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· Mr Fix It
300 Posts
When you turn the key 12vdc is sent to the starter solenoid. It must travel thru various safety switches and wiring connectors. Each connection and switch has a small voltage drop reducing the voltage reaching the starter solenoid which can reduce the voltage enough where solenoid will not operate

A starter assist relay is connected to battery and starter solenoid. It provides a direct path of current to solenoid but is operated by the key switch thru the existing wiring and safeties

The existing solenoid wire is connected to the starter assist relay. Starter assist relay doesn’t need as much power so it will easily operate with the voltage drops of the old wiring and safety switches,

So now you turn key and power flows thru safety switches and activates starter assist relay

The Starter assist relay then sends battery power directly to starter solenoid.

Basically the key activates the starter assist relay which activates the starter solenoid

This is a very clever way to get starter solenoid to work on a machine with old wiring and some voltage drops. Very common fix on old Volkswagens

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