The MGA With An Attitude
ET-211 - Installing a HEATER MOTOR RELAY

The idea here is that installing a relay near the heater motor might make the motor run a little faster by improving voltage at the motor. This is slightly controversial, so I won't get into the pros and cons here. Rather I will just give you the information on how, and let you do it if you like.

Wiring diagram for heater motor relay

Mount a relay near the heater motor. Disconnect the green/brown wire from the heater motor and connect it to one control terminal on the relay. Use a black wire to connect the other relay control terminal to the nearby body ground screw. Run a new heavier gauge wire with a new in-line fuse from a "solid" source of system voltage to one of the contact terminals of the relay. Connect the motor feed wire to the other contact terminal on the relay. That's it. When you switch on the original control switch it now triggers the relay using only a small current. The relay contact will connect the new power wire to the motor input to make it run.

In schematic form it appears that you have mearly substituted a relay contact for the original manual switch contact. For this to make any improvement in voltage at the motor you need to use a larger gauge power wire to reduce resistance, and hope that the new relay contact may have less resistance than the original switch. It may also help to eliminate a few bullet connectors from the circuit, IF you believe those are poor connectors and are adding resistance to the circuit. You should also connect the new power feed wire to a much more "robust" power source, like one of the fat brown wires on the voltage regulator terminal A, or the fuse block terminal A1. This would bypass all of the original wiring from the voltage regulator terminal A1, through the ignition switch, through the original fuse block, through the heater switch, and on to the original motor input connection.

When finished you will have a new relay, a new fuse, and some new wiring lying about. You may try to make the new wires look nice by running them closely along the original harness, but they will be pretty hard to hide. The kicker to all this is that you may get much more improvement in air flow by cleaning connectors and installing an uprated heater motor, and forget the relay and modified wiring. See more information about heater upgrades at

Thank you for your comments -- Send e-mail to <Barney Gaylord>
© 2004 Barney Gaylord -- Copyright and reprint information