Off topic here, sorry.

Helping my dad wire up a pair of diesel heater plugs on his old SABB marine diesel. Running them through a dash mounted switch and heavy duty relay from 12v 'run' on ignition barrel.

Come up with a problem:

the heater plugs are rated at 9.6V each and the battery is putting out 12.8V. No problem i thought, Il wire them up in series so they get 6.4V each and see how we get on... As you have prob realised now this isnt possible as the plugs are earthed onto the engine block and therefore cant be run in series. F**k.

How do I get around this? Do I need to get a large resistor or something to bring down voltage?

Any help much appriciated (like your talking to a small child if possible )

Iain

No idea what current they take I suppose ?

One possibility would be to drop the excess voltage by putting a suitable bulb - sidelight, brake or headlamp - in series with the heaters but all depends on what current the heaters take.

dunno about sabb,s but all the heater plugs ive seen use buz bar to feed them,the voltage drop will prob knock it down to 9/10 volts anyway

As the crkid says....connect the plug across the battery and you'll probably find the voltage dragged down to close to what you're wanting.

Right thats what il try, im not at home so i doubt il have a headlight bulb or soemthing tying around but il see what happens on 12V...

from what i can find out they tend to be about 150W each which makes the pair draw 25A. Il let you know what happens, unless i burn fingers off....

Ta very much for help guys,
Iain

Ian - call NGK Spark Plugs Technical... they may be able to help you.

01442 281000

Sounds like it should ballasted just like a coil, the ballast resistor gets by by passed when the switch is in cranking position as oposed to pre-heat.