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1995 SEDAN DEVILLE-4.9 & 1978 SEDAN DEVILLE-425
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1,673 Posts
Discussion Starter #1
Hi,
My '78 Deville`s cruise control stopped working recently.
I put in another on dash cruise switch a few years back and seems to work ok---all the proper lights light up on
it when button is pushed on stalk control.
I`ve looked at the diaphram assembly under the hood and don`t see any loose vaccuum lines and the diaphram is
still flexible and apparently not ruptured from what I can see.
Don`t know if this is related or not but before the cruise went out I had the car in for a battery check and my mechanic
"manually squeezed the cruise diaphram" to increase idle speed and cruise has`nt worked since.
Any thoughts?
 

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2008 DTS 1980 CDV. Past 1990 Bro. 1981 SDV diesel
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360 Posts
I've squeezed mine also, last week, with no ill affect. My clip that affixed the cable end to the carb was gone. One twisted paperclip & I'm fine. If you can squeeze the diaphragm a little & the rpm increases that indicates the mechanical connection in good.
 

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1990 Brougham, 1990 Brougham d'elgance 1979 Coupe
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1,196 Posts
I think it was pretty common for the module to go bad, it should be right under the steering column accessable with to screws.
 

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1980 Eldorado, 1974 Talisman, 2004 Volvo C70, 1975 Fleetwood 'd Elegance
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7,958 Posts
I would check all your Vacuum lines and connections.
Depending on how quickly he squeezed it, the push of air through the system could have blown apart a weak connection.
Old hard hoses get loose on the barbed fitting but are pulled by the vacuum so they stay in place. The bellows is normally pulled in by vacuum, not pushed in by hand. Squeezing it is putting positive pressure on all the fittings not neg pressure as by vacuum.
 

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1995 SEDAN DEVILLE-4.9 & 1978 SEDAN DEVILLE-425
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1,673 Posts
Discussion Starter #5
I think it was pretty common for the module to go bad, it should be right under the steering column accessable with to screws.
I never knew there is a module under steering wheel.
Is there a picture of it someplace so I know what to look for?
 

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1995 SEDAN DEVILLE-4.9 & 1978 SEDAN DEVILLE-425
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1,673 Posts
Discussion Starter #7

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1926 Model T street rod, 2000 Jaguar XJ8, 1999 Corvette.
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6,737 Posts
The brake light switch above the brake pedal has a vacuum hose on it. It's there to dump vacuum when braking. If that hose is off on either end, it wont work. If the switch is misadjusted, it wont work. It's adjusted by depressing the brake pedal and while depressed, push the switch into its bracket. Then pull up on the brake pedal. You will hear the ratcheting noise of the switch adjusting itself back.
 

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1995 SEDAN DEVILLE-4.9 & 1978 SEDAN DEVILLE-425
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1,673 Posts
Discussion Starter #9
The brake light switch above the brake pedal has a vacuum hose on it. It's there to dump vacuum when braking. If that hose is off on either end, it wont work. If the switch is misadjusted, it wont work. It's adjusted by depressing the brake pedal and while depressed, push the switch into its bracket. Then pull up on the brake pedal. You will hear the ratcheting noise of the switch adjusting itself back.
Thanks for the info---I`ll check that brake pedal vacuum hose too.
I sure hope getting at the red cruise controller unit is not a major job.
I know these old style cruise controls have many vacuum hoses/lines.
 

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1995 SEDAN DEVILLE-4.9 & 1978 SEDAN DEVILLE-425
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1,673 Posts
Discussion Starter #10
I think it was pretty common for the module to go bad, it should be right under the steering column accessable with to screws.
I received "cruise controller module" today.
Is this located at the base of steering wheel under the removable panel under the dash?
 

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1990 Brougham, 1990 Brougham d'elgance 1979 Coupe
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1,196 Posts
I received "cruise controller module" today.
Is this located at the base of steering wheel under the removable panel under the dash?
I think theres just 2 screws and the module is on the cover.

I hope that fixes it, cruise control is a hard problem to diagnoise.
 

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1995 SEDAN DEVILLE-4.9 & 1978 SEDAN DEVILLE-425
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1,673 Posts
Discussion Starter #12
I think theres just 2 screws and the module is on the cover.

I hope that fixes it, cruise control is a hard problem to diagnoise.
You`re right---just 2 screws mount it and module is right on back of cover which came off with 4 screws.
I installed it and nothing changed--so apparently I`ve got a spare controller.
Been going thru the GM service manual from 1977 and the supplement for 1978. Since all the proper lights light up on
cruise dash switch and green light goes out when braked---means fuse and dash switch are ok apparently.
Seems if fuse was bad nothing would light up on dash switch at all.
It just does not engage and points to vacuum related issues.
 

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1995 SEDAN DEVILLE-4.9 & 1978 SEDAN DEVILLE-425
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1,673 Posts
Discussion Starter #13
I had my old school mechanic look at the cruise system today.
He checked all vacuum lines and found 1 that was so soft and would collapse under any pressure---he replaced it but cruise
still won`t engage. I told him I replaced the cruise control module. The servo rubber diaphram and it`s connections are ok.
He`s thinking the cruise servo unit with the solenoids "might" need replacing. He`s not sure of that though.
I`m going to clean the electronic connections on the servo.
 

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1980 Eldorado, 1974 Talisman, 2004 Volvo C70, 1975 Fleetwood 'd Elegance
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7,958 Posts
Don't forget to check the grommets on the valve cover for breather and pcv.
That was where I was losing vacuum on my 87. The rubber boot had split where a metal tube met the valve cover.
I pushed the tube in past the split, and I had cruise.
 

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1995 SEDAN DEVILLE-4.9 & 1978 SEDAN DEVILLE-425
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1,673 Posts
Discussion Starter #15
I brought the cruise back to life today!
Carefully cleaned the 2 electronic connections on the servo unit with contact cleaner.
1 of the connections had some corrosion so I carefully sanded it off with fine grit sandpaper,then I carefully sprayed them with
contact cleaner and reconnected.
Took her out for a highway cruise and was so happy it all worked again!
Feels so good to have fixed it myself.
Thought I`d post the solution here in case anyone else ever has the same problem.
 
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