Diagnosing Parasitic Draw

Nothing can be more frustrating than going out to start your classic car and hearing only the click of the solenoid when you turn the key. Some will assume the battery isn’t holding a charge anymore and after spending hundreds of dollars on a new battery, only to be met the next day with a car that will not start again. The real culprit is a parasitic draw (also known as parasitic drain).

Parasitic draw on a classic car refers to the continuous and abnormal drain of electrical current from the car’s battery when the vehicle is turned off. While modern cars typically have small, expected draws (like from computers or clocks), classic cars—especially those without such electronics—should have little to no current draw when off. If a parasitic draw is present, it can discharge the battery over time, causing hard starts or a dead battery.

We join Mark Simpson in the shop to discuss what parasitic draw/drain is and how to troubleshoot your classic for the source of any problems.

Common Causes of Parasitic Draw in Classic Cars

– Aftermarket accessories (stereos, alarms, USB chargers)
– Faulty voltage regulators
– Old or modified wiring with corrosion or shorts
– Sticking relays or switches (e.g., trunk or glove box lights that stay on)
– Bad alternator diodes
– Clock or radio (if the vehicle has them)

How to Troubleshoot Parasitic Draw

What You’ll Need:

– Test lamp
– Multimeter with DC amperage reading capability (up to ~10A)
– Wrenches (to disconnect battery terminal)
– Patience and a systematic approach

Step-by-Step Troubleshooting

1. Turn Off Everything: Make sure all doors are closed, lights are off, and any accessories are disconnected.

2. Wait a Few Minutes: Allow the vehicle systems to settle after turning off. Some components may stay on briefly.

3. Set Up Your Test Lamp and Multimeter: Disconnect the negative battery cable. First put the test lamp in series between the negative battery post and the cable (one lead to the post, the other to the cable end). If the light is bright your amperage load may exceed the multimeter’s fuse and proceed with the test using the test lamp instead of the multimeter. If the test lamp is dim or no light is visible, Set the multimeter to DC amps (start with a 10A setting). Insert the multimeter in series between the negative battery post and the cable (one lead to the post, the other to the cable end).

4. Check the Reading: A healthy draw should be less than 50 milliamps (0.05A) on a classic car (often close to 0mA). Anything more than this indicates a parasitic draw.

5. Pull Fuses One by One: Start removing fuses one at a time (if the car has a fuse box or disconnect power leads to circuit breakers on non fused cars). Watch the test lamp or multimeter reading. When the current drops significantly, the circuit responsible has been identified.

6. Trace the Faulty Circuit: Once you know the problematic circuit, inspect all connected components: wires, switches, relays, bulbs, and any added aftermarket gear.

7. Inspect Grounds and Wiring: Poor grounds or deteriorated insulation can cause short circuits or small drains.

8. Test the Alternator: Even when off, a bad diode in the alternator can allow current to flow backward and drain the battery. Disconnect the alternator and see if the draw stops.

How to Eliminate the Parasitic Draw

– Repair or replace faulty wiring or switches
– Install a master battery disconnect switch to isolate battery when parked
– Add relays to isolate accessories when the ignition is off
– Use a trickle charger or battery maintainer for long-term storage

Take your time. Diagnosing electrical problems is seldom a fast job, but through the process of elimination you can isolate the problem and quickly resolve the issue, guaranteeing your classic will start when you are ready to enjoy it.