There are major differences between the two basic systems that are on MBZ models beginning in the mid 1970's. Beginning sometime in early 1981, there was a transition from the older, vacuum-type actuator with its accompanying 10-pin amplifier to the motor actuator which has a 14-pin amplifier. We will refer to these systems as the "old" and "new" systems, respectively.
All of the 10-pin amplifiers store the "set speed" as an analog voltage at the junction of a capacitor and the gate of a dual-gate field effect transistor (FET). The 14-pin amplifiers use a CMOS counter and store the same information as a digital number. Naturally, unless the counter or its associated conversion circuitry fails, the latter is a much more accurate method. Along with other system design differences the later systems hold speed much more effectively.
A significant difference between the old and new systems affects the accuracy of the set speed hold in a given vehicle. Both systems amplify and process a difference signal between the set speed and the current vehicle speed. Each system produces an output which increases with increasing difference signal.
However, in the old system, the throttle cable moves with a direct positional relationship to that difference signal. Consequently, on a steep grade, it is common that the vehicle can never quite achieve the desired set speed. There can be a vehicle speed change of as much as 1.5 MPH per degree of grade. This can be worse with a non-turbo diesel. A few degrees of uphill grade can cause a diesel cruise system to drop out entirely. This is normal; the circuit should drop out when the vehicle speed drops to 5 to 10 MPH under the set speed.
The output signal from the new system causes the motor actuator lever to increase throttle as long as there is a difference signal. During accel/set, on most models, throttle increases from zero to full throttle in about nine seconds. Therefore, as long as the actual speed is less than the set speed, increasing throttle is applied. If the vehicle has sufficient power for a given grade, this system holds speed more accurately than the old system.
In addition, the pre-1981 10-pin, vacuum systems are susceptible to a problem external to the cruise system: there must be sufficient and relatively constant vacuum.
It has become clear that a 10-pin amplifier may perform differently when switched between cars with different actuators. The reason is that the circuit samples the output current to the actuator; a portion is sensed as feedback for system stability. The dynamic input impedance of different actuators and corresponding output current from the amplifier varies enough to cause a noticeable variance of set speeds between vehicles.
Furthermore, a variation in actuator current can cause a slow speed oscillation of a few MPH. There are only three possible sources of this slow surge: 1) the amplifier, 2) the actuator and 3) the speed sensing transducer. It is my opinion, after reviewing considerable data, that this problem lies within the actuator. I have found that an actuator that is unstable at times can improve after the engine compartment warms during use or after being parked in the sun. I have found nothing in the 10-pin amplifier circuit that can cause this slow fluctuation.
We now offer a circuit modification to allow an adjustment of the set speed after installation of a 10-pin amplifier. The new systems do not need this adjustment. A quick procedure with a small screwdriver or potentiometer trimmer tool will match our remanufactured module to your customer's vehicle. This adjustment will not cure a surge or oscillation in the system.
The new systems often exhibit a much more pronounced surge. It can be so bad that it can render the system unusable. The cause in these amplifiers is either an instability in a frequency to voltage conversion circuit which is cured by a slight circuit modification or problems with a faulty actuator. The circuit change is included in every 14-pin amplifier that is repaired at our facility. Any problem with the actuator is resolved during rebuild.
We adjust each amplifier using an oscilloscope so that it sets at the exact speed where the lever is released. In other words, on a level road while accelerating to 55mph, the lever is held up in the 'accel/set' position. At 55 MPH the lever is released; the vehicle should return to and hold at exactly 55 MPH after a slight overshoot of a few MPH due to inertia of the vehicle.
Finally, to answer some commonly asked questions, common amplifier failure causes include: cold solder joints, component drift (requires re-calibration), improperly installed components (wrong values and backwards installation) [this happened at the factory!], and failed components (transistors, integrated circuits, even capacitors.)
We don't want to give the impression that the older, 10-pin vacuum systems in the Mercedes from 1976 until 1980 are inadequate. When they have good components they are wonderful; it is simply old 1960's technology. All of the VDO systems are sensitive and each component in the system must be in perfect shape for the system to perform properly. A VDO cruise control in any Mercedes will accurately hold a proper "set speed" all day - or as long as you can drive between rest stops. We even make some of our 10-pin amplifiers adjustable so that they can be indivually matched your vehicle!
Important Note: If you find anything in this article that is not consistent with your experience, please let us know! Thank you.