70's cruise control issues / reapir and how they work

[TJ Hopland]

This pertains to the style with the gear box assembly that goes inline with the speedo cable and is typically mounted on the fender.   When I got the car light worked but nothing else.   Replaced and tested vacuum stuff and it would sometimes seem like it was trying to work.  Found some info on them on a Corvette forum and took the cover off where the gears are and did some cleaning and now it works decent at some speeds like around 55 which is where the manual says to test it.  Below 55 (like 30) it seems to want to go faster, not sure how fast I never let it keep accelerating to see where it goes, I let it get to 50 one time so at least that far.  It will set over 55 so its not like the set point is completely stuck.  Anyone ever see what the bottom area is like?  I am assuming that is where what sets the set point has to be.   There does not appear to be an easy way to access this area and I have never seen a diagram of what's down there so I hate to try and open one and destroy it in the process.   Just seems to me that there has to be something sticking that could likely be cleaned and freed up.

Even after taking the thing partially apart I'm still not entirely sure how the thing works.  If I had never seen one work I would say its not possible.   The basic operation seems to be some sort of a magnetic coupling that has a friction drum that acts on a hoop that appears to be attached to valving that acts on the vacuum levels.   There is a solenoid in the bottom of the assembly that must have to do with how the set point is set.   

Any thoughts or experiences?   

[cadillactim]

I've rebuilt many of these units.  These cruise units give lots of problems with age. 

The solenoid on the bottom causes the loop to "grab" the rubber barrel, and also allows engine vacuum into the unit.  The spinning magnet causes the barrel to rotate.  The faster the speed, the more the barrel rotates.  Once the loop grabs the barrel, that's the set point.  As the car slows down, more vacuum is sent to the servo to increase the throttle.  And when the speed is too fast, less vacuum is sent to the servo, thus closing the throttle.

Vacuum is regulated by the loop that is connected to a sliding sleeve that mixes engine vacuum and air.  This controls how much the throttle is opened or closed. 

Several of the components go bad or get dirty.  Sometimes the magnet deteriorates, and you will see lots of black powder inside the unit.  The barrel will slide on the pin it is mounted to, so it won't rotate properly.  The sliding sleeve gets worn, and not enough vacuum can get to the servo.

The ground circuit for the dash light that let's you know the cruise is engage gets corroded and the light will not work.

There is no easy answer to what is wrong with yours, with so many things that will cause the unit to not work.

Tim

[TJ Hopland]

So it sounds like with the gear cover off I am seeing pretty much everything.   I suppose its then a grip (or lack of grip) issue or maybe that the magnetic coupling part is not as accurate as it used to be.   There was black powder in this unit.

So in later years when they added the resume feature (non Cadillac since Cad went electronic) the 'clamp' stays clamped and they just added an external solenoid to just cut the vac off to the system but as far as the rest of the system knows its still doing its thing, just without vacuum?  You hit resume vac back to system and it all works?

[cadillactim]

If you see black powder, then the magnet is deteriorating.  You can try cleaning it out.  If you look on the cover where the magnet is, you'll see a 3/16 bolt holding a spring.  Remove the bolt, then remove the metal plate and spring.  Then you can examine the magnet better and clean the black powder from the metal plate.

Tim

[TJ Hopland]

I had that area apart.   There was a lot of crud between the plates.  I got the crud cleaned off but there are some grooves into both plates.   I assume there was supposed to be a bearing in the center of the gear side plate?   It kinda rolled out when I took it apart and that was the only place it seemed like it could have gone.   The shaft /pin that goes in that hole on the gear side was way full of crud and seems to have funny wear on it.   I have read that those grooves are not necessarily a problem but there were no pictures so its hard to gauge.    I will go grab the unit again and snap some photos so others can see what is inside these contraptions. 

So you rebuilt these in the past when parts were still available?  Or are they still available?  Or do you just clean up the ones that are not too worn?  If you still work on them do you have a typical cost?

How do those plates wear?  Seems like there should not be any contact.  Or is that the problem there is a narrow gap there and since its magnetic any metallic stuff that gets in there it sticks and messes things up?

Last question for now:  Does sitting effect these things?   Say sitting under the hood of a stored car?  Or a 'rebuilt' unit sitting on a parts store shelf?   Just wondering what the likely hood of getting a good replacement these days would be.

[TJ Hopland]

It looks like contamination is still among my issues with this one.   Its only got a couple hundred miles on it since i had it apart and cleaned it all up and it looks about the same now as it did when I first had it apart last week. 

[bcroe]

Those things are designed to wear themselves out.  The diesel & gas units are a little
different, as I found out converting a diesel to gas.  At the end of the 70s they started
putting that 2-pulse-per-revolution pickup in the speedo head, which drives an electronic
cruise control module in my 79 Eldo.  That couples to the usual vacuum driven throttle
activator.  Probably a lot better design, if you can retro fit a newer head into your speedo. 

The module from my 79 is available.  Bruce Roe

[mgbeda]

One more thing that goes wrong with them is that the insulation of the wires in the coil for the solenoid that clamps onto the drum breaks down.  This makes the solenoid weaker and you get end up with cruise that works sometimes (or not at all).  A classic symptom of this is when the cruise works until you hit a bump and then it drops out.  A solution I used back in the late 80's was to rewind the coil with coil wire from radio shack.  I had a lot of time in those days.

I have an original 45,000 mile Buick that has the same kind of cruise control unit, and it still works.  So I tend to think they mainly wear out from use, not sitting.

-mB

[TJ Hopland]

Spent a little more time playing with the thing and I think I found what wore out and is causing it not to work properly.   The shaft you see in the first photo above fits into a hole in the main body in the 4th photo.  On mine this hole has worn oblong.  The other end of this shaft is a small pin you see in the center of the copper colored disc in the second photo.  This shaft assembly does not spin, it just rotates slightly depending on the speed.  Its magnetically coupled to one of the gears that the speedo cables drive.  That pin fits in a recess in the gear magnet thing that has a single ball bearing.   Where I see the weakness in the design is that pin in the recess against the ball.  The bearing acts as a thrust surface which is fine but it does not give any support other than for thrust.  Radial support has to come from the other end which appears to be some sort of bakelite material.   My guess is this causes that surface to wear and lets that whole assembly wobble.  Eventually it gets like mine and the two plates actually touch and since the one plate is rotating and the other does when they hit it jolts the system.  To work well its got to be smooth and obviously if the plates are hitting from time to time its not going to be smooth.  If these were more rare and I had much smaller machine tools I would try and make bushings but these units still seem to be 'around'. 

If it was not for that wear I think these things would be fairly easy to 'tune up'.   The rubber wheel was slightly glazed on mine but a little scuffing with a worn out wire brush on a dremel tool got it nice and tacky again.  The rest was just cleaning.   

I did find one thing that would die due to age at least on this one, the air filter at the bottom.  This one was foam rubber.  Not sure if they all used this type or when this type started being used but as we have come to expect from anything that was made of foam rubber in the 70's and 80's it just rots.   Headliner anyone?  Hi-Fi speaker woofers anyone?  Most of the crap I found inside the unit appeared to be from this filter.   Any disturbance the stuff just crumbles.   If the rest of the unit was fine I dont think it would have been too difficult to find some newer filter material.  I was thinking of perhaps the cotton like stuff you often find in air compressors.   Chunks of this filter could end up plugging up passages in the unit and will eventually end up being drawn into the engine.  They will be fairly small soft pieces so I would not be concerned about damage to the engine, just plugging up of the passages.  The deteriorating foam also seemed to corrode the electrical connection. 


   

[TJ Hopland]

I also think I got it figured out how the thing works.   If anyone knows better than me and I got part of it wrong feel free to correct me.



This is the end with the speedo gears on it.   The magnetic coupling converts the constant rotation of the speedo gears to just a slight rotation of all the stuff you see sticking out here.  The faster the gears are going the more this assembly rotates.  The white part is fixed to the shaft.  This operates the switch contacts you see in the next photo.  This is is what disables the system under approx 30 mph.  If yours won't engage at all and you have power going to the unit it could be these contacts are bad or damaged.  This is also something you have to be careful of when re assembling so you don't damage it.   The black drum thing is will also be fixed on the shaft if its a non resume model or is spring loaded on a model with resume (like this one). 



When you engage the system by hitting set the solenoid in the bottom pops up assuming you are going at least 30 and the contacts are closed.   This does 2 things, 1 is it moves a valve from blocking the vacuum inlet to blocking a port on the servo side. In rest mode the servo side is open to air so the servo can't operate.  When the solenoid engages it puts vacuum in the system and blocks that servo port.  The servo port is also externally plumbed to the bleed port which is the one on the top near the electrical connectors.  This is also all plumbed into the servo and the brake pedal release.  The second thing the solenoid does is causes that copper loop to grab the rubber drum.  That loop is attached to that black square thing which is a slide valve on that green tube which is what they call the bleed valve.  There are ports on the end of it so depending on where that slide valve is positioned how much vacuum is bleed out of the system.  It appears that when the system is on its always getting vacuum so its adjusted by bleeding some off.   The set point comes from the position that black drum got grabbed.  My guess its that magnetic coupling is very similar to how the speedometer works so if you imagine just attaching a valve to the speedometer needle that is basically how it works.  You change the set point by grabbing the needle at different positions.   On the later models when the added resume to these systems (Cadillac went electronic in 77 or 78) the added an external vacuum solenoid that cuts vacuum to the system when you hit the brake.  Hitting the brake also bleeds the vac from the system but the solenoid still is clamped on the drum so its still holding the set point.  When you hit resume the external solenoid lets vac back into the system and you are running again.   This is apparently why the resume ones have the drum spring loaded so the car can possibly be stopped and still hold the set point. 

[cadillactim]

Your description of how the system works is very similar to what I wrote you earlier, just more complex.

I would not call it magnetic coupling, but more magnetic deflection.  The spinning magnet is deflecting the spring loaded rubber barrel drum.  The drum gets coupled to the loop when the solenoid is energized.

The bleed valve is very sensitive and has to be calibrated to the speedometer, thus the lock nut on the vacuum hose nipple.

As the car slows down and speeds up, the coupled drum causes the loop to pivot at the bottom (either to the right or to the left).  As the loop pivots, the square shaped sleeve (aluminum on some), slides on the bleed valve causing more or less air to be mixed with the vacuum.  This in turn increases or decreases throttle.

On level road, the sleeve gets stabilized, having a "perfect mix" of vacuum and air maintain the set speed.

Concerning replacing the foam in the bottom of the unit, I use sponge weatherstripping to seal window gaps.  Simple solution available at your hardware store.

Tim

[TJ Hopland]

I finally got a rebuilt unit.  Box was not dusty so that is a good sign.   It seems to have the same problem as my old one did.   It will set but does not really seem to care where I set it, most of the time it wants to go faster.  Occasionally it will seem like its working at higher speeds but after a bit it will accelerate, especially if I try to set it lower than about 65.   After I 'fixed' the old one that is pretty much what it did.

Can anyone see that this could be the car's fault?   Electrically it seems to be working.  I would think if it was a vacuum issue it would tend to no hold speed or oscillate which is not the issue.  I'm wondering if I got a bad rebuild?  Or one that has been sitting and has hard grease in it or something like that.   I was going to try driving it again today and see if maybe it will loosen up before I try sending it back. 

[mgbeda]

Perhaps Cadillac Tim had the answer:

"The bleed valve is very sensitive and has to be calibrated to the speedometer, thus the lock nut on the vacuum hose nipple."

-mB

[TJ Hopland]

This one is off by more than a few mph.  It seems to be going to full throttle most of the time.

I just sent the rebuilt one back and they are going to send me another rebuilt one.