Remote PMD mount question

[Am General]

I have a new PMD, and a heatsink to mount it to. I just was wondering what I need to take into consideration when mounting the PMD to the heatsink. I was planning on using heat transfer grease like they use for computers. Is there anything else I need to do? Does the back of the PMD need to stay dry? I notice 2 tiny holes in the diamonds on the back. Would it mess up if water got in there? I think that's the issue with mine since this all started when I drove through 4 feet of water. If anyone has some info as to what I need to do to mount this please help me out. I also have a spare heatsink if anyone needs it. It's way too big, but can be easily cut.

[DA BIG ONE]

Originally posted by Am General:
I have a new PMD, and a heatsink to mount it to. I just was wondering what I need to take into consideration when mounting the PMD to the heatsink. I was planning on using heat transfer grease like they use for computers. Is there anything else I need to do? Does the back of the PMD need to stay dry? I notice 2 tiny holes in the diamonds on the back. Would it mess up if water got in there? I think that's the issue with mine since this all started when I drove through 4 feet of water. If anyone has some info as to what I need to do to mount this please help me out. I also have a spare heatsink if anyone needs it. It's way too big, but can be easily cut. Heat sink paste is always a good idea. You can seal all the edges with silicone sealant.

I have 2 remotes 1st is heath unit for 2wd mounted under front bumper w/o any additional sealant, it's been in deep water w/o any issues. The other unit I sealed, but now wonder of it was necessary, been in deep water too. Had both units submerged for over 30 minutes on one trip.

And for more over kill I still have the pump mounted unit in place.

[Am General]

So it sounds like water is a non issue. I think I'll do a bead of RTV around it just to be on the safe side. Can't wait to see if this fixes my stalling issue. I hope so. Thanks for the info!

[DA BIG ONE]

Originally posted by Am General:
So it sounds like water is a non issue. I think I'll do a bead of RTV around it just to be on the safe side. Can't wait to see if this fixes my stalling issue. I hope so. Thanks for the info! Good luck!

[arveetek]

Before mounting the PMD to the heatsink, pop those diamond-shaped plastic covers off (just use a screwdriver....they'll pop right off!). Then use a nutdriver and tighten the four nuts that hold the transistors on. Just leave the covers off. This will help the heat transfer.

Casey

[BobND]

I have been tightening the nuts on the power transistors securely, then putting a dab of clear silicone sealant on the nuts and threads, to prevent any chance of them shaking loose (although, likely they loosen due to thermal movement of the aluminum heat sink).

Hopefully, the use of a large remote heat sink, and CPU heat transfer compound holds the heat down, and there will be less thermal movement, and stretching and loosening of the FSD-to-heat sink bolts.

I suspect the plastic covers are there to prevent any nut that might shake off from shorting between the metal case of the power transistor and the heat sink, and I feel leaving them off can only aid heat transfer, and with the covers removed, I feel the silicone sealant is good insurance against a nut ever falling off. (The transistor cases are isolated from the heat sink by a mica washer.)

I allow the silicone sealant to cure for 24 hours before proceding.

Then, I fill the pockets in the heat sink around the transistors with silicone dielectric compound, nearly completely full.

Then, I use the metallic silver computer CPU heat transfer compound between the FSD and the remote heat sink, in lieu of the supplied "gasket".

After tightening the FSD securely to the heat sink, I carefully clean up any heat transfer compound, and then seal around the FSD-to-heat sink joint with clear silicone sealer.

I've been told on this forum I am wrong, and the silicon dielectric compound, and the removal of the plastic covers, and the use of the silver-bearing heat transfer compound instead of the "shim" or "gasket" only makes the transistors run hotter, but I have done this mod on close to 10 vehicles, (some of my own, and some for others), so far, I have YET to have a second's trouble with any of them, in several years, and quite a few miles.

So, while I can't provide temperature numbers to back up these successes, the mod seems to be working in the "real world".

I wish I had the time and resources to epoxy temperature sensors to the metal power transistor cases, then bring the leads to the outside through holes drilled in the heat sink, seal around the leads, and then monitor the temperatures, with and without the CPU compound and the silicon dielectric coumpound I have been "bathing" the transistors in, to answer once and for all with which setup the transistors are cooled the best.