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Don, my experiences so far match yours. Lots of flow through the floor vents but poor to the screen.
The following is what have we found out so far having had the whole thing apart and having spent a fair bit of time on figure 90 in the parts book and figure 13-2 in the service manual.
When we put airflow output through the Y connector, we get moderate flow to the screen, not great, but passable flow levels. I would have expected a good blast, but that is not what we get. There are no leaks on either of the defrosters outlet units that we can find so the only thing we figure is that the large chamber below the vent openings slows the flow down a lot. We are going to check for more leaks just the same as it does sound to me that there is air leakage under the glare panel and behind the instruments.
We did find that the system seems to work backwards. That is the valve to the defoster although rigged per the book, operates backwards. When the control is in the up and off position, the valve is open. When the control is in the down and open position, the valve is mostly closed. We have checked and it is almost like the split pin hole that holds the control arm was drilled 90 degrees wrong on the shaft that holds the valve plate or the valve plate holes are out by 90 degrees.
The other control to the flow box behind the right P1 pedal controls the amount of air flow to the thermostat and thus we assume the rate which the heater cycles on and off. Its bracket means the rigging on the defroster flow diverter can only be set up as shown in the manual.
We did find some skat the was a bit ratty and now replaced and the rectangluar duct under the P1 floor was bent inwards meaning there was air flow loss under the floor.
We are now putting it all back together and will try it again, but this time with the various floor heater outlets turned down a bit. Hopefully that will give us reasonable flow through the defroster vents.
The P2 heater vent seems to flow the best of all, so perhaps limiting its flow might also help. I never sit there so I won't get cold feet.
I will let you know what else we find.
Regards - Dave
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