cruise control

[rbp911]

My 76 Eldorado cruise control does not work in spite of my mechanic's best efforts.  We have replaced the transducer, checked the vacuum hose for leaks, checked the vacuum break switch, brake light switch and every other component bur can't figure out why the thing won't work.  Anybody out there with any ideas?
Ron Picchi

[The Tassie Devil(le)]

I had a similar problem with my '72 Eldo.

I checked everything, switches, and vacuum break and replaced all the vacuum lines, and even pulled the module to pieces and checked that. (working from the Shop Manual)

Not finding any reason for the problem, now it is working, but I still have an intermittent problem in as much as suddenly decided to accelerate above the set speed the last time I used it.    Have to look into that.

Bruce.

[rbp911]

What module are you referring to?

[TJ Hopland]

Have you tried holding the set button for an extended period of time?    My 73 for some reason takes like 5 seconds to 'set'.   Don't know why it just does. 

Make sure the body of the gearbox dodad on the firewall is well grounded.   Its supposed to ground through the mount.  Some years had rubber mounts in the bracket that had a thin ground strap to maintain the connection.  I have seen where someone over tightened the screw and broke the strap.    Corrosion and or fresh paint can also keep them from grounding properly.    Add a ground wire for testing just to make sure.

Electrical testing is pretty easy since you can do that with test lights or a voltmeter so I would confirm all that first.   The minimum speed 'sensor' is inside the gearbox unit so you can test that all the electrical functions are getting to the unit.   The servo and and brake release you can test with a hand vacuum pump.   If you eliminate all the external stuff you are just left with the gearbox thing being the problem.   

I tried to get one that style working a few years ago and found out my gearbox thing was just shot (they are not that well made).   I tried 3 'rebuilds' from one parts vendor, all 3 had different issues.    Found a local shop that specialized in cruise controls and such that had a NOS unit which would not maintain a steady speed.   The shop took the NOS unit back and exchanged it for a modern aftermarket unit.   Works great, much smoother than I remember the original ones being even when they were new.    The main unit is a black box about 4 x 4 x 8 inches and I would say and its got about a 3' long throttle cable so you could mount it somewhere that does not really stand out.  Rostra is the brand name.  Summit sells them among others. 

[Jeff Rosansky CLC #28373]

Dont forget the wire in the steering column. Be sure to test it in the position it is in when driving.
Could be a bad spot that doesnt show up until you drop the wheel.
Jeff

[The Tassie Devil(le)]

What module are you referring to? 

The Module I am referring to is the one on the firewall.   The one that the Speedo Cable enters and departs.

The activating switch is pretty easy to test, as all you need to do is lay down on the floor, if you haven't dropped the under-dash cover, and with a multi meter, perform the test as per the Shop Manual.

Bruce.

[rbp911]

Thank you guys.  My week end is all planned now.
Ron

[Scot Minesinger]

You did not mention if you checked the vacuum actuator.  Sometime it is ruptured and will not pull the chain to activate throttle.  That was the problem on one Eldorado I worked on.  Then on anther it was a kinked vacuum line (replaced with too short of one by others before).  Finally and unfortunately the wiring harnesses used in the 75 and 76 Cadillacs (maybe this was all GM during those two years) are of the worst quality I have seen on any car ever.  You will need to check this over carefully, often the spades do not crimp well and the electrical connections back out of the plastic black keepers that plug together.

["Cadillac Kid" Greg Surfas 15364]

Ron,
You did not say exactly how it "does not work". If the two switches (brake vacuum and brake electrical) are good and adjusted correctly and you have a functioning transducer I might suggest you check the little three contact connection where the turn signal stalk switch connects to the harness.  This will be right under the dash and should be clearly visable.  I have found a poor connection there several times myself.
Greg Surfas

[impalamansgarage]

On the same topic, does anyone have experience with aftermarket cruise control systems? Or have a recommendation for a particular brand or model? My 71 did not come with cruise control. Might be nice to add it. Just a thought.

[TJ Hopland]

I have an aftermarket one on my semi daily driver, an 81 diesel Riviera.  Works great.  I have also installed them on a few on other cars that either didn't have em or like mine didn't work right.    Have one on the shelf now ready to go on one of my dad's cars.   On his there is a small ashtray I am going to mount the control switches in.   Pull it out its a convenient location, push it in all looks stock.   I think I will mount the main box under the battery tray.   It will be out of sight and there are other wires there so your eye won't be drawn to wires all by themselves.   

As far as I know Rostra makes em all these days and sells them under various names including their own.   You buy the universal kit, 250-1223, the speedo pickup 250-4160, then the type of switch you want.    The speedo pickup has an adapter that lets it attach to the side of the transmission then the existing cable attaches to it.   You can use pretty much anything you want for switches, you don't have to use theirs.  If you like theirs its just a simple plug in.  If you don't like theirs they give you a few options for normally closed or normally open so you can build something yourself or re purpose existing switches.  Minimum connections are 2 brake wires, on off, set, and power.   There are other wires for other options like gear shift position, tach (rev limit), and clutch pedal but you don't have to use them.   

Install is not too bad.  Under the hood is pretty simple but as always working under the dash is a pain.  You have to attach a couple wires to the brake switch.   On this era GM what I have been doing there is popping the terminals out of the original plug then soldering the kit wires to the factory connectors then back into the plug.  It should be a good connection plus if I ever wanted to remove them I could de solder the wires and shove em back in the plug and no one will know.    You can download the instructions from the site.  Summit racing carries and stocks em. 

http://www.rostra.com/universal-aftermarket-cruise-control-by-rostra.php#2504160

[bcroe]

Just about any electronic cruise control would be superior to those mechanical
ones up until the 80s.  Those speedo cable boxes were very short lived &
troublesome, and made a mess under the hood.  The one on my 79 Eldo is
good, an electronic pickup built into the speedo, and a vacuum throttle
activator.  I guess the electric activator type has the advantage of
not requiring a vacuum release on the brake pedal.  Bruce Roe

[TJ Hopland]

One thing I forgot to mention is if you had cruise to start with (at least the 70's and 80's style) the new speed sensor will screw directly in place of the old unit.   Only catch is the new one is 'inline' vs the original where both cables came off the same end.   This difference requires you to re position things so the cables can connect.   There would be a chance you may need a longer speedo cable from the trans to the sensor to get things positioned so the bends in the cable are not too tight.     Cables are easy, go to a parts store with an actual book and someone that knows how to find and read them.   First page has pics of the cable ends.  You figure out that you need say a type B on one end then a type K on the other.   Say you think 30" would be about right you look at the catalog and see there is a 27" and a 32" so you get the 32. 

If you didn't have cruse to start with the sensor just screws to the side of the trans and the existing cable screws to the sensor.

Someone emailed me asking about the brake switch.  You don't need to add one or change it or one from a car with cruise.   OE systems did have a special switch that had a second set of contacts but this system just connects to the existing switch right on top of the existing wires. 

[cadillacmike68]

You did not mention if you checked the vacuum actuator.  Sometime it is ruptured and will not pull the chain to activate throttle.  That was the problem on one Eldorado I worked on.  Then on anther it was a kinked vacuum line (replaced with too short of one by others before).  Finally and unfortunately the wiring harnesses used in the 75 and 76 Cadillacs (maybe this was all GM during those two years) are of the worst quality I have seen on any car ever.  You will need to check this over carefully, often the spades do not crimp well and the electrical connections back out of the plastic black keepers that plug together.

My 1968 Cruise control has two solid metal rods to link to  the throttle! I replaced a broken control cable and wa-la it started working just fine. Sometimes it wobbles a bit, probably grit in one of the speedo cables, but just check every item to ensure it tests. Sometimes just removing and replacing a wire scrapes off oxidation and the items works once more.