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Under the Hood: Solving a mysterious battery drain

Brad Bergholdt, Tribune News Service on

Published in Automotive News

Q: My 2001 Jaguar Type S has a battery drain that no one has found the reason for. The fuses and relays in all three panels have been pulled one at a time, and the meter reads 0.77 to 0.79. It has the factory alarm system, and will drain overnight if I don't connect the battery tender (charger). I'm hoping you can advise what to look at next. I can't afford a Jaguar dealer.

Anita M.

A: Wow! The good news is the drain seems to be consistent, which will make it easier to find than an intermittent one. The meter you mentioned was likely an ammeter (a function of a basic digital multimeter) installed between one of the battery's posts and its disconnected cable terminal. The reading appears to be about 3/4 of an amp, more than 10 times what's acceptable. With a battery drain this size, assuming the car has a good battery, it's conceivable the car may not start (crank) after perhaps a 24 hour or longer parking. Your battery may be degraded due to too many deep discharge cycles, causing a shorter window of starting opportunity.

Removing fuses one at a time is a great way to divide and conquer a fault like this, as almost the entire electrical system is protected by fuses. When removing a fuse drops the ammeter reading to perhaps 50mA (50 milliamps equals .050A), you've identified the offending circuit and are seeing just the tiny electrical consumption of control module memories still connected. Using service literature allows the specific circuit and perhaps a component to be identified and fixed. This can be something as simple as a glove box light not shutting off to something creepier like the Body Control Module failing to go to sleep. It may help to remember that this elusive electrical consumption is resulting in light, heat, motion, magnetism or sound to occur -- look around and keep your ears trained!

If all of the Jag's fuses were truly removed and the fault remained, one should also consider components such as the alternator, which isn't fused (try unplugging all connections) or perhaps a dealer installed or aftermarket accessory using an elusive-to-locate in-line fuse.

I consulted Shop Key Pro's wiring diagrams, specifically the "power distribution" section to see how and where all fuses and circuits connect to the battery + terminal. The first thing that caught my eye was a path straight from the battery to the "trailer towing power relay, and junction box" that branched off prior to any fuse box connections (if the diagrams are correct). These components may have been missed during the previous fuse and relay pulling, and unplugging them may be worth a try. Another eye catcher/opportunity was the huge 175 amp "clean power" fuse in the rear power distribution (fuse/relay) box, located at the right side of the Jag's trunk. This feeds power to the front power distribution (fuse/relay) box and would be great to temporarily unplug, dividing the car practically in half (if the drain goes away, follow this path!). Were it me, I might have skipped pulling this fuse, thinking it was for the alternator or such.

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I'm wondering if another parasitic drain testing method might point out something missed previously. Measuring the voltage difference across each fuse is easy (use an inexpensive multimeter's millivolt (tiny) scale). With the key off and all doors closed there isn't supposed to be electrical current going anywhere but to module memories. If a tiny or higher voltage drop (difference) is observed across a fuse's slightly resistive element, this means there's current flowing in that circuit. If the reading is true zero, there isn't. I'd try this on all fuses in all three fuse boxes. Hot-at-all-times control module fuses may show a teeny voltage drop due to the memory consumption.


Brad Bergholdt is an automotive technology instructor at Evergreen Valley College in San Jose, Calif. Readers may send him email at; he cannot make personal replies.

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