Concur with auto parts store recommendation--I just posted a question ( http://www.defendersource.com/forum/...ors-40135.html
) re. a similar issue with a 300TDi--volts on the madman ems (which gets its positive juice from the fuse block and is grounded to the engine, so not true across-the-terminals voltage) were reading as low as 12.8V with headlights and A/C running, usually around 13.4 at highway speeds with no serious juice-drainers on.
Went to Auto Zone and they put one of those testers on it, had me take it to 1500ish RPMs and hold it, monitored voltage during starting, and checked battery amperage with the engine off. They said it was good to go--13.4 no load, 13.2 under load (fan and lights on), all within spec, according to the machine.
So that saved me a couple hundred bucks. I'll just keep an eye on the voltmeter and re-engage if the readings drop at all.