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Old 11-19-2014, 02:40 PM   #1
c10noob
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How do I diagnose my heater?

Hey guys I have a 70gmc c15 that isn't putting out any heat, the blower works fine but when I slide the heat lever over there's no change. Where do I begin to try and diagnose this? Hopefully there's a way to diagnose so I don't have to start taking parts off for no reason, from the bit of research I've done that seems to be a pain

Thanks!
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Old 11-19-2014, 02:52 PM   #2
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Re: How do I diagnose my heater?

Lay down so you can see under the dash at the heater control area. Move the lever and make sure the lever moves the cable at both ends (Lever and plenum). That will tell you if the controls are hooked up and not broken.
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Old 11-19-2014, 02:56 PM   #3
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Re: How do I diagnose my heater?

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Originally Posted by dmack91 View Post
Lay down so you can see under the dash at the heater control area. Move the lever and make sure the lever moves the cable at both ends (Lever and plenum). That will tell you if the controls are hooked up and not broken.
Thanks for the reply, I'll check it out tonight
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Old 11-19-2014, 05:24 PM   #4
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Re: How do I diagnose my heater?

-
Also follow the long cable that enters the firewall on the right side of the heater box. There is a plastic plug there, remove it and using a good flashlight see if it is full of mouse nests. That would be my bet...

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Old 11-19-2014, 05:45 PM   #5
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Re: How do I diagnose my heater?

Warm up the engine. Grab the heater hoses with your hand. The hoses should all be hot that you cannot hold on very long. If the hose going into a unit ie the heater core or valve is hot & the one on the other side is not, that is were the restriction is. If none get hot your therm. is bad or the coolant level is low.
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Old 11-19-2014, 09:41 PM   #6
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Thumbs up Re: How do I diagnose my heater?

Thanks all for the replies, when I get home to check it out in a few days I'll post an update.
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Old 11-21-2014, 10:14 AM   #7
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Re: How do I diagnose my heater?

The temp is controlled only through the blend door (blending fresh cold air with air passing through the heater core). Make sure heater hoses are getting hot, make sure the cable actuated blend door actually moves (follow cable and manually move door) and make sure heater box isn't full of crap blocking the blend door or airflow. Its a really basic system to you ought to be able to find the problem pretty easily.
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Old 11-22-2014, 12:09 AM   #8
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Re: How do I diagnose my heater?

Hoses feeding the heater core may be backwards, also check to make sure the core supply line is connected to the intake manifold and not the water pump. Mine were all kinds of wrong backwards at the heater core and feeding from the water pump, switched them back and it was burning me out
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Old 11-23-2014, 10:15 AM   #9
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Re: How do I diagnose my heater?

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Hoses feeding the heater core may be backwards, also check to make sure the core supply line is connected to the intake manifold and not the water pump. Mine were all kinds of wrong backwards at the heater core and feeding from the water pump, switched them back and it was burning me out
The heater core itself is non-directional. The only thing that could happen if you had it hooked up backwards is it not get fully filled with coolant and have a lot of air in it.
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Old 11-25-2014, 01:17 PM   #10
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Re: How do I diagnose my heater?

Thanks everyone for all the replies, I removed to plug and saw that there is no build up so that was ok. All linkage to and from the controls are functioning and intact. The hose to the radiator from the intake gets piping hot but neither of the hoses to the heater core get hot at all, both are slightly warm but I believe its just from ambient heat from the motor running. So I guess the next step is to swap out the thermostat??
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Old 11-25-2014, 04:29 PM   #11
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Re: How do I diagnose my heater?

Have you checked the resistor mounted in the heater box inside the engine compartment? That regulates the blower speed and or temp I believe.
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Old 11-25-2014, 04:41 PM   #12
c10noob
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Re: How do I diagnose my heater?

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Have you checked the resistor mounted in the heater box inside the engine compartment? That regulates the blower speed and or temp I believe.
I have not, Im trying to do everything I can before getting into the heater box
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Old 11-25-2014, 05:08 PM   #13
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Re: How do I diagnose my heater?

If the rad. hose is "piping hot" your therm is probably OK. You have a circulation problem. The core is probably stopped up. Take the hoses off & wrap a towel around an air blow gun & blast a little air through the heater tubes in both directions. That will usually open it up. The inlet & outlet hoses are different sizes so it is obvious which is which.
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Old 11-25-2014, 05:12 PM   #14
c10noob
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Re: How do I diagnose my heater?

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If the rad. hose is "piping hot" your therm is probably OK. You have a circulation problem. The core is probably stopped up. Take the hoses off & wrap a towel around an air blow gun & blast a little air through the heater tubes in both directions. That will usually open it up. The inlet & outlet hoses are different sizes so it is obvious which is which.
Great thanks, judging by your suggestion a radiator flush may not be a bad idea either than I would assume
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Old 11-30-2014, 01:55 PM   #15
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Talking Re: How do I diagnose my heater?

the heater control assembly (levers) sometimes break off as well, their just pot metal, if the cable isn't moving the door on your heater housing, the lever may be broke.
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Old 01-10-2021, 09:08 PM   #16
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Re: How do I diagnose my heater?

Great information. Have to have a good heater.
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Old 01-10-2021, 11:04 PM   #17
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Re: How do I diagnose my heater?

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Hoses feeding the heater core may be backwards, also check to make sure the core supply line is connected to the intake manifold and not the water pump. Mine were all kinds of wrong backwards at the heater core and feeding from the water pump, switched them back and it was burning me out
Hose orientation makes no difference.
Make sure both hoses feel hot. If not clogged heater core is so blend door cable or door itself.
Simple system on these old trucks
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Old 01-11-2021, 09:26 AM   #18
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Re: How do I diagnose my heater?

If you are getting to where you think the heater core is plugged, you can take the heater hoses off and flush water through with your garden hose. With no heater control valve in the way, house pressure water should flow through freely. You can remove your heater control valve if you have one for this test.
Heater cores are a bear so you want to be sure it is plugged before you go swapping it out.

If you end up swapping in a new heater core, order new heater hoses. They are cheap and easy to do at that time.
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Old 01-13-2021, 02:25 PM   #19
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Thumbs up Re: How do I diagnose my heater?

This is what was wrong with my truck!
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