1974 Eldorado cruise problem, hoping for help/suggestions

[Bryan Thompson]

Good morning,

Recently my perfectly operating cruise control started acting weird.  When I set the speed at anything under 55 to 60 mph, when I lift my foot for it to engage the speed drops about 15 mph then catches, goes up to set speed then slows down again different amounts each time, then speeds up to set speed.  When I set it above 55 or so it still drops about 15 mphs then stays pretty close to set speed.

I have a service manual but, I'm not a service technician and don't have all the knowledge or fancy tools.

Anyone ever experience this, and how did you correct it?  Thanks and have a great day.

[cadillactim]

First, make sure your vacuum servo is not leaking. Easy way to test is unplug vacuum line from it, disconnect chain from carb or servo, then connect engine vacuum to servo. Make sure rubber diaphragm pulls all the way in. Then, pinch off vacuum line an make sure diaphragm stays pulled in. If it releases any, then servo is bad.

If vacuum servo is okay, then problem is in cruise transducer.

Tim

[TJ Hopland]

Sounds like its working electrically so your issue is on the vacuum side.

Start with checking the condition of all the vacuum lines and the servo like Tim mentioned.   If the vacuum source is weak getting to the system it can act like you describe.  Same thing if there is a leak in the system.   A crack in a vacuum line can be a big leak at first but as it applies the vacuum it eventually kinda sucks in and seals the crack and thing go back to normal as long as the bad line is under vacuum.   The source line you just have to visually inspect.   The rest of the lines you can test with a handheld vacuum pump/gauge.  They don't hold vacuum forever but if it will hold a couple minutes its probably fine.   

The 'brains' of the unit are in that gearbox thing on the firewall that the speedometer cables connect to.   There is a connection to the intake which is the vacuum source.  There is then an out that goes to the diaphragm that actually tugs on the throttle.  That line also has a T on it that then has a line that goes to a valve on the brake pedal.  With the pedal at rest the valve is closed so its possible to hold vacuum in the system and operate the throttle.  If you press the brake pedal the valve opens which vents the system and releases the throttle no matter what its trying to do electrically.  Its pretty rare to have a leak in that part of the system but it can happen.

If the vacuum lines are all good then its likely a problem with the unit.   There are a couple things that can possibly be cleaned and checked in the unit but the last few I tangled with had wear issues that were not going to be fixed without new parts. 

[waterzap]

You know what. I just got the same issues with my 78. Worked 100%, now not so much.....

["Cadillac Kid" Greg Surfas 15364]

Bryan,
It is quite possible your tranducer is "gunked" up. I would strongly suggest getting a factory service manual if you might consider taking the transducer apart and cleaning it up which is a simple task WITH the manual. The drives tend to wear and debris builds up. As it does so it takes more force (I.e. higher speed) to function. Believe it or not these transducers are still available at auto supply stores, and with the manuals instructions for final adjustments (,,typically not calibrated at rebuilders) a worn out transducer can be seamlessly replaced.
Greg Surfas

[TJ Hopland]

78 its likely a different system.   I think 77 was the change year but I could be wrong.   If you have the speedometer cable gearbox on the firewall for Eldorado or fender for RWD car its the system we are talking about here.   If you don't have that its the newer more electronic system.   The electronic one had a optical electronic sensor in the speedometer head that sent info to a solid state control module under the dash.  That module then operated solenoids under the hood that controlled the vacuum to the servo unit.    Same basic tests and checks apply for the vacuum part of the system including the line to the brake pedal.  After that its following the electrical troubleshooting for the module. 

[TJ Hopland]

I would still start by checking the basic vacuum side of things.   Fairly easy to do, hard to mess up, and cheap and easy to repair if you do find a problem. 

Here is the gear side.   Its basically a speedometer but unlike a speedometer that seems to last a long time these seem to wear out and it does not matter if you use them or not they always turn (and wear) when the car is moving.  The white lever and black disc are basically the needle. 



This is the 'brains' side of the unit.   There is a sliding valve assembly (the black square on the green rod) attached to that copper arch looking thing.   When you set the cruise a solenoid clamps that copper assembly onto the black disc in the first picture which is like the speedometer needle.  This is now your set point.  If the 'needle' goes up the valve slides one way and vents the servo which lets the throttle off a bit.  If the 'needle' goes down it opens a valve to the vacuum source so it sucks in the servo a bit which increases the throttle.   



Overall its a pretty simple but delicate system of balances.    The problems I have seen on them lately is the speedometer side wears so the clutch like discs don't work like they are supposed to.   Its some kind of magnetic process and when the clearances are not right it does not work smoothly and eventually they actually rub on each other, when that happens the thing barely works.  In this photo you can see the wear and how uneven it is.  That disc is not supposed to touch anything.   There are a lot of plastic and soft metals in this unit and as Greg? said it wears and the wear debris just stays in there and wears out the rest even faster so its possible to go 100,000 with no issues then in say 5,000 go from slight hiccup to not working. 



In the last 2 years I have tangled with 3 of these things.  All had similar wear and after taking them apart and cleaning them they worked slightly better but still not really usable.   All 3 cases tried ordering rebuilt units from various sources and every rebuilt unit had different issues.   One case I went through a total of 5 units before giving up.  I know it was not the cars fault because I was able to swap one from another car and it was fine.   Ended up getting an aftermarket unit for 2 of the 3 cars.   I think the aftermarket unit was almost $300 but it works great.  Much smoother than the vacuum systems especially on hill climbs. 

[Scot Minesinger]

Yes, it is likely the transducer, vacuum actuator or the vacuum hoses.  I would replace any questionable or original vacuum hoses.  The way you replace the one going thru firewall to brake switch is: 

1.  Disconnect vacuum hose at cruise unit under hood.  Get a vacuum hose straight coupling and a length of same size hose that will replace the one you just disconnected.

2.  Attach the old hose to the new with the coupling.

3.  Go inside car and above brake pedal you will see the hose connecting to switch above. 

4.  Disconnect the hose from inside car and pull a little to make sure it is same hose connected with coupling.

5.  When verified pull the hose thru from inside the  car.  Sometime it gets snagged and you have to help it from outside.

This takes five minutes and is the easy way to perfectly replace those daunting thru fire wall vacuum hoses. 

If you replace these hoses, generally the car runs a little smoother at idle and even if trouble turns out to be actuator or transducer, it is a good idea to replace these if it has not been done recently.

[Bryan Thompson]

Thank you all very much, i'll start with checking the vacuum hoses:-)  I'll let y'all know.  Leaving for vacation today so i'll be driving mostly highway where the cruise is ok.  have a good Holiday everyone