C3 Plenum Door Operation

[Tom R.]

This is so neat. I’m in the middle of a heater core replacement and have reassembled the box but have yet to install duct work. It’s a 78 A/C car. So, I decided to verify operation of the vacuum operated heat, A/C and ventilation doors. My view was such that I could watch all doors operate as I moved the temperature control lever…pretty cool stuff. But anyway...

Before proceeding, I’m verifying operations of plenum door, defroster door and A/C door. My understanding of what should happen with control set to MAX…is the A/C door opens (closing airflow to defroster and permitting cool air into the duct assembly) and the plenum door closes. This permits maximum cooling by recycling cabin air and prohibiting the introduction of outside air. However, the plenum door is opening! Also, the water shut-off valve shuts hot water from flowing to the heater core, theoretically (and this works as well).

Testing further, plenum door doesn’t open when control set at VENT. It does get vacuum and the actuator holds vacuum…in fact nicely. The vacuum line (orange) is connected to the green which plugs into the back of the switch...per service manual.

I’ve check connections at the back of the switch that leads to the control levers and they are in the correct order according to the service manual.

So my question is…why is the plenum door opening when it should remain closed? What am I missing here?

[Jack H.]

Re: C3 Plenum Door Operation

You're using the term 'plenum door' as a singular rather than plural.... Haven't looked at '78 specifically, but earlier Sharks used a pair of plenum doors (one in the RH kick panel, the other in the outside RH intake plenum) operating in 'flip-flop' fashion....

When recirculate mode is called for, the kick panel door opens and the outside intake plenum door closes. This forces the makeup air to flow from the cabin instead of from the exterior of the car. When 'standard' AC mode is entered, the flip-flop reverses. Kick panel door closes blocking cabin air recirculation and the outside plenum door closes causing all makeup air to enter from the exterior cowl....

Don't know if the '78 AIM (Assembly Instruction Manual) has a vac truth table in it, but earlier Shark cars do for troubleshooting. The truth table lists 'Vac' or 'Vent' by color coded vac hose/control function vs. setting of the interior Heater/AC control mode switch.

[Tom R.]

Re: C3 Plenum Door Operation

Jack:

I was using the truth table in service manual but the table in the AIM is nice and big. I also spot an outside air valve with the the green hose going to another actuator...I'll have to track this one down tomorrow. I've traced the green hose from what I describe as the plenum door (inside door) back to the switch.

So what exactly does 'vent' and 'vac' indicate in the truth table...when vac is being applied? versus vented air?

So if this is the case then the plenum door is properly functioning, opening to allow recir while the door I've yet to find...should be closed.

Thanks.

[Tom R.]

Re: C3 Plenum Door Operation

Yes...78 is no different. The valve and door are located as you described and connected by the green stripe hose all of which traces correctly back to the switch. So, the operation must be such that the blower motor has two sources of air...outside and cabin. In MAX, recir door opens while plenum door closes and blower motor gets air from cabin. In VENT and A/C NORM, doors reverse and blower motor gets air from outside. I believe I've got it but still need clarification on the truth table VAC vs VENT.

[Jack H.]

Re: C3 Plenum Door Operation

Good boyo! My WAG (stretching early Shark to late Shark operation) appears to have been valid....

The terms 'vent' and 'vac' mean exactly what you thought--what is applied to the hose line/function; active vac pressure (vac) or none, plane atmospheric pressure (vent).