Diagnosing Auto Electrical Problems via Voltage Drop
Notes on diagnosing a dead battery in my F150 pickup.
After repeated dead batteries over the course of a few weeks, I decided to investigate the cause.
I disconnected the battery ground. Using a Fluke 179 DMM, I determined that current draw, with everything off and the doors closed was 187mA. 50mA or less is considered acceptable, so it appeared that one or more systems in the truck were drawing more current that expected at rest.
To isolate the source of the current draw, I tested the voltage drop2 across each of the fuses in the two fuse boxes. This was done by setting the Fluke to DC mV and then touching both probes to the test points on each fuse in the fuse boxes.
Fuses in the engine compartment showed little to no voltage drop.
Two fuses in the cab showed significant voltage drop. Specifically, Fuse F14(15A) showed a drop of 0.5mV and F15(5A) showed 1.0mV.
Using tables1 to convert the voltage drop to current draw indicated a 113mA draw on F14 and a 66mA draw on F15, or a total draw of 178mA on both circuits. This is not too far off from the 187mA measured directly at the battery.
Using wiring diagrams, the sources of the majority of the draw were found and corrected.