Faster, More Efficient Glow Plug Heating with a Solenoid

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By Capt. Al, Kindred Spirit #55

The installation of a solenoid to the glow plug circuit will reduce the time to hold the key or push button that powers the glow plugs preheating the engine before starting. Shorter, larger wires = more power to the glow plugs!

Parts list:

  • Solenoid, 12v, O.E.M. Quality Solenoid Ford Starter Solenoid SW3 STD # SS581, Niehoff #FF143A or Wells #F-496 approx. $5.00
  • Maine Sail reports: I use a Cole-Hersee (often the 24059) heavy duty/constant duty solenoids for this as I find they last longer than the no-name intermittent duty versions from an auto parts store... Stu: I used the Wells F-496, it's been working for the past 15 years, easy to buy two and keep a spare, no need for a continuous duty solenoid. Your boat, your choice.
  • Six feet of #8 wire
  • Three large stud round wire connectors (#8) for starter (1) and solenoid studs (2)
  • One small stud round wire connectors (#8) glow plug
  • Grounding wire, two feet #14 wire with end connectors
  • Two bolts, nuts and washers


  1. Mount solenoid with its mounting flange tabs. Most solenoids have two tabs with holes for mounting. These tabs must be grounded for the solenoid to work. Fabricate a plate or find close to matching mounting bolts on the rear of the engine. This was not possible on my M25 so I mounted mine UNDER the fiberglass/wood lip that the engine hatch rests on, above and rear of the engine. This means that it is hanging upside down over the rear of the engine. Two bolts were used to mount it, using one of the bolts to connect the ground wire that is wired to the ground on the engine.
  2. Remove white (I think its white) wire from last glow plug. This is the wire that goes up the wire harness to the glow plug switch. DO NOT cut the end off the wire to make it shorter. If you ever have a problem with the solenoid, then you can connect the system back the way it was. Connect this white wire to the small stud on the solenoid. Some solenoids have two small studs. One is not used. Before installing, ground the solenoid and touch a hot wire to the small studs one at a time. One of them will make the solenoid CLICK. That is the one you connect the white wire to.
  3. Connect the (+) side of the starter to one large stud on the solenoid. Use the #8 wire, cut to size with the end connectors crimped or soldered in place.
  4. Connect the other large stud on the solenoid to the glow plug where the white wire was removed. Use the #8 wire, cut to size with the end connectors crimped or soldered in place, one large and one small connector.
  5. Go to engine panel, turn on key and push the glow plug switch and you should hear a click of the solenoid down by the engine. Hold for 25 sec, shut key off and return to engine and see if you can burn your finger on the top of a glow plug. It still should feel hot. I now hold the push button for only 12 sec instead of 25 to start the engine.

Safety Light:

The engine panel is fused, so the control of the solenoid is but the (+) from the starter through the solenoid to plugs is not. This is a problem if the solenoid should malfunction and remain HOT. You will burn out your glow plugs because you will not notice they are on. I have never heard of this happening but I have a safety light that I have added for this.

Purchase a 12v low amp auto, 1/4 inch, dash light that you mount in the engine control panel. Ground one side of the light at the panel and run a #14 or #18 wire down to the top of a glow plug or that side of the solenoid. When you hold the glow plug push switch on, the glow plugs and the dash light will both be on. Release the button and they both go off.

Additional Considerations:

I have a few more ideas to add to Al's procedure. If you have a M25XP engine, you can mount the solenoid thru the bolts that hold the heat exchanger in place - it saves a lot of work. As I mentioned in the Aug.97 Mainsheet on this topic, don't cut the old wire. If the solenoid fails, you can always connect it back to its original wiring. Take a #2 pencil and spiral the wire around it to take up the slack If you want, you can mount a safety light, but you already have a ready made indicator on the engine instrument panel to let you know if the glow plugs are engaged and when they are disengaged -- it's the voltmeter. When you engage the glow plug switch, the voltmeter will immediately drop about 1+ volts. When you release the glow plug switch, you will see the voltage go back up. This should be a good enough check as to whether the solenoid is working or whether it is stuck. I have always found, the majority of the cost is in time doing the work. As I mentioned in that Mainsheet article, if I had to do mine over again, I wouldn't skimp and use a Napa Ford starter solenoid for $13.99, I'd use a marine grade West Marine solenoid for $19.95. The marine grade is probably a more reliable solenoid in the long run.
Ron Hill, Apache

Here's the wiring diagram that Stu Jackson (Aquavite) sketched (click illustration to enlarge):


NOTE: The power lead from the starter solenoid "B" post to the new glow plug solenoid will be a continuously hot wire -- per ABYC it must be fused at, or within 7" of, the connection to the starter solenoid.

Also, per Mainesail, the glowplugs are 10-volt. Since the new setup will supply a full 12+ volts, don't lay into them for much more than 10-seconds at a time -- otherwise their life may be diminished. [added 7/2014 K Kloeber]


To follow up on the above, I suggest instead using a 30- or 40-amp, continuous-duty relay. These 5-pin relays are readily available from every auto parts store and electrical suppliers, eBay,,, etc., and are used for headlight and horn relays, vehicle audio, and other power uses.
40a relay.jpg
Use a harness/plug with 10 AWG power leads, like this one.
12ga relay socket.jpg

If your source doesn't specify the wire gauge of the power leads, look elsewhere -- check or check Google "Images" for "10 gauge headlight relay socket" Google search and click on one that, like above, looks like it has heavy power leads.

Three benefits:

First, the current necessary to energize the headlight relay is much lower than for a "Ford type" starter solenoid switch. This could be critical if you have (hopefully not) a low battery bank (what, I left the selector switch on "both?!?!"). Since the glow plugs draw less than 20 amps (compared to the high current that a starter draws) the high capacity of a "starter solenoid switch" is unnecessary -- and so (IMHO) that it's closing current is a wasted use of battery energy.

Second, the headlight relay is much smaller and has a much smaller footprint.

Third, the relay uses a plug-in socket and I carry a spare relay - so it's a 10-second job to replace (once I find the spare relay hidden somewhere within the spare parts bin.)

The relay socket isn't grounded, so one of the wires to the relay gets grounded to the engine block or another reliable ground.

CAUTION: Using a slave relay to energize the glow plugs is troublesome -- these are 10-volt plugs, so the voltage loss between the engine and cockpit panel is desirable. If you use a relay the voltage is higher and you run the risk of burning out the glow plugs if you preheat for a "long time."

Ken Kloeber, C-30, M-25. -July 2014