Heater won't work

[SCIV]

For a guy at work -

1999 GMC 1/2 ton ext. cab, and the heater won't work. A/C works. Fan blows on all settings. Discharge cycles through dash, floor, windshield, etc. Won't blow warm/hot air. Been a little chilly even for Texas lately.

I took a dash control unit from another work truck, and didn't fix anything.

Is there a valve that allows hot water to cycle through the core? Something else? Suggestions?

Thanks in advance.

[RedHeartbeat]

Sounds like he's low on water possibly? Could the heater core be plugged up with maybe some stop leak or something like that?

[SCIV]

<div class='quotetop'>QUOTE(RedHeartbeat &#064; Jan 20 2007, 12&#58;28 AM) [snapback]80508[/snapback]</div>

Sounds like he&#39;s low on water possibly? Could the heater core be plugged up with maybe some stop leak or something like that?
[/b]

Hmmm. I don&#39;t know. I will follow up and report back.
Thanks for the 1st idea Red.

[jethro_3]

Did you get your beast back yet?????

Red stole my thoughts, check to see if the out going from the heater core is actually hot.......

[BINNCZ71]

Take both heater hoses off going into heater core and flush...

My 99 quit heating last year. Checked all controls, etc.. All good, flushed the heater core, it ran
me out on the way home. Still working..

Hope this helps..

[badseed91]

If backflushing the heater core doesnt work try un bolting the blend door actuator and watching it move when you turn it to heat. If the actuator moves back and forth, put it back on the blend door and see if its strong engough to actually move the blend door, if it doesnt move you have either low voltage or a blend door that is sticking. Check all connections at control head and actuator for loose pins and corosion. I had one doing the same thing. I went ahead and backflushed the heater core because it was the easiest step and it probably needed to be done anyway. It ended up being a loose pin in the back of the control head causing low voltage to the actuator thus not enough power to move the blend door. Its mostlikely a plugged heater core though so check the hoses going in and out of the heater core when the truck is hot with the heater on and make sure they are both hot like Red said.

If not you will have to investigate further into the source of the problem. Before you bolted the known good actuator up from the other truck to the broken one did you check to see if you had 12v to the actuator and a good ground? Did you watch the actuator to see if it moved when you changed the heater settings on the control head? Did you check to see if you had any loose or corroded pins at the actuator and control head? Did you check voltage comming out of the control head for voltage? Let me know if backflushing doesnt work and i will tell you what else to test for to get the heater working again.