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Marine engine air heater control

Thread starter #1
Hello all,

Got another one for you please.

Don't ask why but I have just discovered that my Yanmar 3JH25A diesel 3 cylinder (ex lifeboat engine) has a heating element in the air intake for cold weather starting. It draws 33 amps. As I am in Greece it is unlikely that it will ever really be used but we do have some cold spells (minus 2 degrees max) in the winter.

As I have already had the dash panel cut and drilled there is no aesthetic place for a push button switch to control it.

As it needs to run (according to Yanmar) for 60 seconds on start up, I thought a timer/relay would be a good idea. This is ok but the relay I found is only rated at 10 amps, so a secondary 40 amps ordinary relay will be wired after it. The 10 amps timer relay can be set to run for 60 secs from on.

Question is, do you think wiring the control of this through the ignition/starter switch would work? Ignition - on nothing, and turn to start to initiate the heater?

I could put a simple on/off switch hidden away somewhere after the starter switch to make it summer/off - winter/on because it really wont be needed for 10 months of the year.

I'm not sure how quickly the heating element warms up and so I could be cranking for a few seconds before warm air is sucked in?

Any ideas please.

Tony K


Well-Known Member
Most Helpful Member
If the recommended heating time is 60 sec, then I think you'll need more than "a few seconds before warm air is sucked in". The heater presumably has to warm up the intake manifold as well as the incoming air.
Ignition - on nothing, and turn to start to initiate the heater?
I don't see that working. Shouldn't it be 'Ignition on: start timer', followed by 'Timer timeout: start engine'?
Thread starter #3
This is the site for the relay.

I don't think it will work with a continuous ignition on current. ie. momentary switch starts it.

The element is 400w and just heats the air being sucked in not the manifold itself.

The only alternative is change the ignition switch to an off/on/heat/start switch but i have already had the panel engraved off/on/start!

Or just fit a push button momentary switch someplace.



Well-Known Member
Most Helpful Member
... Or just fit a push button momentary switch someplace.
Given your infrequent use of the heater (and, I suspect, the boat as well during cold spells), that solution makes the most sense to me.

Use a link from the "Ignition ON" terminal to fire an appropriate relay with the shortest possible heavy duty leads between the battery/relay/heater. Fused, of course.

You'll have to experiment with the timing by trial and error - much simpler, in the long run, than dealing with the installation of an automatic timer.
Thread starter #7
Thanks for that, but if I understand it correctly glow plugs go out automatically when the cylinder temperature is reached. The heater on the boat is a coil of wire in the air intake which goes up to 400W. I think I have decided to use a timed relay to turn it off after say 60 seconds. I can play with the timings but will have to wait for next winter here!
Thread starter #8
This is how I solved the heater connection problem which will work with a timer relay of 10amps connected to an ordinary 40 amp relay. WIN_20170401_09_08_19_Pro.jpg

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