Parasitic Draw

[dr bob]

I use the clamp-on meter at the battery cables, usually the ground. From there I use the voltmeter across the fuses to try and isolate the drain path if it passes through a fuse, then plug a purpose-specific ammeter into the fuse holder, one that has a socket for the removed circuit fuse, and a separate fuse that probably won't protect the meter itself.

There's a collection of 'vintage' Fluke instruments and meters, but I readily admit that I use cheap Harbor Freight meters pretty regularly. The little red $5 (used to be giveaways) meters are too cheap to cry about if I get careless and let the smoke out. I worry more about passing ten Amps through those tiny test leads, but so far no fires or melt-throughs. Surprisingly, the HF meter read the same as my trusty forty-year-old Fluke 77, further backstopping the decision to use the HF ones first.

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There are inevitably flow paths through wires that are not fuse-protected, and at least the 928 is home to several-severals. Each needs to be tested.

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I wouldn't even consider doing much more than some basic measurements without a set of the factory wiring diagrams, along with some feel for the way those diagrams relate to the actual wiring in the cars. Somewhere in the early 1980's, the diagrams changed from the very easy to read versions that isolate loads in specific 'rungs'. For some reason, the drawings then, um, 'progressed' to spaghetti-and-spider-webs drawings. Makes things a lot more interesting, with page-links and all, such that you could easily have several "linked" sheets open at the same time to diagnose just one issue. They must have been trying to save paper or something rather than economizing on ink. Makes things a bit more tedious, and escalates the risks for errors unnecessarily. In my opinion anyway.


All fun stuff, part of the joy of owning cars that are rare mostly because of age-related electrical irregularities.