24v to 12v Glow Plug Relay rated at 80amps help

Thread Starter

Bellaire35

Joined Oct 31, 2019
6
I am working on installing a 12V glow plug relay in my Mercedes. The motor has 12V glow plugs so my idea is to replace the 24V Glow Relay with a 12V Glow relay and install 2 resistors in series before the realy. Hence stepping down 24v to 12v. The relays are rated at 80 amps and from my calculation the watts is 1920w. The online ohm calculators indicate (2) 0.3 ohm resistors.

Any idea on what can be done to address the watts? I might be a little confused as I dont see any 0.3 ohm resistors capable of handling the 1920 watts.
 

Dodgydave

Joined Jun 22, 2012
8,471
I'm confused as to why you want to replace a 24v system with a 12v one, and drop the voltage?? Just keep the 24v relay no wasted heat...

Normally use a resistor of half the coil resistance to drop voltage.
 

Thread Starter

Bellaire35

Joined Oct 31, 2019
6
I'm confused as to why you want to replace a 24v system with a 12v one, and drop the voltage?? Just keep the 24v relay no wasted heat...

Normally use a resistor of half the coil resistance to drop voltage.
The reason in short is i am doing a motor swap. Om602 out and OM605 in. The OM602 has 24V glowplugs and the OM605 has 12V. They do not make 24V glow plugs for a OM605 and my Mercedes ex military G is all 24V. So....I want to keep the 24v system and drop the voltage for the glow plug relay and use a 12V in place of the 24v relay.

The only way I see using my OEM glow plug harness is to drop from 24v to 12v. Inline resistors seem like the key to do this
 

Dodgydave

Joined Jun 22, 2012
8,471
The way I see it, the merc will give a 24v feed to the relay coil, and the relay will feed 12v to the glow plugs, so why can't you swap the relays are they not the same fitting, otherwise use a 12v relay, and measure the coil resistance and use a resistor of half the coil resistance 1W.
 

Thread Starter

Bellaire35

Joined Oct 31, 2019
6
No, the glow plugs are 24v. You can not switch 24V for 12V or 12v for 24V.

Yes, my idea is a 12v relay with resistors before the relay so the feed is from 24v to 12v to relay.

I need to find the coil resistance then.
 

Alec_t

Joined Sep 17, 2013
10,456
Although putting a resistor (or two) in series with the relay coil should be relatively straightforward, the greater challenge is dropping the plug voltage from 24V down to 12V. If the plugs need 80A a PWM approach might be a better bet than using a dropper resistor.
 

crutschow

Joined Mar 14, 2008
23,522
What is the current of the 12V glow plug?
The relay is rated at 80A but what does the actual glow plug require?
That's what determines what resistor you need in series.

But I think that Alec"s idea of using PWM with a 50% duty-cycle to reduce the voltage may be a better way.
That will dissipate very little power.
 

be80be

Joined Jul 5, 2008
1,944
For get all that you have 2 batteries there hooked in series to get 24 volts just use one of them to get the needed 12 volts for glow plugs
 

Thread Starter

Bellaire35

Joined Oct 31, 2019
6
For get all that you have 2 batteries there hooked in series to get 24 volts just use one of them to get the needed 12 volts for glow plugs
On 24v cars it is not good to pull 12v off one battery, you must use both or you will have issues down the road.
 

be80be

Joined Jul 5, 2008
1,944
If you beleave that then I guess that's ok but they been doing it for years and i'm sure the radios should of really caused problems but they didn't
the glow plugs only heat for a short time I don't see that killing a battery or shorting it's life.

We had Mercedes-Benz 2.7L that where 24 volt we ran 12 volt inverter 3000 watt to run drills and charge drill batteries never had a problem with them
and I'm sure the inverter used way more then the glow plugs seeing they sometimes ran for a hour with the motor off.
 
Last edited:

be80be

Joined Jul 5, 2008
1,944
A 384 watt resistor at 1.5 ohm and you could run the 12 volt glow plug off the 24 volt sys.

It be easier to buy 5 more glow plugs and hook in series with the ones in the engine and mount them so they heat the block.
 

Thread Starter

Bellaire35

Joined Oct 31, 2019
6
The reason for use of 24 volt GP is to reduce the draw on the batteries. But since half the energy is wasted in the ballast resistor, there is only the balance of each battery during starting. Since each battery is independently charged by each alternator, if you go with the 12 volt tap for the relay, you can offset this imbalance by using a larger front battery.

With equal sized batteries, tapping off the 12 volt side for the glow plugs will result in voltage depression in the front battery for the starter. This may affect extreme cold weather starting.
 

Thread Starter

Bellaire35

Joined Oct 31, 2019
6
No, OM605 and OM602 use completely different GP’s. And they never made 24v GP’s for OM603/5/6 engines.

I am going to do a complete 24v to 12v conversión. Very easy just cost more. Thanks for all the reply’s
 

be80be

Joined Jul 5, 2008
1,944
Looks like shortbus is right there wasn't any 24 volt glow plugs listed now there was a lot of
completely different kinds of glow plugs
But they all say they heat at 11 volts or more but none look to say 24 volt.
 

shortbus

Joined Sep 30, 2009
7,080
With equal sized batteries, tapping off the 12 volt side for the glow plugs will result in voltage depression in the front battery for the starter. This may affect extreme cold weather starting.
Don't the glow plugs actually turn off when the starter motor is turning? They heat before the motor turns over don't they?
 

shortbus

Joined Sep 30, 2009
7,080
There on a timer and yes most shut off if you start the engine
I know the old ones were on a timer, but some of the new cars they stay on after it starts, until the motor is up to temp. What I meant is when the engine is cranking, lights radio and other accessories usually go off when the motor is cranking over and come back on when it starts, to give full power to the starter motor during cranking. Just figured the glow plugs went out too.
 
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