Vacuum System Question RE: 1987 C68 Vent Position Issue
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Safety Car
Joined: Mar 2003
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From: Indianapolis IN
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Vacuum System Question RE: 1987 C68 Vent Position Issue
I have a problem I am diagnosing with my 1987 C68 system---My control head works as it should, but the air constantly blows out of the center and defrost vents. I want to verify my HVAC vacuum system is working as it should....so my question is:
When testing for a leak, should I be able to apply vacuum to the INPUT side (left side, as seen in picture below) of the vacuum check valve and have it hold? As of right now, when I apply vacuum at that point I can hear a hissing from inside the dash...which I believe is my issue. I also get the same symptom (underdash hissing) when disconnecting the line directly across from the input and applying vacuum directly to it, which I believe to be the supply line for my HVAC controls.
As a side note; I am waiting for my FSM to arrive, so I am operating a little blindly as I do not have a vacuum line schematic to reference.
When testing for a leak, should I be able to apply vacuum to the INPUT side (left side, as seen in picture below) of the vacuum check valve and have it hold? As of right now, when I apply vacuum at that point I can hear a hissing from inside the dash...which I believe is my issue. I also get the same symptom (underdash hissing) when disconnecting the line directly across from the input and applying vacuum directly to it, which I believe to be the supply line for my HVAC controls.
As a side note; I am waiting for my FSM to arrive, so I am operating a little blindly as I do not have a vacuum line schematic to reference.
Safety Car
Joined: Jan 2004
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Vac should hold ,
the hissing would be a good indication a line under the dash , the connections to the vent controller
or one of the vent position control pots the vac operates is leaking
Items 38 ,36,35,14 in diagram
the hissing would be a good indication a line under the dash , the connections to the vent controller
or one of the vent position control pots the vac operates is leaking
Items 38 ,36,35,14 in diagram
Thread Starter
Safety Car
Joined: Mar 2003
Posts: 3,582
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From: Indianapolis IN
Finalist 2020 C4 of the Year - Unmodified
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Thank you for the exploded view of the ducting and for the affirmation that vacuum should be holding on that line--I suspected it should, but the C68 is new to me and I didn't want to chase a dead end. I know that some vacuum systems like the old ThermAC temp sensors on earlier carbureted air cleaners were nothing more than controlled vacuum leaks, and I wanted to make sure I wasn't dealing with something similar here.I suspect once I find the leak and repair it my vent selection problem will be fixed.
My FSM should be here by Wednesday or Thursday at the latest
Race Director
Joined: Apr 2002
Posts: 10,036
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From: Anaheim CA
When you get your FSM, look on pages 1B1-6 and 8A-64-12. They show the vacuum connections for the HVAC system.
Your FSM might have slightly different page numbers, but those will be pretty close.
Your FSM might have slightly different page numbers, but those will be pretty close.
Thread Starter
Safety Car
Joined: Mar 2003
Posts: 3,582
Likes: 504
From: Indianapolis IN
Finalist 2020 C4 of the Year - Unmodified
C4 of Year Finalist (stock) 2019
Cliff, one of your earlier threads from back in 12/2011 ( C68 HVAC Programmer Info & Schematic ) gave me all of the info I needed to fix my problem yesterday. The failure stemmed from unplugged supply lines to the HVAC programmer, as seen in this picture from that thread:
I believe the lines became unplugged after the PO had been behind the radio rifling around during an aftermarket radio install. Given the location of the connector, and all of the slack between the connector and the programmer, there is now way that those lines could just unplug themselves.
While I was at it, I also repaired the cold solder joints on the resistors on the HVAC programmer board as seen in this pic from that same thread: (I will be replacing them with the larger 140 ohms, 5% tolerance, 2 watt resistors in the near future).
I would like to personally THANK YOU for taking the time to create such a terrific write up! It gave me all of the information I needed both written and visual to fix this problem, as well as address that cold solder issue. GREAT JOB!!
Dave
