Something Old, Something New

Now let's look at some traditional procedures for testing parasitic drain:

1. Checking for a draw with the ignition off

Tool: Test light or multimeter set on amperes

Problem: The battery goes dead when your Pontiac is not started or driven for a while.

The first nice day of spring is here and you want to go for a ride. But as you open the door of your Pontiac, a dark dome light and a dead battery greet you. After charging the battery, you use a simple test light and discover a draw. To recap, that means a circuit is not shutting off. Remove the battery ground cable and install a test light in series to the ground path. If the light illuminates, there is a draw. Now start removing fuses from the fusebox until the light goes off. When it shuts off, you found the circuit that is staying powered on with the ignition off.

(On newer Pontiacs, it's better to use an ammeter connected in the same manner to identify the amount of the draw. A modern electronic radio with station memory or a clock will illuminate the test light since they draw current with the ignition off. Not to worry—these circuits use so little energy, it would be hard for them to kill a battery even after months of vehicle storage.)

2. Checking alternator output Tool: Multimeter set on volts

Tool: Multimeter set on volts

Possible use: Confirming charging circuit output

With the engine off, connect the voltmeter leads to the battery's positive and negative terminals. The reading should be a minimum of 12.6 volts and can be as much as 2 volts higher due to the accumulation of what is called a surface charge. Now start the engine and check the battery voltage. The reading should be higher than with the engine off. Next race the engine and have a helper turn on electrical loads such as the headlights, wipers, heater, and A/C. With the engine at approximately 2,000 rpm, the battery voltage should be at or near the reading at idle with no electrical load. If not, then the charging circuit isn't working at its full potential and may have voltage regulator or alternator issues.

3. Check battery cranking voltage

Tool: Multimeter set on volts

Possible use: Determine if the battery has enough voltage to crank the engine

With the voltmeter attached to the battery, have a helper crank the engine over. You may want to disable the ignition coil so you can extended the cranking period. With a fully charged battery, the voltage should stay above 9.6 during crank. If the voltage drops below that, it's possible the battery is weak or the starter is drawing an excessive amount of current. If the voltage stays high and the engine turns slowly, there is a problem with the starter circuit, such as a poor ground, but the battery is fine.

4. Checking a diode

Tool: Multimeter set on ohms

Possible use: Confirmation of an inoperable component that employs a diode

Diodes are used in alternators and many early air-conditioner systems. The diode was usually placed across the compressor-clutch circuit to avoid a voltage spike back into the charging circuit when the clutch shuts off. If the diode is open, the circuit will not work.

Gain access to the diode. A good diode will only pass current one way. This will mean the ohmmeter should read little or no resistance with the leads connected one way and infinity with them connected in reverse. Anything else and the diode is defective and needs to be replaced.

Using the Matco Parasitic Drain Tester for Parasitic Draw

The Matco parasitic drain tester has the unique ability to shunt or electrically remove the fuse from the circuit without physically removing it from the fusebox. This is accomplished by the advanced circuitry in the $199 American-made tester and your programming for the capacity of the fuse circuit being tested. For example, if the circuit being explored has a 20-amp fuse, you should calibrate the tester to tell it that. TH209 has the ability to withstand a draw of up to 80 amps, which is between four and eight times that of even the most expensive multimeter.

Once set properly for the fuse value, the next step is simple. Just use the two leads to make contact with the fuse and read the amount of current (amps) being drawn by that circuit. Go from fuse to fuse until you find the circuit that is being powered and killing the battery with the ignition off.

Next, look in the wiring diagram for your Pontiac to determine what component(s) that circuit powers. You may discover that a particular fuse powers the underhood, glovebox, and trunk lights. Remove one of the bulbs and test again to see if the draw is still present. For example, if you first remove the glovebox lightbulb and the draw on the TH209 tester is still the same, put that bulb back—the problem is not there. Now go to the trunk and remove that bulb; when you look at the TH209 tester, the draw is gone. You have quickly and easily learned that the trunk-light switch is faulty and keeps the bulb on, thereby killing the battery.


4 … and another under the hood.

5. If after setting the correct amperage rating, the instrument’s LCD screen reads 0A (amps), the circuit does not have a draw.

6. With the ignition turned off, however, another circuit reads 1.5 amps, which explains why this Bird’s battery kept going bad after several weeks of vehicle storage.

7. Now it is time to call out the Matco low-current probe/digital clamp multimeter (PN MLCP687). It’s available directly from Matco for $207.