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A few years ago, when I swapped out a PMD in a hurry while at the store, I installed my PMD onto the cooling plate, back inside the front bumper, and I didn't notice these plastic covers.
Truck started stalling while idling a few weeks ago, and continued getting worse for a week or two. So one day, I quickly swapped out the PMD and it's fixed, perfect. Zero trouble since.
I assume the plastic covers, in the pics below, held the heat in and burned up the PMD. I'm surprised it lasted through all the summer heat while hauling, dumping, and running the lime spreader. No other PMD I have ever had, has had these covers. I've only bought two PMD's, ever, and they were used.
This post is a simple reminder to everyone to double check before installing. Pop these off and trash them.
If somehow I'm wrong and they are supposed to be on there, it'll take some convincing. I looked up the covers and they are listed as:
THERMALLOY 8909NB Insulating Cover for TO-3 Semiconductor NSN# 5961-01-103-6222

Product Gadget Material property Font Electric blue
Car alarm Peripheral Input device Guitar accessory Musical instrument
 
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I usually pop off the covers as well before I mount them to the heat sink.Seems to perform well but my last grey stanadyne pmd started giving me grief with low mileage on it even with the covers removed.
 
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1997 Grumman Olson 12 ft step van, NA 6.5 diesel (RPO L57), 4L80E transmission
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That's a pretty specific alloy to be using to make a cosmetic cover, wouldn't you think? Considering the environment where this device is designed to work, in the hot valley of a V-8, I'd bet those thermalloy covers over the power transistors are there to shield them from external heat by shunting that heat around the cases of those transistors to the heat sink to be cooled by incoming fuel. The idea would be to limit the heat the transistors must endure to the heat they generate themselves while operating.

I think.
 

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That's a pretty specific alloy to be using to make a cosmetic cover, wouldn't you think? Considering the environment where this device is designed to work, in the hot valley of a V-8, I'd bet those thermalloy covers over the power transistors are there to shield them from external heat by shunting that heat around the cases of those transistors to the heat sink to be cooled by incoming fuel. The idea would be to limit the heat the transistors must endure to the heat they generate themselves while operating.

I think.
In the original design, that may be the case.
I am not sure if they put any thought on the composition of the cap anymore, though.

In my case, I took it off also before I mount the PMD into the heatsink.
I also use the heatsink compound like the one used in Computers like Arctic Silver.
 
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