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Old 08-20-2012, 08:35 PM
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JamesDean JamesDean is offline
Electrical Engineer
 
Join Date: Apr 2005
Location: NE Ohio
Posts: 5,038
I came across a reply I wrote to a question posted on Benz World about the cruise control system, how it works and what goes wrong.. Figured that would be useful in this thread:

How the unit operates normally:

The amplifier receives a signal that is generated in the tail shaft of the transmission. Once you hit 'accel set' this signal is stored.

The cruise control unit engages the solenoid in the actuator and it holds the throttle linkage in place. When this signal changes to a lower frequency, the cruise module will command the actuator to move the armature forward, pulling the throttle linkage with it and thus speeding up the car. Once the amp sees the signal is back to the originally set frequency it stops advancing the arm and releases it until it returns to its previous position since the linkage is spring loaded the motor doesnt have to do this work.

What goes wrong over the years?

Capacitor Aging
On the cruise control amplifier there are several energy storage devices called capacitors. These devices store the energy needed power the circuitry, engage the solenoid, and move the armature. These devices have a limited amount of life in them when compared to the other components on the board. They have a life rated in the hours 2,000 hours @ 105C is fairly common. Other capacitors on the board are rated for 300,000 hours @ 105 C and will last probably longer than the car itself.

When you have these capacitors age, they loose their ability to hold charge and thus make it difficult to perform necessary operations like power the chips that monitor and compare speed signals. It makes it especially difficult to send current to the motor to move arm and solenoid. That's where you get most of the intermittent behavior.

I've heard some people say that the Germans also had quality problems with their capacitors. I've seen a few different brand capacitors used over the years so there might be some evidence to support this. Additionally, my one 300SD, which was purchased new in 1982 had a repair order in 1983 for a replacement cruise control amplifier. The bill would have been $450 (in today's money). My grandparents never went through with it and thus the car never had cruise really. I removed the board from the car and performed my normal repair routine. Worked perfectly.

Cold Solder Joints
The second problem comes from cold solder joints. Solder connects all the points on the board. I've been told that solder can have a life span of about 20 years or so before it starts to show signs of failure like cold solder joints. The cruise control boards are partially coated with a special type of polyurethane coating. This helps prevent moisture and other outside elements from oxidizing and corroding solder points. Several boards I've worked on have failed to come back to life after a simple capacitor replacement. They needed a full-on resolder of every point on the board to come back to life.

Actuator Failure
This type of failure is actually rare. I've worked on 35+ cruise control units now and none of them have had any signs of actuator failure.

The actuator has a motor and solenoid inside of it. As the motor ages and wears it will start to draw more current from the cruise control amplifier. If this current reaches a suitable high level it can cause damage to the amplifier rendering it inoperable. The output transistors--the devices responsible for supplying power to the motor--are rated up to 4 amps. The actuator should draw an maximum of 220mA or so. Anything higher, say near 250mA and you should consider sourcing a used one. Most of the actuators I've seen draw between 120-220mA. I don't see a problem with that range. Once you get rather high numbers well beyond 250mA the transistors will heat up, they'll start to draw lots of power on the circuit board, this will cause everything to heat up. Some parts of the board are not designed to handle this and they break down and well that's the end of the amplifier!

For information about testing your actuator, see my post here:
http://www.peachparts.com/shopforum/2913150-post5.html

Other Problems

Poor/Bad Speed Signal
This would be more common in 123/124/201 chassis cards with a mechanically driven speedometer. In these cars the tailshaft gear train drives a cable that reaches up to the back of the instrument cluster. This drives the speedometer. There is a small unit attached to the speedometer that converts the mechanical signal to electrical. This unit can go bad, although I've not experienced or heard of it really. I only know of the one that was in my early test bench setup that failed (largely due to my curiosity at its inner workings).

In 126 chassis cars, there is a device on the transmission that converts the mechanical movement of the gear train into electrical signal. From there a wire carries it up to a connector located just to the right of the accelerator pedal, then up into the instrument cluster and off to the cruise control unit. I've not encountered any units that have failed. Only issues with wiring at that connector next to the pedal and that was an isolated incident with my friends 560SEL (it was pretty beat up).

Low System Voltage
If the car's overall voltage falls to below 10 or so volts. The cruise control system will disable. The newer style units may work a little longer than the older ones at this lower voltage however, if you're under 10V you've got bigger fish to fry than the cruise control.

I hope this answered your questions. The short answer is really capacitors and cold solder joints. If you've got anymore questions, please feel free to ask. I'd be more than happy to answer them.

If you're interested in one of the units or a repair for one that's not listed, drop me an jamesdean59@gmail.com or drop me a PM on here.

-Kris
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Cruise Control not working? Send me PM or email (jamesdean59@gmail.com). I might be able to help out.
Check here for compatibility, diagnostics, and availability!

(4/11/2020: Hi Everyone! I am still taking orders and replying to emails/PMs/etc, I appreciate your patience in these crazy times. Stay safe and healthy!)


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