Inside the box: VDO Cruise Control Actuator

[JamesDean]

Hey everyone, forum member shertex was kind enough to send me a cruise control actuator. He mentioned that the unit had taken out one of his cruise control amplifiers. So I figured I'd test it out and crack it open see what all the fuss what about.

The unit tested very similar to several of my actuator. Testing current flow through pin 4 when pin 4/5 are connected to power yielded 190mA, flipping the polarity yielded 200mA.

Applying power to pin 7, ground to pin 6 to activate the solenoid yielded about 290-300mA

The transistors that supply power to these above pins are BD437/BD438 BJT transistors. They are rated at 4 amps.

The transistors are protected via a set of zener diodes and rectifier. I've not yet mapped out the full protection bit so I'm not 100% on it yet. In the event of a high current draw or inductive kickback (voltage spike) I believe they are supposed to prevent the transistors from being blown. Its possible that due to the technology available at the time VDO might not have used the most optimal components for the task. I'll look more into this avenue later.

The actuator itself is fairly simple. A motor drives a stacked gear that spins freely until the solenoid is engaged. When the solenoid engages, it tilts the stacked gear into the large gear. This large gear then contacts the partial gear which is directly connected to the actuators external arm.

On the bottom of this partial gear are two dots, these dots contact a strip on the PCB which create make a potentiometer. The potentiometer tells the cruise amplifier the position of the actuator level.

Here are some pictures:






Here's a brief video of the motor spinning and solenoid engaging (click to play):

[Bio300TDTdriver]

Very interesting. So even though the actuator was in spec according to what you and GDL say, it still took out the amp. Have you figured out why?

[JamesDean]

Originally Posted by Bio300TDTdriver

Very interesting. So even though the actuator was in spec according to what you and GDL say, it still took out the amp. Have you figured out why?

GDL specifies between 120-180mA of current draw. This unit was slightly above that by about 20mA at the highest. Out of spec a little but nothing huge.

I think I'm going to tear into it further and explore more. There were more specifications on the GDL's website such as various resistances of motor brushes, and potentiometer.

If I had to speculate as to why it caused the failure, either some internal is intermittent and I've not discovered it yet or something caused a huge current or voltage spike and the protection bits in the amplifier we not adequate to prevent damage. Perhaps stuck linkage, and a bad potentiometer trace caused the unit to not 'know' where it was and it tried to move against a physical stop area.

More exploration is required for certain.

[JamesDean]

I removed the motor cover. The brushes had wear on them, not sure exactly how much life is/was left in them.

[JamesDean]

Here is a brief write up on how to properly test current draw on your actuator. You want to monitor current when pins 4 and 5 are connected to power. Ideally you should see between 120-180mA or .12-.18. I've see units with .2-.22 that still function normally. If you're up near 250mA or .25 I'd say it's time to consider changing up the actuator.

When you test current you want to place you multimeter in series with the object in test. That means to put the meter in the path of current flow. When you check voltages you put the meter in parallel with the object in test.




1) You'll need access to a power supply of 12-13.5V. Some spare wire and you car's battery will do. I use an old computer power supply.

2) You'll need a meter that will measure DC current. Most cheap meters will do this. For this demonstration I'll use a cheap Cen-Tech meter I bought on Amazon. Here's the one I've got, also available from Harbor Freight Cenn-Tech 90899 I also have a more expensive Triplett 9045 unit.

3) Next you'll want to configure the meter properly. Most meters will have a 10A position somewhere on it. This means it can handle up to 10 Amps of current. You'll want to use this as most of these meters are fused to a low current. This also will avoid confusion trying to interpret the results on the display. The display will show in amps. In addition to setting the dial to 10A you'll need to put the red lead in the 10A position. See photo below.

4) You'll want to wire a solid connection from either side of the battery or power supply. For this demonstration I chose to connect the 12V supply to the actuator's pin 4.

5) Next you'll place the meter in the path of the other wire as shown in the picture above.

6) Turn on your power supply or tap the lead to pin 5. You should hear the motor whur and a reading should appear.

7) Now we want to ensure the solenoid is functioning, to do this simply ground pin 6 and tap 12V to pin 7. You should hear a loud click. Only tap for a brief second.

[Hit Man X]

Cool stuff Kris!

[michael cole]

i would like to remove the actuator from my car 87 300tdt but im not sure where it bolts up.is there a recommended way to get it out?

[Bio300TDTdriver]

Originally Posted by michael cole

i would like to remove the actuator from my car 87 300tdt but im not sure where it bolts up.is there a recommended way to get it out?

It is buried under the intake behind the IP. Why do you need to remove it? Did it test bad?

[michael cole]

the reason my cruise is not working.with 12v across the pins to the actuator i can hear the motor spinning and i can actuate the solenoid but its been in there for 25 years.i thought a look inside might be prudent.i have replaced all the caps on the CC board as per James instuction.the rest of the board looks pristine.no charcoal.i have tried a spare board to no avail but cant verify if its any good.i have tested all the pin outs on the board while installed in the car to see if signals are present from the column stick and brake pedal.i have tested the brake switch and it is good.yesterday i cleaned up the left rear stop light assembly,replaced the bulb and cleaned all the grounds which show no abnormal amount of omhs.the history is the cruise was working albeit intermittanly and with some surging.then it just cut out.that was it

[Bio300TDTdriver]

If you remove the washer fluid tank you may be able to get to it.

[JamesDean]

Originally Posted by michael cole

the reason my cruise is not working.with 12v across the pins to the actuator i can hear the motor spinning and i can actuate the solenoid but its been in there for 25 years.i thought a look inside might be prudent.i have replaced all the caps on the CC board as per James instuction.the rest of the board looks pristine.no charcoal.i have tried a spare board to no avail but cant verify if its any good.i have tested all the pin outs on the board while installed in the car to see if signals are present from the column stick and brake pedal.i have tested the brake switch and it is good.yesterday i cleaned up the left rear stop light assembly,replaced the bulb and cleaned all the grounds which show no abnormal amount of omhs.the history is the cruise was working albeit intermittanly and with some surging.then it just cut out.that was it

Can you post the part number on your amplifier box? I can look it up and make sure that you've got the right box.

Did you re-solder all the contact points, that also helps.

Are you having any speedometer problems? Bouncy needle,etc? The 124s are mechanically driven speedometers, a problem like that might cause inconsistencies/oddities in the output of the hall-effect sensor and confuse the cruise control.

[michael cole]

there are 2 #s on the box 004 545 2432,412 205/7/8 production date 12/87.i have done no soldering on this board except for changing the caps.the speedo shows nothing unusual.i tried to measure the output of the hall sensor at the amplifier whilst the car was at rest no voltage either ac/dc but as i drove the car a voltage seem to be generated but a precise reading was difficult.i assume because my DVM is limited.thanks for your help so far

[JamesDean]

Originally Posted by michael cole

there are 2 #s on the box 004 545 2432,412 205/7/8 production date 12/87.i have done no soldering on this board except for changing the caps.the speedo shows nothing unusual.i tried to measure the output of the hall sensor at the amplifier whilst the car was at rest no voltage either ac/dc but as i drove the car a voltage seem to be generated but a precise reading was difficult.i assume because my DVM is limited.thanks for your help so far

The part number is correct for the 300TDT.

It sounds like your hall-effect is working, at rest nothing is generated as it will only generate when the speedometer cable spins. It will show a varying number as the cable will spin relative to the car's speed.

Try using some stripper and remove the conformal coating on the PCB, then solder every contact point.

When you installed the capacitors, did you be sure to observe proper polarity?

[michael cole]

ill take a good look at all the solder points on the board with a magnify glass.so your saying strip the coating right off the bottom of the board?is there any one particular area that could be involved?id love to get hold of schematics and associated theory of operation.give me a car i can tinker with and im happy.

[JamesDean]

Yes, strip off the coating and then re-solder each contact point. It takes about an hour or so to do.

I used this varnish remover, it worked out pretty well. After it was done I took the board to the garage and used the air compressor to blow of the remains.





I'm not aware of any one particular area, I always re-solder the entire board.

I have a schematic for one of the older board from the early 80s. Nothing for the newer ones.