silicone paste?

Get some silicone grease. It's also known as "tune up dielectric grease" that you put on the spark plug boots to keep them from sticking to the spark plug.

-Bruce

That stuff is just what you DON'T want. It has zinc oxide in it and is formulated to conduct the maximum amount of heat.

Yes. Silicone dialectric grease.

It looks like vaseline. Auto parts stores will have it. Good for the job you mention, waterproofing electrical disconnects, keeping park light bulbs from corroding and siezing in their sockets...lots of other uses. Good stuff.

Actually not that effective. For good heat transfer, you want something like Arctic Silver, which contains colloidal silver. Some of the white stuff is also a titanium compound, tends to be a bit "whiter" than zinc oxide.

Not that I would guess either would be a good idea.

Unless you get it between the battery post and the terminal, where you discover what "dielectric" really means.

You need a thermal paste sometimes called heat sink grease. A new module should come with a sufficient amount. You can get a good compound from Radio Shack, Circuit City, Best Buy, etc. It is widely used in computer CPU's to attach the cooling fan to ensure good heat transfer. There have been many faulkures of ignition modules traced to old dried out grease or, worse yet, no grease used.

Whoever said that the paste is for insulating the ICM from engine heat was dead wrong, and auto mechanics are often wrong about electronics. The purpose of the paste is to conduct heat _away_ from the ICM and into the engine, which, amazingly, is actually cooler, and if the ICM really had to be insulated from engine heat, it wouldn't be mounted inside anything as hot as a distributor. Some transistors are rated for 200 Celcius (not Fahrenheit) continuous use.

Get some dielectric grease, made of silicone oil mixed with zinc oxide. Loctite makes some, and it's commonly applied to spark plug boots to seal them from moisture, but Radio Shack (no. 276-1372) and electronic parts supplies sell it as heatsink grease. Don't get anything runny, like Dow Corning DC4 or DC22, which will drip and attract gas and oil fumes that will form carbon tracks; the Loctite and Radio Shack products are thick enough. And don't ever use any substance that bears metal powder, like Arctic Silver, a heatsink grease made for computer processor chips, because it conducts electricity well enough to create shorts at high voltages. The makers of the product deny that it conducts significantly, but they also said not to use it inside a TV, where some of the voltages are in the same range as those of automotive ignition systems.

I don't know why your previous ICM had no silicone grease on it, but I suspect that's why it failed so soon. I've never handled a factory ICM myself, but I did tinker with a couple of homemade electronic ignitions for myself back in the 1970s, and I always used silicone grease or RTV on the transistors and SCRs. I never had a failure, except when rain leaked in a box I failed to seal and when distributor caps cracked every few weeks from excessive voltage (my first try at CD ignition, adding regulation stopped the cracking).

William R. Watt wrote in article ...

You should ask for silicone heat-transfer compound. It is used to transfer heat away from the module into the breaker plate.

Silicone di-electric compound is NOT the same thing. Di-electric compound has electrical insulation qualities that helps keep sparks from jumping around. That is why it is used on spark plug wires.

Bob Paulin - R.A.C.E. Race Car Chassis Setup and Dial-in Services

I thought it was just to make the boots easier to remove next time and the insulating properties were a requirement for that job.

Jimmy wrote in article ...

Di-electric compound is meant to seal the electrical connection from external corrosive elements. It is used at points other than spark plug boots around the car such as lamp sockets.

Making the boots easier to remove is merely a great side benefit.

Dielectric compound is not _necessarily_ the same thing as heat transfer compound, but in practice it's identical -- silicone oil mixed with zinc oxide powder. The exception are silver-bearing heat transfer compounds marketed toward ignorant computer users. But any silicone heat transfer compound made for high voltage use, like ordinary Radio Shack Transistor Heatsink Grease, Tech Spray Heat Sink Compound, or GC Electronics heatsink grease, will also be dielectric compound. Some of heatsink compounds aren't made with silicone grease but instead with ester oil and are labelled "silicone-free," but I don't know if they're safe for automotive plastics and rubbers, so I'd avoid them. Also some silicone-based compounds contain very little zinc oxide or other oxides and can drip (Dow Corning DC-4 is notorious for this) and should not be used on ignition systems, especially not around the distributor.

You do not want Arctic Silver around high voltage equipment. It is not a dielectric, Arctic Silver co. has warned against using it in TVs, and I know of one horizontal output transistor (voltage similar to ignition primary's) that instantly failed when it was used. It's also false that zinc oxide bearing grease doesn't conduct heat well; otherwise it wouldn't be in the grease used on almost every transistor and power IC mounted against a heatsink, such as inside the better computer power supplies.

Wrong again. The grease will be pierced and squeezed out by mechanical pressure and won't interfere at all with electrical conduction but will simply seal out liquids. This is why high-amp slide switch contacts are often coated with dielectric grease before assembly, and even GE silent mercury wall switches have been. Anyone who coats battery terminals with grease after assembly isn't doing the job right.

from the responses there seems to be trwo functions

1. electrical insulation (di-electric, ie "non-conducting")
2. heat dissipation

I looked at the pastes at Radio Shack and at the auto parts store. The Radio Shack store was a small one and not well stocked but the clerk did find a very small container of the "heat sink grease" for $10. The package did not mention any di-electic property but I think it was reported in this thread that this paste is di-electirc. That's all this radio Shack store had. The auto parts store had two brands of silicone di-electric paste at $5 and $8. The lower priced one was a store brand. Listed on the back of one of the packages were recommended applications including "ignition modules". I didn't read anything on the packages at the auto parts store about heat dissipation.

I'm wondering what it is that gives paste its heat dissipating quality and if the cheaper pastes at the auto parts stores would dissipate heat.

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"The 120 Series Silicone Oil-Based Thermal Joint Compound fills the minute air gap between mating surfaces with a grease-like material containing zinc oxide in a silicone oil carrier. It possesses an excellent thermal resistance of only 0.05[deg]C/W for a 0.001 in. film with an area of one square inch."

|> Can anyone tell me what, if anything, I should ptu between the ICM and the |> distributor? If I were to get something at an auto parts store what should |> I ask for? |>

|> thank you. |>

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Rex in Fort Worth

True.

No, but it IS true that it isn't even in the same ballpark for heat transfer efficiency as colloidal silver.

Ummm, Arctic Silver is a common way to cool a hot Pentium, and is approved by Intel as not voiding the warranty on their Pentiums... at least for an OEM manufacturer that might give them grief about those cheesy thermal pads.

It dissipates heat mostly because it's much denser than air. About any liquid or solid that fills the gaps will be about 50 times as good as air, and in practice anything that doesn't drip or evaporate will work. Metal oxide powders, most often zinc oxide, are usually added so dielectric grease to prevent dripping and improve heat conduction. In a pinch you can use high temperature grease, but it can make damage vinyl and turn it into hard cheese, but I'd probly use silicone rubber sealant (RTV) instead.

The clear "dielectric" grease found at parts stores isn't a great heat conductor because its pure grease. Its questionable whether it might even be WORSE than bare metal-to-metal. The white "heat sink grease" is better because its loaded with fine metal-oxide particles (zinc oxide, I imagine) that do conduct heat much better than the grease without being a good electrical conductor. There are still better heat-conducting greases that can be used where electrical insulation isn't needed or is desired (like silver powder loaded grease and copper powder loaded grease). Those can be used, for example, between a transistor's collector thermal pad and a heatsink in a common-collector ("emitter follower") amplifier circuit.

the di-electric paste at the auto parts store has "heat resistant" written on it which I assume means it will not deform when heated. I hope it does not mean that is acts as a thermal insulator. I do have a pot of heat resistant wheel bearing grease. :)

the ICM is black plastic with a stamped white metal base and two bolt holes lined with metal. around the edge between the metal base and the plastic body is a dried white paste which looks like it bled out and was wiped off when the ICM was made. I can't tell if the ICM was made to be electrically insulated from the distributor body. When the ICM was hooked up for testing one lead was connected to the metal base at the bolt hole. Ground? If so would than mean the ICM is not an electrically insulated design?

When testing, leads were connected to the pickup terminals I guess to stimulate the ICM, and leads were connected to the terminals for the coil, I guess to read the ICM output. There is an additional terminal on the ICM which was left unconnected during the test. the store clerk who did the test on both ICM's said it was "probably just a ground". The wiring diagram in my Haynes manual shows it going to the car's computer. Duh. Makes me wonder about the test. Does it sound reasonable to test the module without hooking up the computer lead?

the stamped metal base on the ICM is flush around the edge and at the bolt holes, I assume for structural contact with the distributor housing it is bolted to, but it is slightly depressed in the central area leaving an air gap between the ICM and the distributor. On the distributor from the wrecker there is a clear (yellowed) paste (like petroleum jelly) where I took the ICM off. (I have the distributor in front of me on the desk as I type.) There is evidence of this clear paste all over the base of the ICM even after I wiped it with a rag to clean it up before taking it for testing, and squeezed into the bolt holes as well. I assume its the original ICM and the paste was put on when the car was made. I tested a drop of the paste off the distributor with an ohm meter and got no current reading but at the low dry cell voltaage that may not mean anything.

It looks like the car came new with a di-electric paste but not a heat transfer paste.

I have a very comprehensive (for me) book on car electronics from the public library with a chapter on electronic ignition but nothing about installation and applying paste. I'd like to read as much information as I can about this situation to avoid the cost of more ICM's ($147 - $291 new locally), and if possible to improve the weak ignition on the Festiva which was noted in the Lemon Aid guide to used cars. Its -10 deg F outside today so it's too cold to work on the car, a good day to stay indoors and think.

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Such ignorant folks as the Intel engineers who designed the Pentium chips and offer engineering support to OEM's that use them. Which pretty much sums up the accuracy of your blathering.

What's its efficiency, about 0.1W/cm^2/cm?

Why has grease laced with zinc oxide been used far more than any other in factory assembled products, and why have auto makers recommended it for their ignition modules since the 1970s? Virtually nothing else was available back then, except silicone grease containing nothing else.

You quoted me out of context, and I was warning against the use of silver-bearing heatsink grease for auto ignition modules, which usually have exposed connections with hundreds of volts on them. As for PC CPUs, nothing more than ordinary heatsink grease is ever needed.