Installing an Electric Fan in Your Spitfire

by Dan Masters

David Moag asked:

While patiently (NOT!) waiting for my body to arrive back on my TR3B so it's restoration can continue, I've turned my attention back to my Spitfire. I've decided to add an electrical fan (in push mode) to it, no thermostat, just a switch for when stuck in stop and go traffic on hot days. My question comes from what relay to use for this. I didn't get too far at the local auto parts store -- since I'm adding something that wasn't meant for the car, they can't look anything up. I did take a look at several relays just to get some ideas. From past postings, I was expecting to find them with four terminals. The ones they had with four terminals all looked like they needed special plugs to attach to them. They had some with more generic terminals (labeled as horn relays), but these only had three (unlabeled) terminals on them.

So --- what kind of relay do I want to get, and how do I determine how it is wired since I have no directions or wiring diagram to go by.

And Dan answers:

First of all, Joe Curry is correct in saying you can get by quite well without a relay for the way you intend to wire your fan, as you won't be using it all that much. I do recomend a relay if you are wiring it to a thermostat and using it as your primary source of cooling, as it will increase the life of both the thermostat and the on-off switch.

For the relay, look in the auxilary lighting section of your parts store - fog and driving lights. The relays of the type you want (four terminal) can usually be found there. In fact, you can often find a lighting installation kit that contains the relay, switch, fuse, and all the required wiring all in one package.

The horn relay will work very well, and identifying the terminals is really quite easy. The method following won't work for ANY three terminal relay, but it will work for any relay with a normally open contact -- that is, the contact closes when the relay is energized, and opens when the relay is not energized. A horn relay will be of the normally open configuration.

To identify the terminals, connect one of them to the negative post of your battery, and another terminal to the positive terminal -- it doesn't make any difference which relay terminals you use, just pick any two. If the relay clicks, you have found the terminals connected to the relay coil, usually identified as 85 and 86 on the relay diagrams. If not, try again til the relay does click.

Inside the relay, one side of the output contact is wired to one of the coil terminals, and the other side is wired to the third terminal. Since we have indentified the other two terminals, we know that the third terminal connects to one side of the contact, so all that is left is to determine which side of the coil the other side of the contact connects to.

To find out, re-connect the relay to your battery so that it clicks, or is energized. Using a voltmeter or a test probe, look for voltage on the output terminal. If you have voltage, the other side of the contact is connected to the same terminal as the plus side of the battery. If you don't have voltage, the other side of the contact is connected to the same terminal as minus side of the battery.

This may be enough information for you to finish your wiring, depending on how familiar you are with electrical circuits. If you're not that comfortable with it, use the following guidelines.

Once you have found the terminal that connects to both the coil and the contact, label this terminal as "common." Label the terminal that goes to the other side of the coil "switch." Label the contact terminal "output."

For several reasons, I recomend wiring your fan setup as follows:

Mount the relay under the hood, close to your power feed and the fan motor.

Connect the "common" terminal to a power source. If the source of power you choose is not fused, add a fuse as close to the power source as possible.

Connect the "output" terminal to the + side of the fan motor. Connect the neg side of the fan to ground.

Connect the "switch" terminal of the relay to one side of the on-off switch, and connect the other side of the switch to ground.

If this doesn't make sense, let me know and I can post a few diagrams on my web site.

Dan Masters, Alcoa, TN

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