Page 19 - Summer 2018 GMC Vintage RVing

For additional technical session presentations and papers, go to gmcmi.com/technical-presentations-papers/Type IIdash and fed by a new channel through the box and part of the old recirc slot through the firewall. To control air through this channel, GM used the vacuum actuator that operated the original recirc door to instead power a new “Max Air” door. Consequently, all air in this design comes from outside the coach. The old recirc door and leftover firewall opening were sealed shut for interim modifications, and later omitted. A service bulletin issued that year references parts to retrofit this Type II design into existing boxes. It was a fairly involvedkit to install, but by 1976 was no longer necessary as the boxes were manufactured to the new design. While an improvement, this version directs air mostly to the center of the cockpit where no one typically sits. The new duct is only for air conditioning, and only when the selector lever is set to “RECIRC” with sufficient vacuum to hold the door open. Otherwise, air is directed through the same oldleaky duct system via the opening atthe top of the box. GM also replaced the blower “OFF” setting with “LO” to makethe fan run constantly whenever the key is on. This was probably intended to combat engine and gasoline odors by creating positive pressure in the cockpit at all times, but many owners have chosen to disable this feature.For 1977, GM almost completely redesigned the system. While the basic black box remained, everything else changed. The recircdoor returned, so the blower side of the box was the same as the Type I boxes, but it connects to a taller recirc opening in the firewall.The back of the pressure side of the box was removed to allow for a much larger opening through the firewall to a new air distribution box and duct system. A divider was added to separate the blower side from the new opening all the way to the firewall. A small sheet metal diverter was added to the face of the box to help push air back towards the new duct system.SUMMER 2018 GMC MOTORHOMES INTERNATIONAL 19