Temp was -26C (well below 0 F), had truck plugged in overnight. Normally it starts no problem, but this morning before trying the big engine, I fired up the APU. One hour later went to start the truck engine and it turned over very slow, didn't start. I'm wondering if it was a mistake to start the APU? Does it circulate cold coolant and takes away the heat that the block heater had? An hour later got it started, but by that time the APU would have been putting warm coolant through the engine.

 

I don't know anything about an APU, but common sense tells me an APU couldn't circulate the trucks coolant until the thermostat opens.

If your truck was plugged in and it wouldn't start @ -26 (-15 f), I would say your batteries are very weak or the block heater isn't working. I've started trucks that weren't plugged in, at those temps.

 

You didn't state the make of your APU..... tri-Pac runs off the trucks coolant system, but many other brands are self-contained.

It sounds to me like your batteries are at fault here. I don't plug my truck in, and after 2 days of below 0 temps, it cranks perfectly, just takes a while for it to start.

 

Its a Carrier and runs off truck's coolant. Even with the thermostat closed it should circulate coolant somehow. I have been doubting my batteries reliability lately, truck is 2 1/2 years old now. Next time at home I'll check block heater; If cold and plugged in and cranks real slow, block heater not working. It has to go to shop for unrelated, so I'll have them check batteries for me. Thanks for replies.

 

Had truck plugged in overnight, started this morning like a summer day. It was -17C (0 F) which is still fairly cold. I'm thinking the APU did take the block heaters heated coolant away.

 

APU isn't that big a thermal load, and once running, should be contributing heat to the coolant going 'round and 'round? At worst, the coolant would get luke warm from hot?

Big difference to the batteries' loss of power between -17 and -26, so a load test of each battery might be in order?

Guys I know have the electric block heater tied into the APU so there's a load on the engine of some sort whenever it is running.

Doesn't your APU also charge the batteries?

Cold weather trouble shooting is a pain!