Fixed VDO 461 Cruise Control Module
 

I have recently spent a lot of time working on the Tempostat Cruise Control Module for my 80SC in order to be able to understand it and hopefully repair the system. My cruise control would engage but not hold speed. In other words, as long as I held the Set/Accelerate switch up the car would accelerate but once released it would disengage. Researching this site and the internet provided a lot of useful information along with information that was not accurate for the module in my car. The Cruise Control Module in my 80SC has the following stamped on the case: 10.79, 12V, 412.203/1/10, and 928.617.127.00. The circuit board is a VDO 461 module and it does not match with any of the schematics that are available via the internet. Granted, there are several failure modes with suggested solutions so a schematic is not really necessary if you can locate certain parts on the module that are known to cause problems due to age of the components such as the electrolytic capacitors. I did all of the basic tests for the external switches and only had to adjust the clutch switch. I also removed and bench tested the tempostat servo motor with a 12 volt power supply and a vacuum pump. The servo motor checked out under vacuum and the servo solenoid engaged. So the only thing left was the tempostat cruise control module.


This is a picture of the tempostat cruise control 461 module from my car.

http://forums.pelicanparts.com/uploa...1551134652.jpg

It is very noticeable that this module has two Quad Op-amps in the circuit where the available schematics for a similar module has only one Quad Op-amp and one single Op-amp along with a variety of components in different locations on the board.

Since it is a 40 year old module that is no longer manufactured and no detailed tech data available we are left with not many choices for replacement or repair. Granted, there are several places that will for a very reasonable price will test and repair the module. As a DIYer my curiosity got the best of me so I set a goal to create a usable schematic and board layout for the circuit in order to troubleshoot and repair my own board. Yes, it would have been easier to send it off and have it repaired but there is no fun in that.

I used a program called KiCad, it is a free software suite that features an integrated environment for schematic capture and PCB layout. Naturally there was a learning curve with using the program. The schematic capture requires that you manually add the components to a blank schematic and add the point to point connections of each component. Once the schematic is completed then the PC Board layout of parts and wiring ratsnest has to be done.

It should be noted that I am not a design engineer so the schematic may not be laid out in the most efficient manner. Also, the KiCad program verified that I do not have any open ended components and it found no errors. The PC Board layout is not exact since the footprints are generic and not specific to each component. I was only concerned with getting the layout close to what the actual board looked like. I believe that the schematic represents the actual circuit board but I am the only one who reviewed it so their was no peer review. If you do notice an issue please let me know.

I tested all of the components that were removed from the circuit board and only came up with three components that needed replacing. The two electrolytic capacitors C8 and C12 did not charge to the 47Uf capacity. Also the C20 capacitor is a small 68PF capacitor that I thought was iffy so I replaced it. All of the other components transistors, diodes, resistors and relays tested good. The module is now working and will set to the desired speed. It initially will cycle by about 2 miles below the set speed and then level out and maintain speed.

I do have one issue with the zener diode D35 I cannot read the rest of the part number after ZTE, so if anyone can provide that part number it would be appreciated.

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http://forums.pelicanparts.com/uploa...1551155243.png

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http://forums.pelicanparts.com/uploa...1551162382.png

http://forums.pelicanparts.com/uploa...1551162382.png

Damn, seriously impressed! This reminds me of what I used to love about his hobby before the run up in values that changed most of our conversations to ones about values, low miles, $1000 toolkits, et al. This will provide real value for those of us who value keeping all of the little systems on our cars working correctly. Thanks!

I bought my 81 in 92 from the original owner. The cruise control was already disabled - he told me that it had never worked properly. The dealer removed the solenoid and some other parts.
I had ambitions for a while of fixing it - I guess I have abandoned those.

You have done a seriously impressive project.

Great work! Any chance you could photoshop a red arrow pointing to the three components you replaced on the picture of the module. With the small print it's real hard to find the components on your board layout,
especially C20 (found C8 and C12).
I have "fixed" several modules that had your same symptoms by removing the varnish coating on the back of the PC board and reflowing every single solder joint on the board (very tedious job). I took this path after I sent two modules to one of the EBAY fixers, and they came back with absolutely NO components replaced, but the board had had all the solder joints reflowed which "fixed" them and made them both operational.

Impressive to say the least. Bravo. Amazing.

Getting so tired of spades on grills, instagram, patina on leather, hashtag, the right color, safari, oil type, flannel, 245/50/16 tires on the rear, etc, etc.


Hats off to you Sir. Thank you.

Great,

I was doing similar work http://forums.pelicanparts.com/porsche-911-technical-forum/1018643-vdo-cruise-control.html
Yet my problems returned, will have to replace the capacitors.

I will try to get the zte diode reference

Any chance you are willing to share the KiCad files ?

I have 3 cruise modules and all 3 work but not very well. All three modules exhibit the same behavior. They engage properly, for example I set the speed at 50MPH and it sets but then the speed drops off by 5MPH down to 45MPH then it slams the throttle and it picks back up to 50MPH then it repeats this nasty cycle 45-50-45-50-45-... very annoying.

I find it hard to believe that I have 3 modules all broken the same exact way?

What I need is another car to test these 3 modules in.

My car is a 1984 with the vacuum activated cruise. I also bench tested the Vacuum device on the bench with a hand vacuum pump and it does not leak.

Thoughts?

Other than not working at all, your three module malfunctions are probably the most common problems. It seems the comparator/feedback ckt is where the trouble lies.
As I have noted in my previous post in this thread,I have fixed several
modules by reflowing ALL the solder joints on the module. Just a "shotgun" approach I know but worked for me. If I knew the area on the board where the affected ckt was, it would make things much easier.
I guess over the 30 or so years these modules have been around,
the solder connections have in some way deteriorated and kinda act like a "cold" solder joint. Sal, I know you have a great deal of technical
prowess, so why not clean off the varnish on the back of one of your modules and reflow the solder connections. I did mine under a big magnifier and bright light using a very sharp pointed solder iron and
very, very fine gauge solder. Tedious job, but quite rewarding.

I have labeled and put red circles on the two electrolytic caps C8 and C12.

C20 is the 68PF cap that I was unsure if it was good (did not get a good reading on a capacitor tester).

I have also highlighted cap C19. This cap is attached to MOSFET transistor M1. I have read that this cap becomes the standard voltage against which the cars throttle is controlled through the servo amplifier. I did not replace the C19 capacitor because it tested good.

http://forums.pelicanparts.com/uploa...1551236840.jpg

Frank,

My only goal was to create a schematic for the sole purpose of troubleshooting my module. In order to create that schematic I had to reverse engineer the physical circuit board using the KiCad program and it took many hours over many days to complete. The KiCad program has the capabilities to create the schematic and the design template for the actual PC board that can be manufactured. With additional work on my files they could be used to create a new PC board so I will not be willing to share the actual KiCad files. I can convert them into a PDF file that would be usable as a schematic for troubleshooting and would be willing to email that to you.

Have you removed and tested the capacitors I have highlighted on the board in another post. Electrolytic capacitors C8 and C12 failed on a capacitor test for my module and I believe they were the reason why my module would not stay engaged. As noted earlier, C19 is attached to MOSFET transistor M1. I have read that this cap becomes the standard voltage against which the cars throttle is controlled through the servo amplifier.

Yes, it's as simple as that, unless you happen to "bridge" a circuit trace! The capacitors rarely fail, unless over-voltaged via
an alternator/charger as their rating is only 16 volts for the electrolytic ones (C8/C12).

Yes, it is as easy as that when you are precise about your work as I'm sure you are! I've been there and done it several times. You just need to be focused. You're not the only one on this board that is competent enough to repair electronic items.

Targa80,

a well readable PDF would be fine! I am just trying to fix my CC module as well (see post here ), had it working for 5 days, now it's back to 'not engaging'.

I have been trying to do the same thing (I have 2 faulty modules) for my '85 Carrera, not trying to produce copies. After a year of working on the car to put everything back to original and in 'new' working condition, this is only one of the two problems I am left with (the other one is one headlamp washer not working) and I am sticking to doing all the work myself.

BTW, I will open my spare box this evening to see if I can find out the label on the ZTE diode...

Nice, didn't think about that voltage limitation ! And I know my car had an alternator failure in its earlier life resulting in a cooked battery. I replaced 'C19' earlier but that didn't fix everything.

Targa80,

As you can see in my earlier post ( here ) I am trying to fix my board myself. I am putting my car (85 Carrera) back to full original and after one year of working I have 2 'faults' left : the VDO cruise control not engaging 'on some days' and one headlight washer not working. Trying to do all the work myself, including the electronics.

So a PDF with somewhat higher resolution (readable numbers) would be just fine for me !

I will open my CC box again tonight to get the label on the ZTE diode that you are missing, with some luck it is readable on mine.

Frank

Hello,
In one of my previous posts, I replaced C8 and C12. The unit was dead as a doornail, when I replaced C8 and C12 it worked like a charm. Recently the unit still works but the speed slowly bleeds off. It makes sense that C19, being the reference for voltage, being old and possibly not working as well as it should, could be my speed bleed. Gonna bust out the Cruise Control brain, fire up my new Hakko FX888D-23BY soldering station, and start melting stuff.
Any suggestions as to types of caps that might be a more modern upgrade? I could just match the old one, but how much fun can that be....

P.S. Paper on lifespan:
https://www.illinoiscapacitor.com/pdf/Papers/reliability_of_capacitors_general.pdf

I have stripped the varnish and reflowed solder on 2 of the modules, same issue.
I also replaced those caps and a few others on one of the modules same issue.

I need to find more time for testing but I think I need to find another donor test car to be sure I'm not chasing my tail with modules that are working and maybe the issue is elsewhere?

I also notice that the problem with the 5MPH up down cycle seems to go away some if you pick a taller gear but I think that just masks the issue.

[QUOTE=scarceller;103705.....

I need to find more time for testing but I think I need to find another donor test car to be sure I'm not chasing my tail with modules that are working and maybe the issue is .[/QUOTE]

Chances are that the tempo stat servo motor is not a problem but it should be tested to eliminate it as a possible fault. Have you bench tested the tempo stat servo motor? You would need a 12 volt power source and a vacuum source such as a vacuum brake bleeding tool. I was able to verify my servo motor worked it held a vacuum once the servo motor solenoid was engaged. You can also test it with just the power source. You would need to cap the vacuum line and press down on the center of the rubber seal/cable connector. The seal should hold its position until you remove power front the solenoid.

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.