The Matco Low-Current Probe/Digital Clamp Multimeter

Let's be clear—the Matco parasitic drain tester identifies which fused circuit is staying powered, but it does not tell you anything more. You need to explore, disconnecting each component in that circuit until the draw stops—unless you have the Matco Low Current Probe/Digital Clamp Multimeter (PN MLCP687), which retails for slightly more than $200.

Doubling as a low-current voltage probe and traditional multimeter, complete with positive and negative wire leads, MLCP687 contains an inductive clamp that allows you to test various wires within a respective circuit to determine the accessory or component responsible for the current draw. When testing circuits on your Pontiac, don't be alarmed if you see minute voltage draws from various wires. Anything greater than 30 milliamps can, however, run down your Pontiac's battery over time.

When diagnosing an electrical problem, the more time spent thinking and analyzing the circuit, the less the Pontiac will be violated and the quicker the repair can be done. The proper procedure for detecting a faulty component should include individually disconnecting various accessories and/or removing light bulbs, while monitoring the meter's digital display. As soon as the numeric readout decreases, you've accurately located the suspect component and the appropriate repair can be performed.

The most common parasitic-drain culprit?

Don't forget, in many instances a parasitic drain can be caused by a bad diode in the alternator. When the ignition is off, it back-feeds to ground. In this case, you would simply electrically disconnect the alternator and check for a draw at the battery. In most if not all Pontiacs, the fuse box would not be the place to test, but a meter or test light in series with the negative battery terminal and cable would be the location.

Using the proper terms

Quite often, both the enthusiast and professional mechanic alike use the wrong terms when describing an electrical problem. They confuse a short circuit with an unintentional ground. Let me explain.

By General Motors' definition, a short is two circuits that touch. It is known as copper-to-copper. For example, you blow the horn and the dome light comes on. This is a short circuit. A short circuit can cause a parasitic drain if the one circuit is powered independently of the ignition switch. Let us say that a wire directly from the battery rubs against the feed wire for the cigar lighter, then the lighter would stay on all the time and kill the battery.

In contrast, a circuit that blows a fuse is an unintentional ground or, as some would like to call it, a short to ground. This bypasses the controlled load, and can be likened to taking a wire from the battery positive and putting it to ground. So if your Pontiac blows the taillight fuse every time you step on the brake pedal, then the voltage is not going to the bulb, but instead right to ground. The load in the circuit is the resistance to current flow—a sheetmetal screw through a wire is a direct path to ground, and the circuit will flow the full battery amperage potential until either the fuse or the wire burns up.

It is important to recognize this. When looking for an unintentional ground, you disconnect the Pontiac's battery; then, using the ohmmeter, you have one lead on the circuit being tested and the other to a ground. If there is continuity, then that circuit is grounded and is the problem.

When trying to find a short, the two circuits need to be identified. If there are any connections/plugs in the circuit, they should be systematically disconnected to close in on the area of the copper-to-copper contact. For example, if a certain fuse is blowing and the wiring harness goes into the car door and has a plug, open the circuit there and retest. If the fuse still blows, then you know that it it's not in the door, so there is no need to remove the door panel. —Ray T. Bohacz


At HPP, we were impressed by the functionality and versatility of both Matco tools and their prices. While either can be used independently, they simply work better as a team. TH209 allows you to quickly and easily determine the offending fused circuit, and MCLP687 allows you to isolate the accessory and/or component. At a couple hundred dollars each, they may seem like a lofty frill, but considering a typical automotive 12V battery can cost $100 or more, and being left discharged for any length of time can compromise its ability to recharge, Matco's latest offerings could be an investment in maximizing the reliability of your prized Pontiac.

8. It has positive and negative leads to test for amps, volts, or ohms, like we describe in this story.

9. Testing for an electrical draw on wires, like the trunk light on this Second-Gen Firebird, is as easy as clamping the instrument’s jaws around the wire. The device used inductive-field technology to accurately measure the draw, if any.

10. The clamp is useful wherever you can access an electrical wire on your Pontiac, like the alternator leads shown here.

Matco Tools
4403 Allen Rd
OH  44224