Used EZL Installation Caution

[Strife]

I got in a 420SEL EZL with the idea of either trying it out in a 560(it appears to have a little more advance than the 117's) or trading it in the future. I notice that the bottom of it was caked with heat sink compound. For non-electronic geek types, I caution that this material, dried and lumpy, is practically worthless for it's intended purpose once removed from installation and fresh material is absolutely needed. Considering the high expense of a new replacement and the rather abrupt failure mode, a little attention here is important.

I suspect that more modern (post-80's EZL's, or new replacements) might have "phase-change" compound under an aluminum, plastic, or silicon sheet. This material also is "one-time-use" and will _never_ be as efficient once deinstalled/reinstalled. These notes do not apply to those types, or to new EZL's.

The EZL contains a very high-power, high-voltage transistor that dissapates a lot of heat, which has to go somewhere or the transistor (and therefore, the EZL) will be destroyed. The heat sink compound fills in the microscopic gaps between the mounting area (the body of the car) and the EZL, allowing heat to spread out to the body of the car. Not applied or misapplied even slightly incorrectly, the efficiency of the thermal transfer drops like a brick.

How to do this correctly:

1. Get some heat sink compound. Radio Shack sells small tubes of it; you might be able to get some from Comp USA or a local computer shop that caters to "geeks" who overclock their computers or build their own. Possibly, M-B or automotive stores sell a kit for this. Hard to beleive, the most common type of this compound is very similar to Desitin, which you put on a kid's bottom to help with diaper rash.

2. Clean the bottom of the EZL spotlessly, and also the mounting area (again, spotlessly). I mean, both surfaces should be as flat and smooth as a piece of paper.

3. Spread the heat sink compound on the EZL with a flat item (bondo applicator, for example). Remember, the purpose of the compound is NOT to "insulate" or act as a major filler on a visible level, but to fill in TINY imperfections. No more than 1/32" of an inch, evenly spread, should be needed.

4. Bolt it in solidly, torquing evently in a cross-fashion. It is NORMAL, and in fact, DESIRABLE, that most of the material you put down will squeeze out.

By doing this properly, you will maximize the lifetime of this part.

[Strife]

I looked into this a little more, and the original MB "interface" from the body of the EZL to the body of the car is a thin sandwich of the following, starting at the body of the EZL:

1. a thin epoxy or plasticisized paint on the bottom of the EZL. I SUSPECT, but do not know for certain, that this part is electrically "HOT" - power output transistors of that era usually were. Do not remove or nick any of this, unlike this writer did! The paint probably provides some insulation (but not all of it, see below).

2. Conventional zinc oxide based heatsink compound

3. The "foil" described in the manual is probably Kapton - a thin plastic, very tough and abrasion-resistant, with good electrical insulation and thermal conductivity properties. This is probably a backup to the paint described above.

It would be VERY BAD to reinstall an EZL without heat sink compound or proper electrical insulation.

[KCM]

Strife,

Came across your excellent thread. How much of the heat sink compound is needed for one module? The local Radio Shack has it in 6 gram tubes. Is this enough? And does the entire rear surface get covered or just one section?

[Strife]

There was a guy on another forum who actually raised the ECU off the body of the car so it would get some ventilation - OWWW! Have you checked the list prices of one of these recently? OMG.

There is, or was, a kit available from MB that contained what you need to mount this unit but I have not been able to find it in the EPC. I'm pretty certain that the entire surface of the bottom would require HS compound and Kapton sheeting.

To quote the service manual:

It is installed with heat-conducting paste to dissapate heat [not true, it conducts heat to the car body]. When exchanging this part, the protective foil must not be removed. The foil does not impair heat dissapation.

To exactly duplicate how a new unit would have been delivered, I think would take a thin layer of heat sink compound spread with a dull, but absolutely flat spatula - like one of those stainless tools used for plaster. By thin, I mean less than 1/2 mm, with few or no voids. Then on top of that (really, the bottom, as installed) would be a Kapton sheet. I've been looking for suitable Kapton sheeting, at a reasonable price and quantity. UPDATE: I've seen some good pieces on eBay, for example, item 180028543577, but it might not be wide enough.

Spread that thin, a radio shack tube goes a long way. But The kapton sheet is really important as a line of defense against a short. If the paint on the body of the car is missing, and the paint on the bottom of the ECU is missing, the only thing stopping a short is the HS compound, not good.

This is a very good article on the subject:

http://sound.westhost.com/heatsinks.htm