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Old 02-10-2024, 20:01   #1
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Perkins 4.108 Starter Solenoid Terminals

Question about the "R" terminal:

I am reinstalling a Perkins 4.108 which has the Delco Remi 20MT starter 1107587. No wiring harness on the motor right now.

The starter solenoid has the "R" terminal in addition to the "S"

"S" will trigger the starter. Is there any use for the "R" terminal on a diesel motor?
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Old 03-10-2024, 20:03   #2
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Re: Perkins 4.108 Starter Solenoid Terminals

At one time the “R” terminal was used to bypass the ballast resistor and supply full voltage to the coil at startup on gasoline engines but the only time we use the R terminal on diesel engines is if there’s a shutdown solenoid with a 3 wire hookup. Typically there’s a momentary “pull in” winding and once the engine is started, the hold in coil takes over and avoids the common overheating and burnout of the pull in winding, the third wire is usually a -ve. R is frequently used on gensets but is seen much less often these days ( there are several other ways to get momentary pull in power to the stop solenoid).
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Old 03-10-2024, 21:32   #3
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Re: Perkins 4.108 Starter Solenoid Terminals

Are we talking about the starter motor solenoid or the fuel stop solenoid?
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Old 03-10-2024, 21:47   #4
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Re: Perkins 4.108 Starter Solenoid Terminals

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Are we talking about the starter motor solenoid or the fuel stop solenoid?
Both. The R terminal on the starter solenoid supplies the momentary power to the injector pump solenoid pull in winding.
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Old 03-10-2024, 22:08   #5
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Re: Perkins 4.108 Starter Solenoid Terminals

Thanks SkipperP, that makes sense now!
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Old 05-10-2024, 07:06   #6
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Re: Perkins 4.108 Starter Solenoid Terminals

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Originally Posted by skipperpete View Post
At one time the “R” terminal was used to bypass the ballast resistor and supply full voltage to the coil at startup on gasoline engines...

Thanks SkipperPete!

So the R is energized only while the starter is spinning the motor? ...and it can help ensure the stop solenoid is positively "out of the way" during cranking ...if I understand this correctly.

I wonder whether the R could also supply power to an auxiliary diesel fuel pump to help the lifter pump during startup?

...or is that too short a duration (only while the starter is spinning) for a meaningful auxiliary fuel volume delivery.
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Old 05-10-2024, 16:34   #7
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Re: Perkins 4.108 Starter Solenoid Terminals

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Originally Posted by bostonmish View Post
Thanks SkipperPete!

So the R is energized only while the starter is spinning the motor? ...and it can help ensure the stop solenoid is positively "out of the way" during cranking ...if I understand this correctly.

I wonder whether the R could also supply power to an auxiliary diesel fuel pump to help the lifter pump during startup?

...or is that too short a duration (only while the starter is spinning) for a meaningful auxiliary fuel volume delivery.
Correct, and it’s relatively useless apart from the application I mentioned….Even then it could be eliminated by installing a relay on the S terminal to obtain momentary power…. or even better, use a single wire shutdown solenoid like on the Northern lights small gensets.
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Old 05-10-2024, 17:27   #8
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Re: Perkins 4.108 Starter Solenoid Terminals

Got it. Thank you!
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