Cruise control amplifier:
For these tests you will need a multimeter and two or three clip leads (or pieces of stiff wire) that can plug into the socket that has been removed from the amplifier. 12-gauge Romex (house) wire works well, you only need a few inches of it. You can wad up other types of wire to fit in there as well, but I find the Romex to be particularly suitable. (I also have a bunch of it.)
The connector that plugs into the vacuum unit amplifier looks something like this (facing the little sockets):
o	2	O	1	o
o	4		3	o
o	6		5	o
o	8		7	o
o	10		9	o
o	12		11	o
            	
2		 	1	
4	Decel	 	3	Actuator +
6	Brake	 	5	Power
8	Cancel	 	7	Actuator –
10	Resume	 	9	Accel
12	Ground	 	11	Speedometer
The first series of tests is done with the key off. The resistance between Pins 7 and 3 should be approximately 12 ohms and there should be no continuity to power or ground. If it's substantially more than this 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. Pin 12 should be grounded, a resistance test of it to the car's chassis should show little/no resistance. (Leave the ground lead of the meter in this socket for the next series of tests.)

The next series of tests is done with the key on (though the engine need not be running). Pin 5 is power, it should register +12 volts. Pin 6 is the brake lights, it should register 0 V normally, and +12 volts when the brakes are applied. Pin 8 should register +12 V unless the stalk switch is moved to Cancel. Pin 10 should register +12 V when the stalk switch is moved to Resume. Pin 9 should register +12 V when the stalk switch is moved to Accel. Pin 4 should register +12 V when the stalk switch is moved to Decel.

The next test is done with the engine running. Connect one jumper between pins 6 and 7, and another between pins 9 and 3. When the Accel switch is activated the engine should rev up; releasing the switch 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 vacuum supply to the actuator, or its mechanical linkage to the throttle.

The last tests are 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.)

An inadequate supply of vacuum can cause strange problems with sinking set speeds, surging, etc. If this is your problem tee a vacuum gauge (a common vacuum/fuel pressure gauge or a MityVac will do nicely) into the black/yellow line that feeds the actuator, as close to the actuator as you can make it, and place the gauge where you can see it safely while driving. (A long hose can be used to snake the gauge into the passenger compartment, or you could try strapping the gauge to the wipers.) Try to use the cruise control normally. If the vacuum supply drops below 5" (Hg) during operation the cruise control cannot work properly: the value should normally be twice this number in operation. A low vacuum supply can be caused by leaks or occlusions in the piping feeding the actuator, and must be corrected before suspecting electrical problems. Of note is that the system consumes the most vacuum while holding a set speed, not while accelerating, so the ability of the system to accelerate well does not prove that the vacuum supply is good.

The connector that plugs into the servomotor unit amplifier looks something like this (facing the little sockets):
o	2	O	1	o
o	4		3	o
o	6		5	o
o	8		7	o
o	10		9	o
o	12		11	o
o	14		13	o
            	
2	Decel	 	1	Power
4	Accel	 	3	Cancel
6	Resume	 	5	Clutch
8	Brake	 	7	Motor +
10	Motor –	 	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. 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. Send it in! (If you wish to read about some of my personal trials with these systems, refer to here and the 240D's log starting here for about two weeks for the vacuum system; refer to here and the 380 SL's log starting here for the servomotor system.)

from:  http://userweb.windwireless.net/~jimc/mamerepairs.html#testing