I did test the servo motor as described. I applied vacuum with a hand pump then gave it 12v and the cable pulled all the way in and held for several minutes. I think the Servo is fine.

You just need to be able to bench test using a 12V PS, a square wave generator, and a 10-20 ohm load resistor
to simulate the actuator. Using a scope, you can monitor how the duty cycle of the actuator signal varies with
a frequency change of the speed signal.

Hey,
Being a diagnostician at heart, failure modes will dictate the problem area for replacement. It seems there are multiple possible failure modes. In reading a bunch of posts from 2008 there are several types of failures, capacitor, and a whole bunch of other magic related parts. I am willing to bet there are date ranges for certain types of failures. While flying transport category airliners, I had a series of hydraulic pump failures over a very short time period. Talking to the mechanics, they said they were overhauled by a company that used a series of bad seals. Component failure, or poor assembly methods will exist until someone corrects them. In 25 years of flying this series of hydraulic pump failures were my only hydraulic pump issues. Try a cruise control brain from a different, but applicable year.

The below picture is of electrolytic capacitors C8 on the bottom and C12 on the top. They are both 47UF 16 Volt. It was easy to diagnose that C8 blew its top sometime in the past 40 years.

http://forums.pelicanparts.com/uploa...1551301676.jpg

Interesting that they consider the calendar life to be 15 years +/-.

Aluminum Electrolytic Capacitors
Reliability
MTBF (Mean time between failures) is the most commonly used reliability rating used today. Aluminum electrolytic capacitors do not failure in a manner where MTBF can be used accurately. Instead of using MTBF we use load life rating.
In aluminum electrolytic capacitors as they are used the capacitors slowly degrade over time and once a capacitor has degraded beyond a specified amount the capacitor is considered to be a failure. Most capacitors are considered a failure when the capacitance has changed by 20 to 25% of its initial value.
Aluminum electrolytic Capacitors load life’s ratings are generally expressed between 1000 and 10000 hours at there rated voltage and temperature rating. This means that the capacitance of the capacitor will not change by more than the amount indicated under the load life rating when the capacitor is operated at its rated voltage and maximum temperature rating. Although the life expectancies appear be a short amount of time the following can increase them.
When the capacitor is operated at temperatures other than the maximum rated temperature for the capacitor the expected life of the capacitor will increase. The rate of increase in expected operating life is for the life to double for every 10°C decrease in temperature.
The above is expressed mathematically as:
L2=L1*(Vr/Vo)*2x
Where L2= life expected at ambient temperature. L1= Load life rating of the capacitor. Vr= Rated voltage of the capacitor.
Vo= applied voltage
X=(Tm-Ta-Tr)/10
Tm= maximum rated temperature of the capacitor. Ta= ambient temperature.
Tr= temperature rise due to ripple current
Reducing the amount of voltage applied to the capacitor can also increase the expected life of the capacitor. The expected life can be increased by the ratio of applied voltage to rated voltage for the capacitor. The expected life by voltage derating is limited to a 2:1 ratio even if the ratio is more than 50% of the rated voltage of the capacitor.
3757 W. Touhy Ave., Lincolnwood, Il 60712 ● (847)675-1760● Fax (847) 675-2850 ● www.illcap.com
By de-rating, the life expectancies beyond 15 years can be achieved. When life expectancies exceed 15 years the expected life of the capacitor should be limited to 15 years mainly due to the sealing materials will deteriorate over time.
The life of the capacitor can be reduced if the amount of ripple current becomes excessive causing the capacitor heat up from its ESR. Care should be taken to limit the temperature rise (Tr) due to the ripple current and ESR to a maximum of 10°C above the ambient temperature the capacitor is operating in.
It should be evident that a 10°C temperature rise due to the ESR will reduce the expected life is half. Other factors that can reduce the expected life of a capacitor are:
• High leakage current.
• Frequent charge and discharge cycles.
• Excessive reverse voltage.
• Application of voltages greater than the rated voltage of the capacitor.
• AC voltage exceeding the limits of the capacitor.
• Operation at temperatures exceeding the maximum temperature rating for the capacitor.
3757 W. Touhy Ave., Lincolnwood, Il 60712 ● (847)675-1760● Fax (847) 675-2850 ● www.illcap.com

Sal,
Check the wiring connectors that are located on the speedometer. Mine were loose and did not make a good connection causing wacky speed oscillations, as well as intermittent capture problems, the unit would accelerate but not hold any speed. That happened during “the good years” on my last repair. Some minor crimping on the connectors removed that wackyness.
I think the imprecision in the system comes from the electromechanical/vacuum interface. Speed changes are relatively instantaneous, and vacuum and engine speed are not. Which I would gather they changed to the electronic type system.

It's a JFET (n-channel junction FET).

Actually, the key problem with the vacuum actuator system is that it's an analog system design versus a digital system design as used
in the later Porsches. The analog design stored the desired speed as a capacitor's voltage. The later digital design stores the speed
as a digital value in a register. The latest Porsches starting with 996s don't use a separate module, but have the cruise control function
integrated with the electronic throttle body module.

Thanks for the correction on the type of FET transistor. It has been a long time since I have dealt with the highly sensitive to static electricity components. In my younger days (40 years ago) they were mostly MOSFET transistors if my aging memory is correct. They need to be handled with care or you can damage them by your body's static electricity.

Actually, JFETs came before MOSFETs. JFETs are infrequently used in designs versus MOSFETs today.

Capacitors are usually the weak link in all these old circuits....including old Stereo's etc. Macintosh AMPs etc. everything. My Cruise C is on the shelf, removed it during a weight saving program years ago.. Wonder if it works? Seems like I could utilize that diagram if I desired, but for now, WOW, I am just really impressed.

Yesterday I took the car for a ride and hooked up a picoscope to my laptop via usb and was able to capture the following images.

The scope probe was connected to J1-11 (+A Speedometer input) and the scope ground was connected to chassis ground. The car was moving at 35~40 MPH.

http://forums.pelicanparts.com/uploa...1551550783.jpg

The scope probe was connected to J1-7(+Pilot Valve Current Loop Output) and the scope ground was connected to chassis ground. Car was moving at ~40 MPH

http://forums.pelicanparts.com/uploa...1551550783.jpg

Now, very slowly vary the speed input signal (+/-) and see what happens to the signal to the actuator (simulate with a 10 ohm resistor),
when bench testing.

Great, I was planning on doing that myself but don't have a digital scope. Now I know what the voltage looks like, it seems I can quickly build a 'simulator' for that input from an arduino. The X-axis is in milliseconds right ?

Frank,

In the lower left corner of each screen shot it states MS for the time horizontal (X) sample. I believe a variation in speed will either increase or decrease the time interval between pulses. I.e. slower speed wider time interval, faster speed shorter time interval between pulses. I do not believe that the pulse width varies with speed. At least I did not notice any significant change in the pulse width. Also the voltage of the pulse was constant. This analogy is from my memory of the testing I did over a month ago.

This is a great cruise control thread! It just kind of stopped. Did some of the posters resolve their issues? The easiest path is to send the module off to a couple of known reputable Pelicans to repair, but it is fun to figure this stuff out and fix on your own. Part of the hobby.

I’m chasing a similar CC behavior that the OP had. I opened up my box and found C8 and C12 capacitors have slight bulges. I don’t have an instrument to test the capacitors, but I know from years ago that a bulging capacitor could indicate failure.

http://forums.pelicanparts.com/uploa...1597836636.jpg

Just replace each of the electrolytic caps since they are the week link in the circuit especially if they are popped like in the picture I posted previously. Granted there may be other issues with your module but the bad caps will cause the unit to function poorly (not hold speed) or not function at all.

Thanks! Then more i look at the circuit board the more I see. Looks like there may be a few other misshapen components.

During my time creating the schematic I removed all the components and tested what I could. So in essence I reflowed the circuit board when I reinstalled all of the good parts and the new capacitors. If the caps don't fix the issue then you could reflow the solder joints. Also do a magnified inspection of the solder runs to make sure you do not have any breaks.

You are a brave and smart man!! I’m guessing you have an engineering background.