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Discussion Starter #1
I have a 1998 Eldorado. The upper ac vents don't open all the time. Some times after driving for a while they open. I have checked the vacuum going into the firewall and its perfect. I went under the dash and checked the vacuum line to the canister and didn't get vacuum, I switched the floor vacuum line to the upper vent canister and moved the mode switch to the floor position and the upper vents worked. My question is can the mode switch be the problem?
 

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The parking brake auto release is vacuum operated and on the same vacuum circuit. Check that for leaks.
 

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Discussion Starter #3
Sorry for not responding to posts. I'm not very handy with the ability to enter posts. I have tried to do several and I don't get it right. It's been a couple of months since I posted my problem with my upper vents not opening. I have measured the vacuum and it is good. The canisters are working properly. My question is it the mode switch is working then not working?
 

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I switched the floor vacuum line to the upper vent canister and moved the mode switch to the floor position and the upper vents worked.
That kind of sounds like a programmer (#10) problem, or a vacuum leak on one of the vacuum lines or connection (#9).

576746
 

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Master of the Dark Art of Diagnostics
2003 DHS - two-2002 DHS, 2003 SLS, 1995 Sedan DeVille, 1989 Coupe DeVille
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Sorry for not responding to posts. I'm not very handy with the ability to enter posts. I have tried to do several and I don't get it right. It's been a couple of months since I posted my problem with my upper vents not opening. I have measured the vacuum and it is good. The canisters are working properly. My question is it the mode switch is working then not working?
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couple things you MUST keep in mind -

1) YOU have no manual control WHERE the air blows from - except for DEFROST -
2) warm/hot air is only delivered to the floor vents (and defroster)
3) you only get cold air - or ambient/outside air from the dash vents -

vacuum motors control the MODE doors -
electric motors control the temperature mix doors -

depending on desired cabin temp - and ambient air temp -
it is quite possible for the automatic climate control to command heat -
thus diverting air flow to the floor vents -
then switch to ambient air -
thus diverting air flow to the dash vents -
and back to heat -
 

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Discussion Starter #6
My 1998 Eldorado has vacuum canisters, not electrical motors to open the doors to allow the air flow. I believe that later year models were electrical. As I said I tested the vacuum for proper vacuum. All the lines to all canisters are fine except the line going to the upper vents. But it has vacuum some times but quits working then starts working again while driving. If I pull the line from the floor vent and hook it up to the upper vents and put the mode switch to the floor vent the upper works all the time. So that's why I am so perplexed. Can it be a bad mode switch?
 

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Master of the Dark Art of Diagnostics
2003 DHS - two-2002 DHS, 2003 SLS, 1995 Sedan DeVille, 1989 Coupe DeVille
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My 1998 Eldorado has vacuum canisters, not electrical motors to open the doors to allow the air flow. I believe that later year models were electrical. As I said I tested the vacuum for proper vacuum. All the lines to all canisters are fine except the line going to the upper vents. But it has vacuum some times but quits working then starts working again while driving. If I pull the line from the floor vent and hook it up to the upper vents and put the mode switch to the floor vent the upper works all the time. So that's why I am so perplexed. Can it be a bad mode switch?
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My 1998 Eldorado has vacuum canisters, not electrical motors to open the doors to allow the air flow.
CORRECT - that is exactly what I wrote -

VACUUM motors control the MODE doors -
ELECTRIC motors control the TEMPERATURE mix doors -

I believe that later year models were electrical.
NO - the Eldo was already destined for the chopping block and never got the HVAC upgrades
that the Seville got in 1998 and the Deville got in 2000 -
eliminating ALL vacuum motors -
AND allowing full manual control of the MODE at ANY temperature -

As I said I tested the vacuum for proper vacuum.
All the lines to all canisters are fine except the line going to the upper vents.

and as I said - except for DEFROST - you have no control over WHERE the air blows from - the MODE -
HEAT only comes from the FLOOR -
COLD only comes from the DASH VENTS -

I'll add this -
IF the HVAC system looses vacuum -
the MODE will default to the DEFROST position -
 

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2002.5 F55 CORSA STS, 2014 Explorer XLT FWD
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basscatt is informing you that the car - not you - makes the decisions for where the HVAC air goes to/comes from.
 

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As I said I tested the vacuum for proper vacuum. All the lines to all canisters are fine except the line going to the upper vents. But it has vacuum some times but quits working then starts working again while driving.
Locate the cabin vacuum supply line at the firewall on the passengers side. Follow that line back to the vacuum reservoir canister. Look for a leak at the canister or in the vacuum line.
 

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Is that the vacuum line that comes off the front (passenger end) of the intake manifold? We just fixed one in the shop on Wednesday. Busted off at the rubber connector - Ranger's picture.

overpressure valve closed.JPG
 

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Discussion Starter #11
I have have tested all the vacuum lines with a vacuum gauge and everything is ok, except the intermitten of the line that goes to the vacuum canister that opens upper vent door. It works then does not work.
 

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Discussion Starter #12
The only vacuum line that comes from control module and the mode switch that does not get vacuum all the time is the one that opens the upper vent door.
 
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