heat, vent, defrost - The 1947 - Present Chevrolet & GMC Truck Message Board Network

chris989 · 02-13-2017, 02:27 PM

Hello All,

I searched and read a few post about the heat/ac not switching from defrost to floor or vent. Most post say the diaphragm that actuates the damper door has failed.

I am not getting vacuum to the diaphragm. i have vacuum under the hood to where is goes through the firewall.

is there another point of failure i should consider before i tear my dash apart? Does the main control in the dash sometimes fail or become clogged? When i switch from heat to defrost i can here one of the diaphragms move but i still get 90% of the air out of the vents.

Additional note,
I feel i have a leak somewhere because now if i am going downhill with my foot off the gas the engine will surge.

Thanks,
Chris

Dead Parrot · 02-13-2017, 05:56 PM

Yes, the main control can fail, all or partial. A vacuum leak in the AC controls shouldn't let enough air in to have much impact on the motor. A failing brake booster can impact idle behavior.

Check this tread out: http://67-72chevytrucks.com/vboard/s...d.php?t=460942

chris989 · 02-14-2017, 11:01 AM

Quote:

Originally Posted by Dead Parrot

Yes, the main control can fail, all or partial. A vacuum leak in the AC controls shouldn't let enough air in to have much impact on the motor. A failing brake booster can impact idle behavior.

Check this tread out: http://67-72chevytrucks.com/vboard/s...d.php?t=460942

The motor runs great, idles perfect. It is only when going downhill or slowing i get some surge.

Thanks for the link.
Chris

wilkin250r · 02-14-2017, 03:39 PM

I think what Parrot is saying is that it's highly unlikely the two problems are connected.

While a broken or disconnected vacuum line inside the cab is technically a "vacuum leak", it's not significant enough to cause any engine problems. You can't pull enough air through that maze of lines to actually affect the performance of the engine.

If you DO have an actual vacuum leak, significant enough to cause the engine to surge, it's someplace other than your AC controls.

chris989 · 02-21-2017, 01:01 PM

does anyone have a parts source for these items. i searched rock auto and LMC but do not see any listing.

Chris

chris989 · 04-08-2017, 03:06 PM

Update,

the small plastic piece that connects the rod to the actuated door in the heat system failed and that is why i had no heat/defrost choice.

Also, the 1/8 plastic tube/hose from the intake manifold that runs the heat/ac system had melted on the ERG valve and was causing my surge.

two two issues coincidentally happened near the same time.

02-13-2017, 02:27 PM	#1
chris989 Registered User Join Date: Dec 2011 Location: Boise, Idaho Posts: 589	heat, vent, defrost Hello All, I searched and read a few post about the heat/ac not switching from defrost to floor or vent. Most post say the diaphragm that actuates the damper door has failed. I am not getting vacuum to the diaphragm. i have vacuum under the hood to where is goes through the firewall. is there another point of failure i should consider before i tear my dash apart? Does the main control in the dash sometimes fail or become clogged? When i switch from heat to defrost i can here one of the diaphragms move but i still get 90% of the air out of the vents. Additional note, I feel i have a leak somewhere because now if i am going downhill with my foot off the gas the engine will surge. Thanks, Chris

02-13-2017, 05:56 PM	#2
Dead Parrot Registered User Join Date: Aug 2012 Location: Oklahoma City, OK Posts: 2,611	Re: heat, vent, defrost Yes, the main control can fail, all or partial. A vacuum leak in the AC controls shouldn't let enough air in to have much impact on the motor. A failing brake booster can impact idle behavior. Check this tread out: http://67-72chevytrucks.com/vboard/s...d.php?t=460942

02-14-2017, 03:39 PM	#4
wilkin250r Registered User Join Date: Sep 2010 Location: Lake Tahoe, Nevada Posts: 755	Re: heat, vent, defrost I think what Parrot is saying is that it's highly unlikely the two problems are connected. While a broken or disconnected vacuum line inside the cab is technically a "vacuum leak", it's not significant enough to cause any engine problems. You can't pull enough air through that maze of lines to actually affect the performance of the engine. If you DO have an actual vacuum leak, significant enough to cause the engine to surge, it's someplace other than your AC controls. __________________ I know a little about cars, but if you have a question about electricity or sport quads, I'm your man!!!

02-21-2017, 01:01 PM	#5
chris989 Registered User Join Date: Dec 2011 Location: Boise, Idaho Posts: 589	Re: heat, vent, defrost does anyone have a parts source for these items. i searched rock auto and LMC but do not see any listing. Chris

04-08-2017, 03:06 PM	#6
chris989 Registered User Join Date: Dec 2011 Location: Boise, Idaho Posts: 589	Re: heat, vent, defrost Update, the small plastic piece that connects the rod to the actuated door in the heat system failed and that is why i had no heat/defrost choice. Also, the 1/8 plastic tube/hose from the intake manifold that runs the heat/ac system had melted on the ERG valve and was causing my surge. two two issues coincidentally happened near the same time.



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