PeachParts Mercedes-Benz Forum - View Single Post

Maxbumpo · #2 04-11-2018, 10:16 AM

Following copied / pasted from the personal website of a fellow I've know via internet for years.

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The connector that plugs into the (cruise control) servomotor unit amplifier looks something like this (facing the little sockets):

2 1
4 3
6 5
8 7
10 9
12 11
14 13

The pins correspond to:

2 Decel 1 Power
4 Accel 3 Cancel
6 Resume 5 Clutch
8 Brake 7 Motor pos.
10 Motor neg. 9 Feedback Pot
12 Ground 11 Speedometer
14 Ground 13 Feedback Wiper

The first series of tests is done with the key off. Pins 12 and 14 should be grounded, a resistance test of them to the car's chassis should show little/no resistance. The servomotor's clutch unfortunately has a series diode, so its resistance is difficult to measure. You can either connect an ammeter between Pins 1 and 5 and see approximately 300 mA, or you can defer testing it to a later step. (If it's broken the feedback resistance test will fail.) If you have a diode test range on your meter you can test it as a diode, though you won't see the actual resistance of the clutch coil. The resistance between Pins 7 and 10 (the motor) should be approximately 5 ohms, and there should be no continuity to power or ground. The resistance between Pins 9 and 14 should be approximately 3000 ohms, as should be the value between Pins 13 and 14, and likewise there should be no continuity to power or ground. If any value is substantially more than specified the actuator under the hood is broken, it's unplugged, its connector is extremely dirty, or the wiring to it is broken. Find out what's wrong and correct it.

The next series of tests is done with the key on (though the engine need not be running) and the ground lead of the meter in Pin 12. Pin 1 is power, it should register +12 volts. (Battery voltage, that is, the exact value will vary with the circumstances.) Pin 8 is the brake lights, it should register 0 V normally, and +12 volts when the brakes are applied. Pin 3 should register +12 V unless the stalk switch is moved to Cancel. Pin 6 should register +12 V when the stalk switch is moved to Resume. Pin 4 should register +12 V when the stalk switch is moved to Accel. Pin 2 should register +12 V when the stalk switch is moved to Decel.

The next test is done with the key on and the engine definitely not running. Connect one jumper between pins 5 and 3, another between pins 8 and 10, and another between pins 7 and 4. The ohmmeter should be connected between pins 13 and 14. When the Accel switch is activated the measured resistance should decrease smoothly to zero. Moving the switch to Cancel or pressing the brakes should return the resistance to the approximately 3000 ohms value. Repeat this test several times, especially trying to 'tease' it slowly. If the resistance jumps to a large value at any time the actuator itself has suffered internal damage to its feedback potentiometer, usually due to wear. I have heard of this being repaired by painting conductive substances on the damaged resistance track, but I have never done so. Procuring another actuator (used?) may be the best option at this point.

Next start the engine, leaving the preceding three jumpers in place. When the Accel switch is activated the engine should rev up; moving the switch to Cancel or pressing the brakes should return the engine to idle. Be particularly careful on gasoline cars as there is no governor to prevent the engine from over-revving. If this doesn't work check the actuator and its mechanical linkage to the throttle.

The last test is done while driving the car. Pin 11 is the signal from the speedometer, an AC voltmeter between it and ground (pin 12) should show a voltage that rises with speed. (And if your meter also measures frequency, like mine does, it certainly should show a rise in frequency with speed; frequency, in fact, is what the circuit is actually reacting to.)

If your car passes all these tests and the cruise control still doesn't work right, then the amplifier is definitely at fault.