wish I was better at diagnosing that stuff. However, yes it's possible something shorted it from underneath. It might be a good idea to check under it with an inspection mirror. The A/C won't come on for other reasons too, like not making the pressure switch. The fan doesn't come on also until the temperature switch on the tstat housing is made(approx 52C). The best suggestion I can give you though, is to post this problem on the diesel discussion board. It has a lot of traffic and there are folks there who know much more about this.

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'82 300SD - 361K mi - "Blue"

"Good judgement comes from experience. Experience comes from bad judgement."

listen, look, .........and duck.