Parasitic Draw Testing

October 2015  
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Rob Thompson's Automotive Technology Education Newsletter

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This newsletter is to share teaching ideas for auto tech instructors and to provide informational updates on what is happening in CT education around the country. 

Parasitic Draw Testing Using Voltage Drop

Several years ago at a NACAT session, I learned about testing parasitic draw by measuring the voltage drop across each fuse. For some unknown reason, having sat through the session and even seeing the test in action as part of the session, I didn't start using this method until recently. 

To put the test into practice, I used a vehicle recently donated by our city, a 2000 Olds Bravada. The battery will go dead if left sitting in the lab, so this seemed to be a good candidate for the test.

To begin, I used a DMM to measure the voltage drop across each fuse in the underhood fuse box, shown in Figure 1. All the other fuses read 0.0mV except for one, the TCB fuse, shown in Figure 2

Figure 3  shows a DMM connected in series at the battery negative terminal and negative cable to measure actual current draw. This is for demonstration purposes since the voltage drop method does not require measuring the current flow from the battery.The second DMM is showing the voltage drop across the suspect fuse. The DMM shows a voltage drop of 1.28mV, not a lot but none of the other fuses showed any voltage drop at all.


Figure 4 shows the current draw with the fuse removed. 

Using this method is a quick way to narrow down a parasitic draw. My plan is to make or adapt a tool to match up with the openings in the fuse bodies to make testing even faster.

More to come next month.
Stay Tuned!

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