Page 20 - Summer 2018 GMC Vintage RVing

Technical Corner MAINTENANCE, PARTS, TIPS, INNOVATIONS AND MOREType IIIThe top opening in the old black box remained, but only for defrost. Instead of a flimsy plastic door in the duct, a new metal defrost door was mounted inside the box.Probably to accommodate the new distributionbox on the firewall, ride height controls movedto the driver’s side arm rest. The inside air vents moved more towards the driver and passenger side, except for a new central heat vent below the dash. The “LO” blower setting remained, with some air by default always coming out of the central heater vent except when in AC mode. At the control panel, “MAX AC” on the mode selector opens the recirc door, so “RECIRC” was removed from the temperature slider. There were also exterior baffles added to improve air flow to the condenser and radiator, as well as other smaller changes. For more details, see the 78 Maintenance Manual (page 1B-2). For further study, there are also lengthy discussions in the manuals about the theory and operation of HVAC systems.77-78 Duct System20 GMC VINTAGE RVING MAGAZINE | GMCMI.COM