single speed circuit has only one wire in the harness between the motor and switch, black with a white stripe, which is switched to ground to make it run. If you have read the previous article you know about the 12-terminal 4-pole 3-throw toggle switch which emulates the circuitry of the MGB steering column mounted multi-function switch. That requires 4 wires to the switch, none being grounded. There is also a 3-position toggle switch used for the wipers in the 1968-1972 MGB and Midget (European cars, not North America?) which does the same thing and is more easily procured. This article should make the circuitry simpler to understand, and maybe also simpler to install.
The diagram on the right shows a 4-terminal 3-position slide switch, which does exactly the same electrical function as either the MGB switch or the multi-pole toggle switch. In the "off" position it connects the low speed motor winding to the parking switch in the gearbox. When the motor runs to the park position the internal switch will disconnect power and ground the motor winding to activate dynamic braking. The switch center position connects power for low speed, and the third position connects power for high speed. This one is simple to understand..
The diagram below shows a connection method using a standard MGA 3-position lighting switch in place of the original 2-position wiper switch. To make this work I would attach two miniature relays to the wiper motor. By making the relays part of the motor, it becomes a direct interchange for the original MGA unit, with only one additional wire required from the motor to the switch to operate the second speed. All of the 12 volt power connections will be wired together at the motor. The additional wire to the switch will be switched to ground like the original wire, so this wire should also be black with a stripe (different color stripe than the first one). I would like to suggest soldering all wire connections on the relays to minimize the number of terminal connections for best reliability.
In the off position both relays are at rest (de-energized) as shown, with relay R1 making the connection between the low speed motor winding and the parking switch. This enables the motor to run to the park position, and then switch itself into dynamic braking. The center position of the pull switch (first pull) will trigger relay R1 to disconnect the dynamic brake circuit and connect power to the low speed winding, making the motor run continuously in low speed. Final position of the pull switch (second pull) maintains the first connection (to defeat dynamic braking) while also triggering relay R2. This switches power from the low speed winding to the high speed winding, making the motor run continuously in high speed.
The purpose for using the relays is to allow use of a standard lighting switch for original appearance on the dash, and to minimize the number of new wires required in the harness.