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Trouble with heat
#600015 January 24th 2013 7:47 am
Joined: Oct 2011
Posts: 69
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journeyman
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journeyman
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Joined: Oct 2011
Posts: 69
Hey all,

I'm having some trouble with my heat in my 91 G20. The heat will cut out sometimes and blow full force other times. The first time it happened I checked the fuses and I guess messing with them got it working. Any suggestions where to look? I do have a coolant leak at the heater control valve. New heater core was put in Feb last year.

On a side note, my 98 cavalier does the same thing and hitting the dash (or a pothole) usually gets it going. I changed the blower motor and blower motor resistor but it still does the same thing. Any suggestions there too?

Thanks


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Re: Trouble with heat
Moliva1568 #600018 January 24th 2013 8:39 am
Joined: Sep 2009
Posts: 1,051
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old hand
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Joined: Sep 2009
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I just went through this on my 85, sometimes work, sometimes wouldn't. I found a lot of corrosion where the plug goes on the resistor. Gm placed them where any rain that runs in sits in this area. I replaced the resistor, which seemed to help but I ended up putting a new fan motor on it and cleaning my ground wire attachment point, haven't had any issues since. Mine is a no AC van, corrosion might not be an issue with AC vans as the resistor is in a different location.


'85 GMC Shorty
'98 Suburban K2500
'98 Pontiac G8 GT
'77 Pontiac Astre
'95 Jeep Wrangler
"I thought you weren't allowed to talk to guys with vans??"
Re: Trouble with heat
Moliva1568 #602508 February 15th 2013 3:57 pm
Joined: Feb 2013
Posts: 52
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journeyman
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Joined: Feb 2013
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The air ducts in your HVAC system are controlled by vacuum switches and actuators, as you slide the lever from one to the other you are simply opening one vacuum circuit and closing another, on your intake manifold threaded into a coolant passage or water crossover at the front of your manifold is a vacuum ported switch that is controlled by coolant temp, you will have manifold vacuum going into 1 side of the switch and lines coming off the other side that trace back through the firewall and into the HVAC system, that ported switch is kinda the brains of the whole operation, if it's not working correctly nothing in the HVAC system will work correctly, also keep in mind that the only time you have manifold is at idle or steady-state cruise, when the engine is under load you will have no vacuum and all those air doors behind the dash go funky until you let of the gas.. hope this helps


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