How to find the problem area - without wasting fuses.

First off, it's no use replacing fuses because they'll keep on blowing. Instead, make yourself a test light consisting of about fifteen feet of line cord (lamp cord). Enough to walk around the car with it. Put alligator clips on one end, and a stop/tail lamp on the other. Wire both filaments together as shown in Fig. 1, to increase the current drain.

Be sure to wrap insulation tape around any exposed connections, including the body of the lamp. See Fig. 2.

Operation

Clip the test lamp across the offending circuit (with the fuse removed).

Assuming that the tail/parking light circuit is the problem, turn on the parking lights. If the short is still there, the test light will glow brightly. Ohm's Law states that the sum of the voltage drops (R1, R2, etc.) must equal the supply voltage. Therefore, for all practical purposes, the test light will have 12 V across it under short circuit conditions.

Now go around the car and jiggle the wiring around while watching the test light. Pay particular attention to wiring connectors, light fittings, behind the dash connections, and places where wiring goes through sheet metal.

When you get to the intermittent, the test light will dim, indicating that R2 (see Fig. 1) has become unshorted, and is doing its share of dropping the supply voltage.

Once you've located the physical area of the short, it's usually obvious where and what the problem is ( a loose wire from the instrument panel lights touching the sheet in my metal case). Or maybe a wire has come adrift in one of the tail lights.

This method is completely safe, provided that you have insulated the test light properly.

It can be used on all fused circuits. However, when checking head light circuit shorts, the test light will not dim as much when the short is located, because of the lower resistance of these units (Ohm's Law once again).

Other Uses

Checking for 12V power. Clip one end to chassis and touch the other end to the point where the power is supposed to be.

Checking Fuses. Turn the circuit on, with one clip grounded to the chassis, the test light should light when the other clip is touched to either end of the fuse holder ( with the fuse in place of course).

Static Ignition Timing. Clip one end to the chassis and the other to the wire going to the distributor (not for electronic ignition). Turn on the ignition. Push the car forward while in 4th gear. Bulb should light when the pointer on the front pulley indicates "F".

CAUTION: Remove the high voltage lead from the coil or distributor cap before doing this. Note: this is not the best use of the light because the wattage is higher than is should be for this purpose - But it works.

Trouble light. Need to change a wheel at midnight? Clip one side to the chassis and the other to a convenient 12V source and you're in business. Might keep you from getting run into as well?