Author Topic: 75 Eldorado - hissing noise near temperature control  (Read 156 times)

Offline pplaut

  • Posts: 4
  • Name: Peer Plaut
75 Eldorado - hissing noise near temperature control
« on: November 12, 2017, 12:45:16 PM »
I just got my temperature control unit serviced by cadillac Tim. Now the functions work much better.
However, in a couple of modes there is a loud hissing noise coming from around the Control Module
- which was present before Cadillac Tim rebuild.
Selection OFF - No Hiss
Selection Economy - No Hiss air volume seems proper
Bi-Level
Low - Loud Hiss
Auto- loud hiss
High Low hiss
Bi Level- after Cadillac Tim blows air from vents - Air Volume Seems Low
Defrost - No Hiss - air volume seems proper

The vacuum harness - "looks new"
Where do I go next?
Thanks!
P.

Offline "Cadillac Kid" Greg Surfas 15364

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Re: 75 Eldorado - hissing noise near temperature control
« Reply #1 on: November 12, 2017, 01:05:25 PM »
Might suggest you call Tim. He would be my choice at this point.
Greg Surfas
Cadillac Kid-Greg Surfas
CLC #15364
66 Coupe deVille (now gone to the UK)
72 Eldo Cpe  (now cruising the sands in Quatar)
73 Coupe deVille
75 Coupe deElegance
76 Coupe deVille
79 Coupe de ville with "Paris" (pick up) option and 472 motor
514 inch motor now in '73-

Online cadillacmike68

  • Posts: 1139
  • Still finishing up this 1968
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      CLC Member #15823
  • Name: M Santos
Re: 75 Eldorado - hissing noise near temperature control
« Reply #2 on: November 12, 2017, 02:16:30 PM »
It is Definitely a vacuum leak. I remember my 1970 & 73 would hiss for a second or so when changing modes. The 1968 does it too, I think, but in all cases only for a second or so as the rotary switch, well, rotates, and vacuum is bled off.
Regards,
"Cadillac" Mike
Current:
1968 DeVille Convertible
1996 Fleetwood Brougham
2009 STS NorthStar Platinum ed RWD
2011 CTS PRemiun ed Sedan RWD
Past:
2008 CTS Premium ed Sedan AWD
2005 CTS Hi-Feature Sedan RWD
2000 ElDorado ESC Hard Boot Convertible
1995 Fleetwood Brougham
1973 Sedan DeVille
1970 Fleetwood Brougham
1969 DeVille Convertible

Offline The Tassie Devil(le)

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  • Name: Bruce Reynolds
Re: 75 Eldorado - hissing noise near temperature control
« Reply #3 on: November 12, 2017, 03:30:30 PM »
There is no actual sealing membrane between the sides of the vacuum control switch, and the faces are just diecast to diecast.

Yesterday I dismantled the dash controller from a '72 Camaro and pulling the "switch" apart, I couldn't believe just how clean it was.   Even after sitting for some 30 years, it still sealed like new.

Bruce. >:D
'72 Eldorado Convertible (LHD)
'70 Ranchero Squire (RHD)
'74 Chris Craft Gull Wing (SH)
(Past President Modified Chapter)

Offline Mike Josephic CLC #3877

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Re: 75 Eldorado - hissing noise near temperature control
« Reply #4 on: November 12, 2017, 04:20:40 PM »
To echo what has already been said, it sure seems like a vacuum leak due
to the hissing sound you're hearing.

However, it may not be the control unit.  Your under-dash vacuum lines being
that old could easily have a cracked line or damage more during the removal.
There are a bunch of them under there.

Before tearing that out again and sending it back, I would check all of those lines
to insure that the problem does not lie there.

Since the sound was present PRIOR to the rebuild, that's all the more reason
to check.

Mike
1955 Cadillac Eldorado
1973 Cadillac Eldorado
1995 Cadillac Seville
2004 Escalade
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Past: VP International Affiliates, Museum Board Director, President / Director Pittsburgh Region

Offline Scot Minesinger

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Re: 75 Eldorado - hissing noise near temperature control
« Reply #5 on: November 12, 2017, 08:42:38 PM »
You typed it works much better, so do the functions work as they should? In other words if you could not hear the hiss would everything be OK?

If you hear a hiss and everything works that is one thing, and quite another if hisses and does not work.  In any event agree with the rest, you probably have a vacuum leak and you should be able to easily find it if the hiss is that loud.
Fairfax Station, VA  22039 (Washington DC Sub)
1970 Cadillac DeVille Convertible
1970 Cadillac Sedan DeVille
1970 four door Convertible w/Cadillac Warranty

Offline cadillactim

  • Cadillac Tim
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Re: 75 Eldorado - hissing noise near temperature control
« Reply #6 on: November 14, 2017, 07:44:31 PM »
Peer

I would suggest you take some needle nose pliers and one by one pinch off each vacuum line at the control valve until the hissing sound stops (except port 1 which is the vacuum feed). When you find the line that stops the sound then trace that line to find the leak(s). You might try the same technique at the programmer valve as well.

Tim
Tim Groves

Offline pplaut

  • Posts: 4
  • Name: Peer Plaut
Re: 75 Eldorado - hissing noise near temperature control
« Reply #7 on: November 15, 2017, 06:43:45 PM »
Tim-
I was thinking of this... pinch them off one or more at a time.. :)
Especially since it makes no hissing noise in the off position.
Thanks to everyone else for your advice. I will post the remedy when I figure it out. :)