Tech Tip: Polarity of the Coil

As our Classic Thunderbirds get older and older, and the mechanics who grew up working on these cars become fewer and fewer, simple common repairs and procedures become overlooked or forgotten. This is the case with the installation of your car's coil. The coil is the cylindrical object which is mounted on the top of your engine. It has a short "spark plug" type wire attached to it which connects the coil to the center of your distributor. The coil has two other wires attached to it. These wires attach to positive and negative terminals on the coil. The order of placement of these wires on the terminals of the coil is what determines the coil's polarity. The coil is made to conduct current in one direction, from the battery to the distributor. If your wires are connected to the wrong terminals, your car may still function, but you will be conducting current backwards through the coil. This is not very good for your car or your coil.

Incorrect polarity of the coil is a common problem for original six volt system 1955 Thunderbirds. The six volt Thunderbird uses a "positive ground" as opposed to the 1956 and later Thunderbirds with use a "negative ground." What this means is that the positive terminal of your coil, marked with a "+" or the letters "POS", should be connected to the small wire which goes to your distributor. The negative terminal of your coil, marked with an "-" or the letters "NEG", should be connected to the small wire which goes to the "hot" or battery side of your starter relay.

On 1955 Thunderbirds which have been converted to a twelve volt operating system, and on al1 1956 and 1957 Thunderbirds, the correct polarity of the coil is obtained by attaching the wire from the positive terminal of your coil to the "hot" or battery side of your starter relay. There will also be another wire attached to the positive terminal of your coil which should be connected to your resistor. The wire from the negative terminal of your coil should go directly to your distributor. As most cars today operate on a "negative ground" principle, only a mechanic familiar with a "positive ground" system would know the proper polarity for a 1955 Thunderbird coil. For this reason, you should check to make sure your coil's connections are as they should be. I thank Ken Smizinski for this month's "Tech Tip".

source: CTCC Tech-Tip Manual 1993-1997