Agreed with above, check connections, and the batteries, just in case. I've had the same problem on both my diesels. My 6.0 has 2 batteries, and when the connections get corroded, or the batteries start to get weak. I can run the glow plugs, but don't have enough reserve left to crank the truck to start. The 6.0 has to build oil pressure to start, so it does not start as fast as the IDI's do.... but on that truck, if it is cool out, and the batteries are weak, it will not start, period. Put my 200 amp boost charger on it, and it fires right up.
My VW IDI is doing the same thing right now. If I park in the garage at home, it only runs the glow plugs for a second or so, it will start right up. But at work last night, it was right at 32 F. Turned on the key and it ran the plugs for 6 or 8 seconds, hit the key and I got maybe 3 revolutions out of the starter. Got out my little 70 amp boost box, it ran the plugs again for 4 or 5 seconds, and then cranked right over. Battery is old, and just has no reserve left in. Getting a new battery tomorrow on payday...
BTW - Reserve is the measure of how long it can maintain output. So simply stating, the higher the reserve, the longer you can crank on it before you loose power. Just remember thought, just because you can crank longer, the starter still needs a rest. I believe most hold to the 30 seconds of cranking followed by at least 2 minutes of rest to cool down the starter. If you do not let the starter rest, you will let out the "magic smoke".... Also, a DC motor takes a certain amount of power to work. This is voltage and current combo. So, the lower the supply voltage is, the more current it will draw to do the same amount of work. More current, more heat, burn it up faster. Voltage and current are inversely proportional in the power equation.
P=I * E that is P power (in Watts) = I current (in Amps) * (multiplied by) E voltage (in Volts)
So lets say that a truck starter draws 600 amps at 12 volts. That would be P=600A*12V or P (in Watts) is 7200 Watts to start the truck.
Now lets say that you are now at 9 volts due to a weak battery. The starter is still going to want 7200 Watts to start the truck. so that makes our equation look like this. 7200W=(?)A * 9V or 7200W / 9V = 800A to start the truck......
Same way we figure out how big of cord / breaker to use for the block heater. Most block heaters are 1500 Watts. So that gives us 1500W = (?)A * 120V. or 1500W / 120V = 12.5 Amps. So you have to plug into at least a 15 Amp breaker, and use a big enough cord. Also you have to take that in mind if you buy a timer for the block heater, a little 5 Amp timer for a 60Watt desk lamp isn't going to cut it. To further the example, now lets say you wanted to run that same heater at 12Volts the truck already has on-board. That would be 1500W = (?)A * 12V or 1500W/12V = 125Amps to run the same heater. Power is a really fun thing to work with. I would love to work on the power crew at work, you just have to respect it, and know it will kill you without any remorse.
Sorry for the long post, but I am sure not everyone out there understands how power works.......
