You don't HAVE TO ground out the multimeter on the battery, you can use any good ground. The readings you get may not be as precise, however as in my recent case, it doesn't really matter how precise you're resistance readings are when 3 GPs are completely open (infinite resistance). If you have some GPs on the brink of acceptable resistance you may want to ground directly to battery to get a more reliable reading.

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'85 300td Frybrid WVO Kit