Block heater on GM vehicles
GM has installed temperature sensing circuitry into the plug on factory installed block heaters. It prevents the heater from operating unless it is colder than 0°F or -18C. It may seem insane, but it really does make sense if you understand why. If you install an aftermarket block heater without this circuitry, the PCM will see warm coolant, but a cold engine and cold air coming into it. Remember, it’s the PCM’s job to determine the correct air/fuel mixture. A cold engine block and cylinder head act as a fire extinguisher when you first start up the engine. So the computer commands a very rich mixture. How does it know what temperature the engine is? It used to look at only the coolant temperature. But now it looks at both coolant temp and the temp of the metal itself. If it see a discrepancy between the two numbers, and the outside air temp coming in through the intake is 0° or above, it assumes there’s something wrong with the coolant temp sensor and sets a trouble code.
Bottom line, if you add an aftermarket heater, it must have the temperature sensing circuitry in it or you WILL set a trouble code. Also, if you ever have to replace the power cord to the factory block heater, you must use a new GM power cable.
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© 2007 Rick Muscoplat
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Rick, your explaination of what happens when a block heater is used below 0 degrees on a newer GM vehicle is misleading and not entirely correct (Block Heater, Block Heater Cord Replacement-posted 8-25-09 10:51 AM). When a block heater is turned on, it heats the water in the engine AND the metal of the engine (heads and block). It would be impossible to heat the water in the water jacket without heating the metal that surrounds it. That’s why an engine is plugged in—to warm the combustion chamber so that it will start much better and not have to have as much initial fuel enrichment. That all worked perfectly, in my opinion, until 2005 or so when something went wrong somewhere between real life and GM Engineering. For some reason, they tied the air temp and the water temp sensors together with code that said if the two didn’t read the same, or very close, at startup and the air temp sensor said it was above 0 degrees, there had to be a problem. It promptly set “check engine” light and turned off the tempurature gauge. For the life of me, I can’t understand why they did this or let it happen, but their fix was simply not to let you (the person who spent thousands of $$ to buy their car) warm up the engine until it got below 0 degrees. And since this problem started, they have not felt the need to fix the code/sensor problem. Here in Alaska, we don’t like to wait until it’s below 0 to warm up our engines, in fact, our largest City, has a highly advertised and strongly suggested program that your vehicle should be plugged in below +20 degrees. Anchorage has a pollution problem from “air inversion” and all those vehicle starting on the vehicles “cold startup program” really pollutes the air. GM has no answers or seamingly no interest.
I don’t mean to unload my opinions of this problem on you, but I did want to comment on your posted explaination of “warm water, but cold metal”. thx DAB
DAB,
You assume that the engine coolant temp sensor is always located in the block. Sometimes they’re on the radiator. Others are at the Thermostat housing which isn’t a real good place to get an accurate reading for block heater warm up since the T stat is usually closed. The whole point of the article was that you have to replace the cord with a genuine GM product. As for the way it’s designed–you’re right, it sucks. It doesn’t suck as badly as their anti-theft system, their leaking intake manifold gaskets, their transmission problems, or their inferior window regulators, but put it all together is the only thing that amazes me is that they didn’t go bankrupt earlier.
Does anyone have the correct wiring schematic for the block heater cord on a 2010 Chev Suburban with a 5.3 L motor?