Classic Mercedes-Benz Cruise Control E-Tempomat System

Table Of Content

• References
• Other Related Systems
• Related Features:
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• LIM – Electronic speed limiter

However, they will not apply the brakes in the event of overspeeding downhill, nor stop the car from going faster than the selected speed even with the engine just idling. As mentioned previously, the actuation switch has four inputs to the amplifier, on Pins 2, 3, 4 and 6. Pin 5 is the actuator clutch power output mentioned before, while Pins 7 and 10 control the actuator motor. The voltage on these two pins varies both in magnitude and polarity, since the DC motor used to actuate the throttle needs to be able to run in both directions and at variable speed.

References

Another inventor named Harold Exline, working independently of Riley, also invented a type of cruise control that he first installed on his car and friends' cars. Again, while these older models probably don’t make up the bulk of your customer base, we all know just how enthusiastic and passionate these classic-model owners can be. Your efforts at keeping their “baby” operating as designed will be appreciated by your customers, and likely profitable for you.

Other Related Systems

Most systems do not allow the use of the cruise control below a certain speed - typically around 25 or 30 mph (40 or 48 km/h). The vehicle will maintain the desired speed by pulling the throttle cable with a solenoid, a vacuum-driven servomechanism, or by using the electronic systems built into the vehicle (fully electronic) if it uses a 'drive-by-wire' system. All cruise control systems must have the capability to be turned off explicitly and automatically when the driver depresses the brake pedal and often also the clutch. Cruise control systems frequently include a memory feature to resume the set speed after braking and a coast feature to reduce the set speed without braking.

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The system ground (at W1) is connected to amplifier Pin 12 and (originally in production) Pin 14. At the time, there was a Dealer Technical Bulletin explaining the issue, but that document is no longer available. The system works by measuring vehicle speed and actuating the throttle linkage to increase or decrease engine output.

Daimler and HERE to bring HD Live Map to future Mercedes-Benz models

While most of your shop’s daily work is probably on newer Mercedes-Benz models, there are many customers with classic models, and their cars still need regular service and maintenance. With that in mind, StarTuned plans to deliver a series of occasional articles on some of the classic systems used in these older models. Today, we will explore the E-Tempomat cruise control system used in vehicles from the mid-1980s until the early 1990s.

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Many customers love their cruise control, using it as an easy way to control vehicle speed, reducing both fuel consumption and the possibility of a speeding ticket. Not just for long highway cruises, even the daily commute is made easier, allowing the driver to focus on tasks other than speed control, which research shows also helps make for safer driving by reducing critical workload. Automatic braking systems use either a single or a combination of sensors (radar, lidar, and camera) to allow the vehicle to keep pace with the car it is following, slow when closing in on the vehicle in front, and accelerate to the preset speed when traffic allows. Some systems also feature forward collision warning systems, which warn the driver if a vehicle in front—given the speed of both vehicles—gets too close within the preset headway or braking distance. In the digital cruise control amplifier, Mercedes-Benz decided to use a so-called Reference Resistor to set the characteristics of the amplifier, allowing a single part number amplifier to be used in several different models. When replacing an analog-type amplifier, be sure to also order the correct reference resistor or the system won’t work.

Components

Cruise control is less flexible on vehicles with a manual transmission because depressing the clutch pedal and shifting gears usually disengages the cruise control. The "resume" feature has to be used each time after selecting the new gear and releasing the clutch. Therefore, cruise control is most beneficial at motorway/highway speeds when top gear is used virtually all the time. Also note that other systems, notably ABS and the Anti-Theft Alarm, also connect to the brake light circuit, but rarely cause problems with cruise control. The cruise control amplifier, as an electronic control unit, cannot be tested directly, but we can easily say that if all of the inputs and outputs to the unit are correct, either the system is operating correctly or the amplifier needs to be replaced. The unit only has 14 pins, so it is fairly quick and easy to check every one of them and come to a conclusion.

When the cruise control is engaged, the throttle can still accelerate the car, but once the pedal is released, it will slow down the vehicle until it reaches the previously set speed. Tipping the 4-way lever upwards sets the desired speed, or increases the set speed by a small increment. Exactly how much the speed is increased or decreased depends on the E-Tempomat version, but it is generally either about 1 MPH or 1 km/h. Holding the lever either up or down increases or decreases the vehicle speed at a set rate, although on a downhill slope the speed may not decrease as quickly. Pulling the lever towards the driver resumes a previously-set speed, while pushing the lever away from the driver switches the system off.

Other ways to switch the system off include tapping the brakes, pressing the clutch pedal (in manual-transmission vehicles), or if the system detects an unreasonable deceleration (about 1.5 m/s2). In 1965, American Motors Corporation (AMC) introduced a low-priced automatic speed control for its large-sized cars with automatic transmissions.[10] The AMC "cruise command" unit was actuated through a push-button on the dashboard once the car's desired speed was reached. The throttle position was automatically adjusted by a vacuum control that opened and closed the throttle based on input from the speedometer cable rather than through an adjustable control on the dashboard.[10][11] The unit would shut off anytime the brakes were applied. The cruise control systems of some vehicles incorporate a "speed limiter" function, which will not allow the vehicle to accelerate beyond a preset maximum; this can usually be overridden by fully depressing the accelerator pedal. Most systems will prevent the vehicle from increasing engine speed to accelerate beyond the chosen speed.

Now you know everything you need to know about the Mercedes-Benz E-Tempomat cruise control system to keep the cruise control system, and your customers, happy. Modern cruise control (also known as a speedostat or tempomat) was invented in 1948 by the blind inventor and mechanical engineer Ralph Teetor.[4][5] He came up with the idea due to being frustrated by his driver's habit of speeding up and slowing down as he talked. So now we know how this relatively simple system operates, from both the customer’s view and as a system.

If you are replacing a digital amplifier, you should swap the reference resistor over to the new amplifier. One frequently-heard complaint is a ticking or clicking sound coming from the actuator, along with the complaint that cruise control is inoperative. This is a symptom of a problem with the feedback potentiometer, where the amplifier believes (inaccurately) the throttle position is too high, causing the amplifier to run the actuator motor in an attempt to bring the throttle linkage to idle position. The clicking is the ratchet in the actuator clutch, since the actuator motor is also disengaged in this situation.

Cruise control (also known as speed control, cruise command, autocruise, or tempomat) is a system that automatically controls the speed of an automobile. The system is a servomechanism that takes over the car's throttle to maintain a steady speed set by the driver. Another complaint we’ve heard is that the cruise control system operates more-or less fine, but tends to ‘hunt’, or constantly change throttle position from a little too much to a little too little, almost continuously while in operation. Found especially in higher-mileage vehicles, this is a clear symptom of a worn feedback potentiometer in the actuator. What happens is that the potentiometer resistance surface eventually wears out in just one small spot, and the only solution is a new actuator. That covers all 14 pins of the amplifier, and diagnosis is as simple as taking 14 readings (and 2 of them are grounds!).

But as we mentioned, there are other components related to this system which may need to be checked as well. Using your ohmmeter, you can verify that all the switches are switching when they are supposed to, and by actuating them several times (including with more and less force) you can check for intermittent contacts. Because the switch’s connector contacts (down in the steering column) can also contribute to the problem, you might consider also measuring the switch values directly at the amplifier plug.

This symptom is often caused by a poor ground to the actuator at Pin 1 of the actuator connector, located in the engine compartment. Originally, the E-Tempomat control amplifier was a purely analog circuit, with a different part number for each vehicle. Since vehicles have different power-to-weight ratios, a different reaction by the amplifier is needed – for example, a 190D 2.2 needs to approach the throttle more aggressively when the speed drops, while the same reaction in a 560SL might be a little scary. Later versions, including the replacements available today, were digitally controlled, and a coding plug was attached to the amplifier to set the specific throttle response and other parameters.

An mentioned, vehicle speed is a series of pulses, with increasing frequency indicating increasing speed. In Mercedes-Benz vehicles with a mechanical speedometer cable, there is a sensor on the back of the speedometer that generates these pulses, while in electronic speedometer models the speedo re-forms the pulses received from the inductive sensor at the transmission output shaft. Since several other systems use the vehicle speed signal – CIS-E engine control, fuel pump relay, air conditioning and radio to name a few – there will be other systems affected if the vehicle speed signal is faulty. The cruise control takes its speed signal from a rotating driveshaft, speedometer cable, wheel speed sensor from the engine's RPM, or internal speed pulses produced electronically by the vehicle.