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12V - STS-M12 24V - STS-M24
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FAA-PMA APPROVED for Certified Aircraft

Continuous Duty (Battery/Master Solenoid). Available in 12V and 24V. Activation: -(Ground) to S-Terminal. (Floating ground frame).

All stock meets Service Bulletin 72.


  • Silver Contacts provide 300A in-rush & 150A continuous-duty capability Much higher than OE units
  • Light spring and low-current coil for excellent engagement and holding capability
  • Single coil terminal (other end of coil connected to Battery terminal)


  • FAA-PMA Certified - stop using automotive parts and operating in the 'gray area' (aka. non-conformance) of the FAR's!
  • Lamar has retooled & upgraded the GOOD, "old-school" aircraft solenoids!
  • High Quality Manufacturing designed to take the beating our piston aircraft dish out
  • Upgraded to perform best with today's lightweight, high torque starters
  • Not automotive store parts with extra mystery terminals
  • Same OE terminal layout used for decades on our aircraft - simple to install
  • Silver contacts (vs. copper) for high-current capability and long-life
  • All-metal case construction
  • Customers Also Viewed
  • Frequently Purchased With


Exact fit and replacement for my old one. Works perfectly so far.

Richard C
August 17, 2020

Product came as Advertised. As always I never had any problems with the great service Aircraft Spruce provides. Thanks

Peter S
June 8, 2020

so far so good

Richard J
March 15, 2020

Good fit. Solid construction. No more jiggling battery leads to get relay to engage.

Steven A
February 19, 2020

to replace an original equipment solenoid in my OLD Mooney, this is the only game in town (and at a reasonable price too). it fit just fine and operates exactly as expected. as usual, no complaints with anything from Aircraft Spruce.

T. D
March 7, 2019

Just Fine

February 3, 2019

This product fit in the mount and to this point accomplishes the intended task at a fraction of the OEM part.

July 20, 2018

Good Part- standard replacement.

May 14, 2020

We installed them on 3 of the Cessnas in our rental fleet. Had problems with one of them dropping out in flight. Complied with the service bulletin and Hartzell sent 3 new units. One of the new units in a Cessna 150 failed ON after about 60 hrs. We were able to bring a flat battery back. Hartzell replaced the unit for free. The replacement unit has now also failed ON after 45 hrs.

Cas L
September 12, 2018

This is a replacement for an identical master solenoid (STS-M12) that was in operation for less than 6 months. The previous solenoid would make a loud click when I turn it on but would not make contact across the primary voltage wires. This would happen intermittently. I inspected all connections and battery - which were all like new. The description states the Silver Contacts provide 300A in-rush & 150A continuous-duty capability Much higher than OE units. I removed the cap on the solenoid and did not see any silver contacts - They are copper contacts. I would like to have an explanation of what do they consider silver contacts. I certainly did not find silver inside. Very disappointing.

Richard G
July 2, 2020


Q: What is the total current draw of the solenoid when actuated?

The total current draw is 0.8 amps.

Q: What is the difference between STec-A12 and STec-M12 relays?

The STS-M12 is for the battery master. The STS-a12 is for Auxiliary master, like a GPU.

Q: What size are the power and coil terminals? Also what is the recommended mounting orientation? I note that the 11-03161 (White-Rogers) unit recommends Cap Down orientation.

The coil terminals are #10's and the power terminals are 5/16ths. It is recommended to mount per the airframe specifications. If none, it is recommended to mount with the cap up.

Q: Does this Lamar solenoid include the diode? If not do I need it and what part number would it be?

Per Vendor: This product does not include the diode. You do not need the diode for the part to work correctly, it is simply a safety precaution some like to take. The airframe manufacturer might have a diode they recommend.

Q: What is the warranty for this unit?

12 mo from date of installation of product or 500 hrs - whichever comes first. Log books are required for warranty

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