Trying to diagnose the A/C in my '85 300D.

Installed a known working temp regulator (from my other car!) in the bad car. As soon as I applied power.... I heard a loud POP from the temp regulator box! (Now this box rattles just like the one that I pulled out did!)

Anybody have any idea what could cause the temp regulator box to blow like that?

I'm afraid I've let the magic smoke out of too many of these boxes!

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'83 300D
'85 300D
'01 VW TDI