Measuring Parasitic Load

Temporarily connect a jumper wire c'3.5 mA before connecting the ammeter

If an aftermarket alarm system is installed, connect a jumper wire first. Then, connect the ammeter in parallel. You can now remove the jumper wire and measure the parasitic load.

Temporarily connect a jumper wire c'3.5 mA before connecting the ammeter

Determining the Location

All that is left now is a process of elimination. Since you know which fuse is connected to the problem, you now need to find which circuits are connected to that fuse and disconnect the circuits one-by-one until parasitic load drops off. There are two different "strategies" that you can use to pinpoint the location of the parasitic load:

• Disconnect components that are fed by that fuse. Look at Section H Power Source (Current Flow) to find the components which use that fuse, and one-by-one, disconnect these components until the parasitic load drops off. This simple, straightforward approach can have some time saving advantages if there are not a lot of components that are connected to the fuse (too many connectors to disconnect), and if most or all of the connectors are easy to get to.

When disconnecting the components, choose each one strategically. Go first to the components that are the easiest to get to, or to components that have a history of causing these unwanted "draws". Areas to check first include lighting circuits (trunk light, vanity light, interior light, etc.), and aftermarket accessory installations.

• Follow the current flow through the Junction Blocks

If there are a very large number of individual components which use the fuse, you may want to isolate the junction block used by the problem circuit. By finding the junction block, you will be able to narrow down the number of component connectors you will have to disconnect. The procedure to follow is listed below. Note that this is a time consuming process, and should only be used if there are too many components that would have to be disconnected, or if the component connectors are not easy to get to.

Procedure for Mapping Current Flow through the J/B's

1. To determine which Junction Block connectors are fed by that fuse: Look at each System Circuit Diagram for that specific fuse at the top of the page. Note any Junction Blocks or Junction Connectors that are used, and write down the connector and terminal numbers. (This is a time consuming step, but it has to be done.)

2. Disconnect each junction block connector individually until the parasitic load drops to a normal level. By doing this, you are identifying which connector provides power to the problem circuit.

3. If a single J/B connector has two or more pins which branch into other circuits, you can isolate the individual circuits on the J/B connector by carefully removing the specific terminals, one at a time. If you have an inductive ammeter which is sensitive enough to measure the parasitic amperage, simply clamp around the specific wires to determine which one is connected to the problem.

4. Look at the list of J/B connectors and terminal numbers that you wrote down earlier. See which circuits use that specific J/B connector and pin.

5. Isolate individual components in each of those circuits. Disconnect the connector at any of the loads or at a wire harness-to-wire harness connector. Watch for the parasitic load to drop to a normal level on the ammeter. When this happens, you know that you have disconnected the problem from the circuit. Again, you can also use an inductive ammeter (if the amperage is high enough) to pinpoint the problem wire.

6. Reconnect the connector, and strategically disconnect other connectors until you isolate the problem.

7. Once the location of the short causing the parasitic load has been isolated, make the repair.

Diagnosing Short-to-Grounds

A short-to-ground occurs whenever a circuit finds a path to ground before going through the load. Because current flow is no longer controlled by the resistance of the load, excessive current flow forces the fuse or circuit breaker to "blow", avoiding damage to the wiring.

Short-to-Ground The process for diagnosing a short-to-ground has similarities to diagnosing a Diagnostic Strategy parasitic ioad. The major differences are:

• You know exactly which fuse the problem is connected to.

• You need to connect a load (such as a test light, short finder, or headlight) in place of the fuse while isolating the location of the problem.

• You know that the short-to-ground will be located in either the load itself or in the wiring before the load. The problem can never be on the ground side of a load. Because the short-to-ground could potentially be located somewhere within the harness, the number of possible causes is multiplied.

Selecting a Load

A load of some type must be used in place of the fuse in order to diagnose the circuit. It is a common practice to use an ordinary 12V test light. But be aware that not just any test light will work. In fact, if the fuse circuit you are testing is connected to a number of unswitched parallel branches (especially lighting circuits), an average test light will be ON at all times, even if the short-to-ground is fixed!

In general, it is better to use a load which requires a few amps to operate, such as a sealed beam headlight. With a sealed beam headlight, you will see a "bright light" go "dim" when the short-to-ground is disconnected. But there are alternatives to this approach.

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  • Antti
    How to measure parastic load?
    4 months ago

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