@nioko Labs boat electronics gadgets mod Projects Uncategorized

Eberspächer heater 24V to 12V conversion

The previous owner of our boat installed an Eberspächer Air heater, which was meant for a lorry, and therefore 24V. Our boat only has 12V Batteries / Alternator. As far as I know he used another battery in series with the auxiliary battery to power the heater, but this battery obviously wasn’t charged when using the heater – I didn’t like that solution, the heater wasn’t used for about 10 years, and now I’ve decided to build a solution.



The heater has a small controller unit + the heater itself. My idea was to find a 12V to 24V step up converter, and run everything on 24V. This didn’t work out, as I didn’t find an affordable 12V to 24V converter, the one I bought said 150W output power, but that just isn’t enough for the initial glow.

2nd idea:
I thought, why not just replace the glow plug, with a 12V version (didn’t know if it exists back than), and use a relay to switch a 12V line with the 24V from the controller unit.

Doing it:
At this point I’ve decided to unmount the heater, as i needed to find out what kind of unit I have, and which glow plug I actually needed.

What I found was even better:
There’s a huge resistor (or call it a spring) in series with the glow plug – the glow plug is actually a 12V plug, you just need to rewire it to not use the resistor, and install the relay as mentioned above (I obviously don’t know if this is with all the Eberspächer heaters, but it’s worth looking).



Open controller unit:


Controller unit with attached step up converter + relays:





Close-up of the modification:

The yellow line goes to the glow plug (already connected directly on the picture, the yellow line in the background goes to the resistor and back):

5 replies on “Eberspächer heater 24V to 12V conversion”

Hi, i have a D1L which i have tried to convert. I used a cheap chinese step up which worked and i by passed the resistor with a direct feed to the glowplug. It did work but won’t glow now, even with a new plug.

Did you make that step up convertor?

Hi, my step-up is Chinese as well. So you basically do the same, power a relays from the 24V that would go to the glow plug, and switchting a 12V line with that relays to power the glow plug?
Maybe your step-up is too weak, and the eberspächer is shutting down? I’ve had this, when I’ve tried to run the plug on 24V as well… you should monitor your input Voltage, if it goes below a certain value (don’t remember it), the control unit will stop the preheating, and the fan.
Regards, Nico

Hey Nico – Many thanks for the writeup; looks great! Everything still working well? I’m looking at a similar install on my sailboat but have a few questions. Can I shoot you an email? Cheers.

Hi Nico,

I do have an Eberspacher 1DLC 12V but without but control Unit.
All my van is 12V.
So I would like to know if it’s possible to modify 24V control unit (easily to find) to 12V.


Hi Muzo

I don’t know any other models than the X2, but what I did should be possible with any unit, you just need to make sure, that no 12V parts are powered by 24V (Glow plug is clear if you do the same as I did, but the Pump + Fan on mine are 24V). You’ll need more relays (one per part that the unit controls).
Cheers Nico

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