Air to cab declines in driving mode only

EdAC42 on Thu August 19, 2010 3:32 AM

Year: 1997
Make: Chevrolet
Model: 3500
Engine Size: 454
Country of Origin: United States

When sitting idling there is plenty of cool air coming in through the vents (ran for an hour). Once on the road the amount of air starts to decline. After about 5-minutes hardly any air is coming through the vents - Blower motor running like gang busters. I have cycled through the various modes but still no air. After the truck sits for a bit everything works OK until I drive it.
I have checked the electric door actuator and linkage and it is working fine. There are no vacuum lines on this model (except for purge tank). Is there another door I'm not seeing that could be causing the problem? Got me stumped and HOT.

Dougflas on Thu August 19, 2010 5:17 AM

give us some pressures and temps. It is possible your evap is freezing up due to low charge or cycling problems.

GM Tech on Thu August 19, 2010 8:22 AM

Open hood when no airflow---and compressor is running-- are your low side lines frosty white? what you describe is typical evap freeze-up-- probably need a new cycling switch on accumulator. always confirm first...

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The number one A/C diagnostic tool there is- is to know how much refrigerant is in the system- this can only be done by recovering and weighing the refrigerant!!
Just a thought.... 65% of A/C failures in my 3200 car diagnostic database (GM vehicles) are due to loss of refrigerant due to a leak......

EdAC42 on Thu August 19, 2010 5:24 PM

It's not the temperature of the air that declines. It's the VOLUME of air coming out of the vents. As mentioned, sitting idling for an hour I got plenty of cold air. Once I start driving the volume drops to a trickle whereas I can hear the blower going full strength. I took off the electric door actuator and manually moved the lever to the various positions - still no volume coming out. I was thinking there was possibly another door (other than the ones actuated by the door actuator) hidden somewhere in the mechanism or ????

mk378 on Thu August 19, 2010 5:39 PM

When the fins on the evaporator become covered with ice, airflow will be blocked. So do what the others said and check the lines under the hood for freezing up. The pressure control is there to prevent overcooling and freeze up.

iceman2555 on Thu August 19, 2010 5:57 PM

Why not simply test the vehicle as GM stated. This is one of the easier performance test to complete. Another would be to place the vehicle in park....start the engine....max air, high blower, rpm 1200-1500....allow the vehicle to operate for several minutes...once the interior begins to cool....decrease fan speed to low.....now watch the evap outlet and accumulator....is it begins to frost up....and the compressor does not cycle off....the LPCO is gone...toast....finis.....needs to be replaced. Easy to do...simply purchase a new LPCO.....remove wiring connector.....twist the old switch off.....and then install the new unit....re attach wires and re test.....no need to recover refrigerant....evac or recharge.....a very simply repair.
If you have gauges....perform the same test....except monitor the low pressure gauge....the compressor clutch should disengage at...app....25 psi.....re engage at...app 42-45 psi.
Total agreement with GM...sounds like a in-operative LPCO.

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The strongest reason for the people to retain the right to keep and bear arms is, as a last resort, to protect themselves against tyranny in government.
Thomas Jefferson

EdAC42 on Thu August 19, 2010 11:06 PM

Thanks, I'll give it a try in the am.
Ed