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The package the EZL is in is a lot like the package for an industrial semiconductor (and some of those can run into the hundreds for ONE TRANSISTOR, beleive it or not), which I'm pretty familiar with. The flatness and even the polish of both surfaces and the heat sink compound qualities are absolutely crucial. A gap that you can't even really see can kill the thermal transfer to the heat sink (the car body, in this case) and kill the part, either quickly or slowly. I THINK, but don't KNOW, that MB sells a kit for reinstallation, and I've been thinking about redoing the mounting of the one in my car (a 107, 20+ years old). I got in a used part of that era and the heat sink compound (besides being put on way too thick) had turned to brittle chalk; I suppose the one in my car is like that also. The compound appeared to be conventional zinc oxide based compound; there are "better" materials today but I don't know how they hold up under temperature extremes and exposure to moisture.
BTW, don't scrape off or damage the epoxy painted surface of the bottom of an EZL; the base of an EZL is probably electrically "hot" underneath and shorting it to ground would be "very bad". There is a very thin plastic over the heat sink compound that is under the EZL. It's very flimsy, and I have reason to think that it is electrical insulation - you don't really need much at 12 volts.
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