I too wondered about these battery terminals on top of my ISC-350 do and what they were for? So I did some research and this is what I found... thanks to another forum member. Note: This information was written for Cummins diesel trucks, but I think it will apply to our RV applications as well:
A 12V grid heater is not the same as a 120V block heater.
And in general, diesel engines need heat to ignite the fuel.
A cold diesel is hard to start because the air entering the combustion chamber may be too cold to allow for the ignition of the fuel. Thus, diesels need some method to warm the air in the combustion chamber for the fuel to ignite. A) They can use the old style method of "glow plugs." B) They use a 12V grid heater. Or C) The owner can plug in a 120V block heater if installed.
Note: In hangar type maintenance facilities, huge room heaters assist in engine starting.
Cummins uses a grid heater type and this heating element will turn on-and-off until the compression in the cylinder alone is enough to heat the air and allow combustion to occur.
The heater grid heats the air as it passes over the grid, but it will not come on until the outside air temperature drops below 59-degrees. (Typically.) This means you will not see a voltage in 59+ degree weather.
Does anyone know what part measures temperature and what part controls the grid heater? Also, there has to be a relay/solenoid for this function. So I guess I need to find a circuit diagram for the grid heater too?
Both glow plugs and grid heaters will cycle even after the engine is running until a predetermined temp is reached. How hot is the air heated to? ...I don't know, but Id be willing to guess that it will get hot enough to cause serious burns if you were to try to hold one in your hands while heated.
Q: In cool and cold weather, my voltmeter jumps up and down. Is a belt slipping?
When the temperature is below 59 degrees, the intake manifold heater cycles to warm the air entering the cylinders. This grid heater may draw more current than the alternator can produce when running, so the voltmeter will drag-down when the grid heater cycles on-and-off. And this cycling may continue for several minutes to 15 minutes or more in cold weather.
Q: Will the diesel start in subzero weather?
The Cummins B will reliably start at temperatures to -20 degrees F without the block heater, but the engine is much happier if the block heater is used.
Plus it may take several manifold heat/start cycles before the engine fires and continues to run. You can also expect a large smoke cloud when the engine starts, and the you will hear accessory whine, knocking, and vibration until things warm-up.
The engine block heater usage is recommended as follows:
1) With temperatures above 40 deg F, the block heater is not needed.
2) With temperatures between 40 deg F and 0 deg F, the block heater is suggested.
3) With temperatures below 0 deg F, the block heater is required.
For more information, here's a website link:
Cold Weather Diesel Operation
Linear Mode
