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The "S Wire" (starter solenoid) Circuit and Engine No-Crank Issues



- K Kloeber 8/25/18

Note: This is for older engines and harnessesnot for the "B series" (e.g., M-25XPB) engines.

There are several potential issues with the "S" circuit that can cause an engine no-crank situation.


What's an "S circuit" or "S"  wire?

The "S circuit" energizes the starter solenoid, which slides the starter pinion gear into the flywheel and powers the starter motor.  It gets its power from the start switch (whether yours is the key switch or a pushbutton.)  By ABYC convention the wire **should be** yellow with red stripe.  However, note that on some harnesses/panels/engines, the wire colors from your the panel itself can change as they go thru the panel gummy bear plug, to the extension harness, and thru the engine gummy bear plug, to the harness on the engine itself.

In other words, the harness installed between the panel and the engine wasn't necessarily constructed to match all the wire colors on the panel that came from Seaward or all the wire colors on the engine that came from Universal!  Big D'OH


Possible issues with the "S" wire circuit that can cause a no-crank situation are:

* "S" wire gauge.

* Burned contacts on the key switch and/or pushbutton start switch.

* The Gummy Bear plugs.

* The Fuse on the "S" wire.

* The Terminals.on the "S" wire.

The Key Switch and/or Pushbutton Switch

Whether you start using a pushbutton (Off / Momentary-on)  or the key switch (Off / Run / Momentary-start) the power to the "S" wire is initially supplied through the key switch "Run" position.  Burned or corroded contacts on either of these switches can cause a no-crank issue.  Replace them if needed with a quality Cole-Hersee marine-grade switch with a rubber weather boot to help prevent corrosion.  

Wire Size

Some older harnesses have a 16 gauge "S" wire.  The solenoid depends on full voltage to pull in the high-amperage switch and make good contact, and voltage loss in the "S" wire coupled with loss through a burned start switch contact or other poor connections can cause a no-crank issue. 

Newer harnesses had a 14 awg "S" wire.  Whenever I build a harness the customer gets nothing lighter than a 10 awg "S" wire -- and if you make an upgrade use nothing less than that to add a factor of safety on that circuit.

Gummy Bear plugs

There will be voltage loss thru the gummy bear plugs (aka trailer plugs aka RV plugs) on the OEM harnesses, so especially corrosion or either/both (rememeber there's TWO plugs) loosening can cause a no-crank issue.

Additionally, the plugs can deteriorate from engine heat, overheat from high amperage thru them and melt, and cause a fire.  They need to be eliminated (a different article.)

Note: If you think it's too much trouble to spend an afternoon fixing them, it's okay to burn up your Catalina -- but please be a good boat neighbor and warn your dock mates that your boat is a fire hazard and you're too lazy to fix it. Then it's their choice whether to move.

The "S" Wire  Fuse
The OEM harness has a barrel fuse on the "S" wire, just before the solenoid, and the contacts can loosen or corrode and cause a non-crank issue.

Photo from Rod Collins aka MaineSail aka Compass Marine aka

The fuse is there for two reasons -
If the "S" wire falls off the solenoid "S" terminal and in the unlikely event it contacts an engine ground while you are trying to start, but the second reason is that, if the start switch sticks or the solenoid locks up, the fuse will blow before the starter burns up your starter and wiring.  See:

So what to do? Replace it with a quality weathertight fuse holder?

That's what RC did:

Photo from Rod Collins.

Note  - One could use yellow heat shrink and a red permanent marker on the red fuse holder pigtail so the entire circuit conforms to ABYC colors. 

But wait - there's another problem with the OEM harnesses.  The #10 red power lead to the cockpit is unfused, so you have a constantly hot wire (when your battery switch is "on") running to the cockpit with absolutely no overcurrent protection.  Can you say fire hazard?  

That wire needs to be fused (a different article.) 

The "S" Wire Terminals

All the terminals on the OEM panel from Seaward, and the engine harness from Universal, were open to air, non-marine-grade terminals that will corrode over time.  Couple that with the OEM harnesses being un-tinned copper wire, and you have a potential problem.  So the terminal at the start button can cause a no-crank situation.

The wire terminal at the solenoid end can also be an issue.  The solenoid has a 1/4" male quick disconnect (QD) terminal or as I call it, a quick fall-off.  It's basically open-air and can corrode, and expansion/cooling from engine heat and vibration can loosen the mating female terminal on the "S" wire.   

A marine engine should never have a terminal like that -- it should it be a threaded post that accepts a ring terminal and a lock washer and nut.

So, how to fix that?

There are two ways.  I supply customers a replacement solenoid that has the threaded post terminal, but if you don't need to replace your's here's another permanent fix:

  • Make up a 6" pigtail of yellow wire with:
    • One end a crimped-on UNinsulated 1/4" female QD terminal.

    • The other end a FULLY insulated, adhesive heat shrink, 1/4" female QD terminal.  You'll also need a mating FULLY insulated male terminal (the type of quick-disconnects below) so that the connection is fully covered to prevent corrosion.




  • Pull the starter.  It's relatively simple, held by two M8 hex bolts (you need a socket extension to get at the inside bolt.)
  • Apply and carefully solder the UNinsulated QD terminal and pigtail to the tab on the solenoid.  Just enough to affix it -- not gobs of solder or excessive heat. 
  • Heat shrink over the bare QD terminal and solenoid tab to prevent corrosion.
  • Reinstall the starter.
  • Crimp the FULLY insulated, heat shrink, male QD terminal onto your "S" wire.
  • Plug the "S" wire into your pigtail.

The insulation-within-insulation on the fully insulated type QDs, blocks moisture plus the friction won't allow them to separate.  Having the QD terminal there is priceless if you ever need to troubleshoot the solenoid or energize it from the cabin (rather than from the cockpit.)

(Note that any terminals must NEVER have a strain on them.  Zip-tie the harness so it's supported and there's no pull on any terminal, whether it be  QD or a ring terminal on a post.)

Whether you use a fuse on the pigtail (see above) is Your Boat, Your Choice.