Integrated Magnetic Switch On Heavy Truck Starters

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May 10, 2005
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Toronto, Canada
I do not usually encounter IMS on starters. I take care of a small fleet of Freightliner Medium Duty trucks and the 28MT, 29MT and the 41MT starters they use don't have the IMS. Years ago we had a International MD truck with a IMS.

I came across this set of instructions for a 28MT starter.

NOTICE! Some applications use a starter with a magnetic switch (relay) directly attached. Replacing it with a starter without an attached magnetic switch (relay) is not recommended, because it requires installing a separately mounted magnetic switch (relay) and potentially upgrading the solenoid control circuit wiring. The switch and wiring must meet or exceed a current rating of 300 amperes for 12V starters and 200 amperes for 24V starter.


I do not understand the 300 amp wiring requirement for solenoid control. My Sierra starter solenoid draws a total of 18 amps (pull in + hold) current and the 28MT draws 58 amps (pull in + hold). Even the 41MT starter, which is a monster starter weighing about 80 lbs, draws 75 amps (pull in + hold).

300 amp wiring requirement for a 58 amp current draw seems unreasonable and overkill.
 
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Does seem odd. I could see 2x, but several x seems counter to the point.
 
I emailed Remy


ME Hi. The instruction sheet for the 28MT calls for minimum 300 amp wiring for the solenoid control circuit if the integrated magnetic switch is bypassed.
I measured the pull-in + hold current for the solenoid and it measured at 58 amps. Why would a 58 amp draw necessitate 300 amp wiring?

REMY The instructions is a caution to not install a starter with a Integrated Magnetic Switch(IMS),that’s attached to the starter to replace with a starter that does not have a IMS like the 28MT. The 28MT non-IMS starters solenoid circuit is a 100A while IMS starters control circuit is a 300A circuit. Sorry for the confusion and I hope this helps.

ME Thank you for the clarification. Let me see if I understand the situation correctly. The IMS starter solenoids need high current (up to 300 amps) to operate. Is the high current requirement due to a different design of solenoid or these solenoids are bigger and need to generate high forces to move the starter pinion into engagement with the ring gear?

REMY You are correct the high current requirement is due to different solenoid design necessary to generate the current necessary to rotate pinion to engage ring gear.


I stopped at this point and thanked him. Not sure what the different design solenoid is. He mentions rotating the pinion. Bendix Drive?
 
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