BostonETC97 Posted July 23, 2006 Report Share Posted July 23, 2006 The a/c in my 97 Eldorado is making me crazy. When cruising, especially uphill, the A/C will stop blowing out the vents, and switch over to the defroster. This lasts a minute or so, then usually switches back to the vents again. If the car points downhill, or if I pump the brakes, the air is directed back to the dash again. I know the system is vacuum operated, but I can't find a leak anywhere. The vac reserve tank holds vacuum, the parking brake works fine, and all the lines are intact to the best of my knowledge. I can't find any vacuum leaks, but I know there must be one somewhere! I'm starting to think the problem is on the backside of the firewall, in the HVAC unit itself. Is there any easy way to access the vacuum actuators behind the dash, and do they ever go bad? Anybody with a similar experience ever find a good solution? Thanks for the insight! -Scott Black 1997 ETC, 90k miles. Link to comment Share on other sites More sharing options...
Texas Jim Posted July 23, 2006 Report Share Posted July 23, 2006 The a/c in my 97 Eldorado is making me crazy. When cruising, especially uphill, the A/C will stop blowing out the vents, and switch over to the defroster. This lasts a minute or so, then usually switches back to the vents again. If the car points downhill, or if I pump the brakes, the air is directed back to the dash again. I know the system is vacuum operated, but I can't find a leak anywhere. The vac reserve tank holds vacuum, the parking brake works fine, and all the lines are intact to the best of my knowledge. I can't find any vacuum leaks, but I know there must be one somewhere! I'm starting to think the problem is on the backside of the firewall, in the HVAC unit itself. Is there any easy way to access the vacuum actuators behind the dash, and do they ever go bad? Anybody with a similar experience ever find a good solution? Thanks for the insight! -Scott Black 1997 ETC, 90k miles. Scott, Not on a Caddy, but the exact same thing on my Chevy AstroVan. I tried to find the leak, spent several hours on it, scratched and scraped my arms pretty bad by reaching into places that were smaller than my arms. Finally took it to the Chevy dealer. "Oh yeah, we know that problem. Vacumn leak." It was the lines on the rear of the intake. $110.00 bucks... it is fixed. That was about a year ago. Still working fine. Link to comment Share on other sites More sharing options...
BodybyFisher Posted July 23, 2006 Report Share Posted July 23, 2006 I am experiencing the same problem. I have replaced the black plastic nylon vacuum line (not cheap) that goes from the check valve to the pressure tank that sits under the RF head light area and I have replaced the check valve and associated vacuum hoses with no change in the loss of AC flow when the engine labors uphill creating a low vacuum condition. My brake release is slow to release also and I replaced the release servo, with no change. I have tested the system by creating a vacuum with hand pump with gage from the check valve to the tank and it holds pressure without leaking. I am thinking that I have a leak in the ACM, the ACM is related to the HVAC system and supplys vacuum to the parking brake release. Its possible that there is a dry rotted rubber vacuum hose in there... I found a dry rotted rubber hose under the hood and replaced it already. The ACM sits behind the glove compartment I think. Hope this helps, I need to look at the ACM vacuum connections. Maybe the vacuum hose that goes through the firewall is worn from rubbing, these lines are plastic nylon.... Pre-1995 - DTC codes OBD1 >> 1996 and newer - DTC codes OBD2 >> https://www.obd-codes.com/trouble_codes/gm/obd_codes.htm How to check for codes Caddyinfo How To Technical Archive >> http://www.caddyinfo.com/wordpress/cadillac-how-to-faq/ Cadillac History & Specifications Year by Year http://www.motorera.com/cadillac/index.htm Link to comment Share on other sites More sharing options...
Dave in Texas Posted July 23, 2006 Report Share Posted July 23, 2006 For a person that is a DIR'er, a good investment is the Mity-Vac. It's a hand-operated vacuum pump. It has a pressure/vacuum guage and it comes with lots of attachments. Use it to track down vacuum leaks in hoses and vacuum motors. It can also be used to bleed brakes. Good instructions too. A very handy piece of equipment and it's in the $30 price range. GM Reman 4.1 engine Dec '08 Link to comment Share on other sites More sharing options...
BostonETC97 Posted July 23, 2006 Author Report Share Posted July 23, 2006 After spending some time troubleshooting, I'm starting to think the problem is with whatever activates (opens) the flap directing the air to the dash. If I set the air to come out the floor, it will never flux between the defrost and floor. If I set the system on bi-level, the air still comes out the floor, but will flux back and forth between the dash and defroster as I drive. If it was a true vacuum leak, would the floor position still hold? My parking brake pops up quickly and fast (unfortunately, that's my only 'vacuum gauge' right now ), and if you pump it while the air is coming out the defrost, it still has strong vacuum and releases fast. I figure I have nothing on the afternoon agenda, and I'll start taking the glovebox and lower dash apart. Maybe I'll get lucky and find a rotted out or kinked line. Who decided a vacuum system was a good idea for HVAC controls? Seems like such a fragile and trouble-prone system to control so many small aspects of the car. (Well, I guess I'll be thankful I don't have the vac-operated power locks like the old Benz's have! ) Link to comment Share on other sites More sharing options...
BodybyFisher Posted July 23, 2006 Report Share Posted July 23, 2006 For a person that is a DIR'er, a good investment is the Mity-Vac. It's a hand-operated vacuum pump. It has a pressure/vacuum guage and it comes with lots of attachments. Use it to track down vacuum leaks in hoses and vacuum motors. It can also be used to bleed brakes. Good instructions too. A very handy piece of equipment and it's in the $30 price range. Thanks Dave, that is what I am using, I couldn't remember the name of it... Pre-1995 - DTC codes OBD1 >> 1996 and newer - DTC codes OBD2 >> https://www.obd-codes.com/trouble_codes/gm/obd_codes.htm How to check for codes Caddyinfo How To Technical Archive >> http://www.caddyinfo.com/wordpress/cadillac-how-to-faq/ Cadillac History & Specifications Year by Year http://www.motorera.com/cadillac/index.htm Link to comment Share on other sites More sharing options...
BodybyFisher Posted July 23, 2006 Report Share Posted July 23, 2006 Cadillac and most manufacturers have used vacuum operated HVAC controls for years they are pretty good, prior to that they were manual cable operated if you recall. You and I have old cars however and as things get older they break down. My 66 Caddy had vacuum door locks! The vacuum hose IN the hinge area went bad from opening and closing the door... Good Luck with your diagnosis... Pre-1995 - DTC codes OBD1 >> 1996 and newer - DTC codes OBD2 >> https://www.obd-codes.com/trouble_codes/gm/obd_codes.htm How to check for codes Caddyinfo How To Technical Archive >> http://www.caddyinfo.com/wordpress/cadillac-how-to-faq/ Cadillac History & Specifications Year by Year http://www.motorera.com/cadillac/index.htm Link to comment Share on other sites More sharing options...
Ranger Posted July 23, 2006 Report Share Posted July 23, 2006 Don't want to get off subject here, but if you think vacuum operated HVAC controls are a PITA, you have never expirienced vacuum operated windshield wipers. From what you say, I would locate and concentrate on the bi-level vacuum circuit and or door diaphram (aka vacuum motor). Link to comment Share on other sites More sharing options...
BostonETC97 Posted July 23, 2006 Author Report Share Posted July 23, 2006 My conclusion: Vacuum Sucks! (Bad joke...) I just took out the glovebox, checked all over for vacuum leaks, and can't find any. My diagnosis is that the vent/defrost actuator has failed, as it was the only piece of the puzzle that doesn't add up. I haven't tested it, but after listening to it, it sounds very slow and weak. The other actuators have a crisp "pop" to them, and activate quickly. The defrost/vent actuator sounds like its dying, and takes a long time to swap between settings. The good news is that I have an extended warranty on the car, and the parts/labor should be covered (The extended warranty has paid for itself 3 times over already! Replaced alternator, a/c compressor, fuel level sending unit, and power window motors!). I'm guessing a large portion of the dash needs to be removed to access that particular module, since in the manual it looks like it's mounted high up near the firewall. Guess I'll find out this week! Thanks for all the help this weekend, I'll let everybody know how it turns out! -Scott Link to comment Share on other sites More sharing options...
Ranger Posted July 23, 2006 Report Share Posted July 23, 2006 Don't overlook the parking brake diaphram. It may hold enough vacuum to release but still have a slow leak. It's easy enough to check. Link to comment Share on other sites More sharing options...
Bob*97 SLS Posted July 24, 2006 Report Share Posted July 24, 2006 Here is my post when I had this problem earlier this year. I leave my repairs to my Caddy Dealership and Mr. Goodwrench! I hope this helps! **My Seville started moving the air delivery from the dash (center) to defrost vents this weekend. I dropped the car off at the dealer this morning at 8am and picked it up at noon. They had to replace a part called the "AC Mode Switch Solenoid." Link to comment Share on other sites More sharing options...
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