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Hi BB,
Regarding Zinc Paste:
In older cars they used brass or copper alloy (copper mixed with other metals) to prevent corrosion at the metal contacts. You know this from the "yellow-tinge" of the metal. In modern cars such as 900 series most electrical contact are already zinc-plated. Thus there is no need for zinc paste. In older cars yes. How to know if its already zinc plated? Look at the colour of the contact metal, if its "silver-white" then its already zinc plated.
The contacts of AFM, CPS, O2 sensor, ECU, ICU, fuel injector, idle valve and some relays are ALL zinc-plated.
The bullet type contacts at the firewall engine harness are ALL brass thus you could see some corrosion there. The RSR electrical contacts are also brass. Needs cleaning once in few years.
Thats why you'll see posts about reflowing the RSR once every few years. Once every few years is not long enough to cause any solder connection to crack. Its just that by removing and reinserting the RSR from its connector cleans the brass contacts.
A better than zinc contact plating is gold. Unfortunately its expensive for automotive use thus only available for more specialised items like for audio connectors in my hi-fi hobby.
Rhodium plating is also better than zinc but still expensive. Also used on some audio connectors in hi-fi and some very specialised use such as reed relays because rhodium is also a hard-wearing metal for the relay's internal contacts.
Don't use any abrasives (steel wool/fine sandpaper/glass fibre pens) to clean the zinc-plated contacts as the plating is very thin (few microns thick) and could wear them out. Use some ISOPROPYL alcohol on Q-tips and blow (or let) them dry.
Many many years ago I got rid of those brass contacts of engine firewall harness. I solder those wires one-by-one and put some heat-shrink over them. Spent the entire weekend doing this from morning to evening. Until now i have no issue with firewall harness. I figured long ago that harness replacement its not going to happen in lifetime of the car. And the 900 series harness is not biodegradeable type.
Hope these helps,
Amarin
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