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A common problem on many marine diesel engines is getting enough current to the excite terminal of your engines starter motor. The factory wiring harnesses use small gauge wire, wire which is barely adequate for the current needed to pull in the starter solenoid under perfect conditions, let alone poor conditions. We then add years of being in a corrosive environment and you now have starting issues. This is usually not the fault of the starter motor, but rather the wiring trying to energize it.
The CMI Start Assist Solenoid is a simple three wire unit that gets mounted to the engine or a bulkhead nearby. The mounting ear of this relay is the negative/ground point and the "S" terminal is used to operate the relay. You then interrupt the start wire with the relay, as shown in the wiring diagram (see photos), and connect a fuse and 10GA wire between the large starter motor lug where the main battery cable is and then one 10GA wire to the starters excite terminal. This means the start wire now only needs to do is energize the tiny coil in this relay, and this is very low current. The relay connects the two large studs and the motor starts with plenty of amperage to the excite terminal.
Yanmar engines currently ships almost all of their engines with a start assist relay as does Westerbeke & Universal. If your old engine lacks a start assist relay, and many do, this one is easy to install and reasonably priced too. This is a "generic-fit" relay and can be used on any marine diesel engine.