: '78 Deville cruise control problem.



pompste
06-02-10, 01:12 AM
I found a replacement in dash cruise control switch--thinking the original was the problem.I installed the replacement switch and still have the same problem.Both switches lit up--amber light at top and green light when i pressed the stalk button to the first detent position-----but the cruise does`nt work at all.
It worked when i bought the ride in January---sometimes,more often than not.
I looked underhood at the main unit attached to the valve cover---all lines are attached,i cleaned all electrical connections,and the rubber diaphram looks good---still pliable and no visible holes/leaks.

I`m stumped at this point.

Any suggestions for a solution will be appeciated.

77CDV
06-02-10, 02:53 AM
If both lights are working showing an engaged system, then there must be a leak or disconnect in one of the vaccum lines. The only other option is that the system is disconnected from the throttle.

pompste
06-02-10, 03:25 AM
If both lights are working showing an engaged system, then there must be a leak or disconnect in one of the vaccum lines. The only other option is that the system is disconnected from the throttle.

All the vaccum lines look good as far as i can see in there--looks like some lead to the firewall area way down under.I wish the hoods on these models opened up higher than they do so i could look in there closer/deeper.
How could you tell if the system is disconnected from the throttle?

77CDV
06-02-10, 03:36 AM
It's connected with what looks like a beaded chain.

gornati
06-02-10, 10:42 AM
All the vaccum lines look good as far as i can see in there--looks like some lead to the firewall area way down under.I wish the hoods on these models opened up higher than they do so i could look in there closer/deeper.
How could you tell if the system is disconnected from the throttle?

check on top of the valve cover (driver side) there is a rubber diaphragm connected with a little chain to the throttle control.

pompste
06-02-10, 02:26 PM
It's connected with what looks like a beaded chain.

Is that the 2"- 3" or so little linkage that connects to the CENTER of the rubber diaphram? If so,it`s connected----i gently pulled on it and there is tension on it and it is connected to the rubber diaphram.I pulled on it and it pulls back.

77CDV
06-02-10, 02:36 PM
The FSM has a huge three-page diagnostic flow chart with procedures for checking all the cruise system components. The upshot is, if all other components are intact and functional, then the electronic controller (the "brain" of the system) is bad and must be replaced.

pompste
06-02-10, 02:51 PM
The FSM has a huge three-page diagnostic flow chart with procedures for checking all the cruise system components. The upshot is, if all other components are intact and functional, then the electronic controller (the "brain" of the system) is bad and must be replaced.

I always thought these model Deville`s were basically all "mechanically" made and free of all the more complex "electronic brain/computer" stuff more common to the '80`s and up models.
Is the "brain" located in the big unit that mounts to the valve cover?

carnut
06-02-10, 08:11 PM
Get under the dash, drivers side and verify the vacuum hose is connected to the brake pedal cruise release switch thats on the brake pedal support. If its split, off, kinked all electrical will work, but the unit wont engage. Thats the hose you say disappears under the firewall. Remove each vacuum hose one at a time where they plug in and look closely for a split , kinked hose. Make sure they go on the nipple tightly. Many years of heat will loosen these hoses.

77CDV
06-02-10, 09:42 PM
There's no computer control (unless you have fuel injection), but there's a huge amount of electronics in these cars. It's what makes a lot of the gadgets, like cruise, climate control, twilight sentinal, Guidematic, etc. work. But, it's all very rudimentary, archaic by modern standards. There's very little in the electronics of these cars that can't be fixed with a solder gun.

jayoldschool
06-02-10, 10:36 PM
Get under the dash, drivers side and verify the vacuum hose is connected to the brake pedal cruise release switch thats on the brake pedal support. If its split, off, kinked all electrical will work, but the unit wont engage. Thats the hose you say disappears under the firewall. Remove each vacuum hose one at a time where they plug in and look closely for a split , kinked hose. Make sure they go on the nipple tightly. Many years of heat will loosen these hoses.

Would this apply to the cruise light solid yellow when on, but the green light only briefly but no cruise?

pompste
06-02-10, 11:50 PM
Get under the dash, drivers side and verify the vacuum hose is connected to the brake pedal cruise release switch thats on the brake pedal support. If its split, off, kinked all electrical will work, but the unit wont engage. Thats the hose you say disappears under the firewall. Remove each vacuum hose one at a time where they plug in and look closely for a split , kinked hose. Make sure they go on the nipple tightly. Many years of heat will loosen these hoses.

Thanks for the detailed instructions! I`ll have a look at the brake pedal connection tomorrow.
I did notice a slightly loose vacuum line that`s attached to the cruise unit on the valve cover.It stays on the nipple somewhat,not split, but it`s not a real snug fit-----i`ll try to get a zip tie on it if there`s room for the width of it.

pompste
06-03-10, 12:04 AM
There's no computer control (unless you have fuel injection), but there's a huge amount of electronics in these cars. It's what makes a lot of the gadgets, like cruise, climate control, twilight sentinal, Guidematic, etc. work. But, it's all very rudimentary, archaic by modern standards. There's very little in the electronics of these cars that can't be fixed with a solder gun.

I have the 4 barrel carb.I like that these vintage Caddy`s just have the simpler electronics that,as you say,can be fixed with a solder gun.I also like the ample room under the hood to actually be able to work on this ride myself!
Even my '95 Deville is a PIA to work under the hood on compared to my '78 Deville.

carnut
06-03-10, 10:06 PM
A zip tie will probably split the hose. Just replace the hose. With that old of a car, taking sample diameters of underhood vac hoses to the parts store and buying 4 or 5 ft of hose is a pro active approach. Very easy replacement, just one hose at a time. When done, they'll go another 30 years.

pompste
06-04-10, 03:32 AM
A zip tie will probably split the hose. Just replace the hose. With that old of a car, taking sample diameters of underhood vac hoses to the parts store and buying 4 or 5 ft of hose is a pro active approach. Very easy replacement, just one hose at a time. When done, they'll go another 30 years.

Actually carnut---the somewhat not so snug hose is in real good shape---looks like it`s fairly new.So i did snug it on the plastic nipple and put the slimmest zip tie i could find on it.Almost directly below this hose is an electrical connection with a very small metal nipple right next to it.It had a white cracked holder that held a very small piece of "foam" directly in front of the very small metal nipple.When i touched the cracked white holder for the foam--it fell off,along with really dirt filled very small piece of foam.The metal nipple is now exposed.
Long story short--i took it out for highway ride and the cruise ACTUALLY WORKS---for now anyway! I will leave well enough alone as long it continues to work.
These older cruise controls are really rudimentary----whatever speed i set it at,it drops 5 mph then it kicks in.
I`m happy for now! I also replaced the blower motor with a new one and a new blower relay that blows out every 3 months or so.

75sled
06-04-10, 08:59 AM
I had the same problem and the diaphragm had a leak internally. The way I discovered mine was by removing the vacuum line, compressing the diaphragm, plugging the vacuum feed with my finger, then letting go of the diaphragm. Mine would decompress immediately. I found one at the JY and did the same test, and it stayed compressed.

77CDV
06-04-10, 11:42 PM
The 5mph drop off after engagement is called "lock-in error". There's a procedure in the FSM to adjust the system to eliminate it.