One of the most frustrating issues for any Pontiac owner is an electrical problem, especially one that drains the battery. You didn't leave the headlights on or the door open, so why is the Pontiac stone dead? Thinking it is just a fluke, you charge the battery or jump start the engine, and all seems good, but overnight it is dead again.

In the parlance of automotive repair, this condition is identified as a parasitic drain. In lay terms, it describes a circuit that is powered even though the ignition is off. It can be a frustrating and often confounding issue, since you really do not know where to look for the cause.

The basis of any electrical-circuit diagnosis is accurate information. Every person who has ever tried to diagnose an electrical problem has experienced the frustrated feeling of staring at a circuit and not being able to see what is wrong.

Electricity cannot be seen as it flows through a wire. So in order to diagnose a problem, it is necessary to see the manner in which the circuit reacts to electrical flow. Whenever electrical systems are being diagnosed, you need accurate instruments and wiring diagrams. These aids will allow you to look inside the circuit so you can interpret its function.

The Matco Parasitic Drain Tester

Professional-tool supplier Matco Tools is constantly looking for ways to make automotive repair easier, quicker, and more accurate, all while being less invasive. This is especially true when dealing with a parasitic drain or most other electrical issues. The task can be likened to trying to find a water leak in a house. Have you ever made a hole in the wall or ceiling only to find that the problem is not in that location? The same often occurs when working on electrical issues. You remove your Pontiac's interior (or some other part) only to find the issue is not there. Then, due to the nature of cars, the area doesn't go back together the same way since the parts are old and brittle.

HPP was very interested in the new Matco Parasitic Drain Tester (PN TH209). With it, a user can unobtrusively check each circuit in the vehicle without even removing the fuse to quickly determine the open circuit or circuits. (Matco's don't-remove-the-fuse feature is of great value since classic Pontiacs employ a Buss-type glass fuse that is often hard to remove without cracking the glass. Later Pontiacs employed a bayonet-style fuse; it too was fitted very tightly for reliable service.)

Before we explore the virtues of the Matco parasitic drain tester, let's review traditional electrical-diagnostic steps that can be used on any Pontiac and contrast them with the new technology of the Matco tester. We don't want to falsely represent that an electrical draw cannot be diagnosed without the Matco tool—the tool just makes it faster and easier. There are other electrical issues that can happen in a Pontiac that still require the use of a basic ohmmeter or voltmeter, and these will be briefly reviewed. In HPP tradition, a complete representation will be presented.

Volts/Ohms/Amps

The most practical meter for any enthusiast is a multimeter. This tool combines a voltmeter, ammeter, and ohmmeter in one unit, either analog or digital. The one disadvantage of a multimeter is that the measured current draw is usually limited to 10-20 amps. Therefore it is good for almost every electrical circuit in an older Pontiac except the starter.

An ohmmeter measures resistance and can only be used on an unpowered circuit. It has no polarity, so it doesn't make any difference which lead is connected to either end of the circuit. The only time this doesn't apply is when checking a diode. For example, if you want to check the amperage draw of an electric antenna, the circuit must be disconnected and the meter installed in series, the ignition turned on, and the antenna activated to obtain a reading. If the resistance of the power-antenna motor was to be checked, the component must be unplugged and an ohmmeter lead attached to each wire in the plug of the motor.

An ammeter measures the load that is being consumed or used by the circuit. It is polarity sensitive. To obtain a reading, the circuit must be connected and in operation. A traditional ammeter needs to be connected in series with the circuit. That describes the circuit being broken and the meter installed as part of the current path. Some more expensive meters (such as Matco PN MLCP687) use an inductive clamp that is just clipped around the wire and does not need to have the meter become part of the circuit. This design reads the magnetic field that is created around the wire from the electron flow.

A voltmeter is connected in parallel; the circuit remains intact and isn't invaded to obtain a reading. Like the ammeter, it requires proper polarity when being connected to the test point. For example, if you want to check the voltage of your Pontiac's 12V battery, you attach the meter leads to the proper terminal without disrupting the connections at the battery.

Since each meter is slightly different in operation, you should read the owner's manual and become very familiar with the scales, settings, and connections before attempting any diagnostic procedure. Nothing is worse than trying to diagnosis an electrical problem as you try to figure out the test instrument.


1. The Matco parasitic drain tester makes troubleshooting electrical problems easy. It’s available directly from Matco for $199.

2. When testing for a parasitic draw, the fuse box needs to be accessed. The Matco parasitic drain tester is 100-percent compatible with classic Pontiac fuseboxes that employ Buss-type glass fuses, like those shown here.

3. Some late-model Pontiacs have two fuse boxes—one on the side of the dash, like the Firehawk shown here, or in the glovebox…