"Engine Coolant Hot" warning - no heat in cabin

[Hacker-Pschorr]

2003 Denali XL

It is 3 degrees with a -16 windchill.

My wife took my truck to work today, on the way home she wasn't getting any heat in the cabin and then the display said: "Engine Coolant Hot" then changed to "Engine Overheating". She immediately pulled over and shut down the engine.

I get there, pop the hood, the coolant level is fine, I don't see any obvious leaks and turning the key to the on position the temp gauge was just under 12 o'clock.

I start the truck, sounds normal. The voltage gauge is off the meter, well past the last number, just stuck there. It wasn't like that the last time I drove it.

I drove home with no warnings, temp gauge perfect and zero heat coming out of any vents.

I'm guessing some sort of heater valve must be stuck closed, how would this cause the engine warnings? My wife pays attention to gauges and said she noticed the voltage gauge and never saw the temp gauge to any higher than maybe a tick above 12 o'clock.

Odd to say the least, any ideas?

 

[Hacker-Pschorr]

The plot thickens.....

Searching around the forum I found a few people say their heat only worked on auto mode, since I never use auto, figured it was worth a try.

First I wedged my head under the dash to see if any moving parts are visible. I saw a white / black half gears on the side of the "HVAC box" sitting in the middle of the adjustment range. With the key on, I moved the temp controls from high to low, the gears didn't move.
I push the auto button.......they rotated all the way down.

Went for a drive to warm up the engine, once up to 210 I turn on the system in auto mode.....yay - heat!!!

I put it back in manual mode, vents on - more heat.

Anyone know what those gears are for? Back in the garage with the vents on in manual mode they were back in the middle setting and still hot blowing out the vents.

I'm wondering if something didn't freeze up and overnight in the garage thawed it. I did check my coolant before winter with one of those bubble gauges and it showed good down to the coldest setting. Maybe a good flush just to be safe.


This still doesn't explain why my voltage gauge is now pointing straight down and the engine warning's my wife saw last night. Unless the frozen theory somehow effected the sensors for whatever triggers those warnings.