I think the biggest obstacle to overcome for everyone interested in this mod is going to be the ability to disassemble the passenger side factory dash
without destroying it. Mine started breaking the moment I tried, and so it came out in about two minutes. In too many pieces to even contemplate
salvage. So I am building an all new dash, driver and passenger side, that will allow easy opening and access to everything.
But once the dash is out, all the mods are relatively easy. Not technically difficult, merely time consuming.
The Jeep system is a defrost default system, meaning a loss of vacuum shifts the system to defrost mode, because the defrost flap is open all the time
until vacuum closes it. My 77 GMC is a dash air default, with the defrost flap held shut by a spring until vacuum opens it. I had to reverse the
action of the defrost actuator to close it upon vacuum signal, instead of opening it.
I've been told by someone on the forum ( I need to look up who) that there are vacuum solenoids that can use a vacuum signal on one port to apply
vacuum on another port. If this is the case, then one of these could be used to pull open the defrost flap on the GMC when the Jeep control calls for
defrost. In that mode, the Jeep control has NO VACUUM applied to the defrost actuator, because on the Jeep, its normally open with no vacuum applied.
So you would need a solenoid that activates with a lack of vacuum at the sensing port and applies vacuum at the operating port, which would open the
GMC defrost flap.
If one of these solenoids is available, then the defrost actuator on the GMC would not have to be modified.
This still leaves the modification of the dash air flap, which is held open by a spring/pivot wire. This is what makes it dash air default. You have
to remove that spring and replace it with a 1/4" rod, and then move the actuator 5/8" to the other side of the 1/4" pivot rod, which reverses the
action of the dash air flap, making it closed with no vacuum. You have to be able to get to the air box to do that, at least on a 77. I am not
familiar with the earlier coach systems.
Again, I am more than happy to reply to any questions here on the forum, or a via phone, if someone has an interest and needs more information. I'm in
the middle of the dash build, so until I get that done I won't have time to do a full write up. But I plan to.
--
Greg Crawford
KM4ZCR
Knoxville, TN
"Ruby Sue"
1977 Royale
Rear Bath
403 Engine
American Eagle Wheels
Early Version Alex Sirum Quad bags
without destroying it. Mine started breaking the moment I tried, and so it came out in about two minutes. In too many pieces to even contemplate
salvage. So I am building an all new dash, driver and passenger side, that will allow easy opening and access to everything.
But once the dash is out, all the mods are relatively easy. Not technically difficult, merely time consuming.
The Jeep system is a defrost default system, meaning a loss of vacuum shifts the system to defrost mode, because the defrost flap is open all the time
until vacuum closes it. My 77 GMC is a dash air default, with the defrost flap held shut by a spring until vacuum opens it. I had to reverse the
action of the defrost actuator to close it upon vacuum signal, instead of opening it.
I've been told by someone on the forum ( I need to look up who) that there are vacuum solenoids that can use a vacuum signal on one port to apply
vacuum on another port. If this is the case, then one of these could be used to pull open the defrost flap on the GMC when the Jeep control calls for
defrost. In that mode, the Jeep control has NO VACUUM applied to the defrost actuator, because on the Jeep, its normally open with no vacuum applied.
So you would need a solenoid that activates with a lack of vacuum at the sensing port and applies vacuum at the operating port, which would open the
GMC defrost flap.
If one of these solenoids is available, then the defrost actuator on the GMC would not have to be modified.
This still leaves the modification of the dash air flap, which is held open by a spring/pivot wire. This is what makes it dash air default. You have
to remove that spring and replace it with a 1/4" rod, and then move the actuator 5/8" to the other side of the 1/4" pivot rod, which reverses the
action of the dash air flap, making it closed with no vacuum. You have to be able to get to the air box to do that, at least on a 77. I am not
familiar with the earlier coach systems.
Again, I am more than happy to reply to any questions here on the forum, or a via phone, if someone has an interest and needs more information. I'm in
the middle of the dash build, so until I get that done I won't have time to do a full write up. But I plan to.
--
Greg Crawford
KM4ZCR
Knoxville, TN
"Ruby Sue"
1977 Royale
Rear Bath
403 Engine
American Eagle Wheels
Early Version Alex Sirum Quad bags
