Additional CC diagnostic for Non Drive by Wire NDBW
Additional Cruise Control Diagnostics.

a-aranda set the bar with his excellent Cruise Control diagnostic. I'm happy to provide this addition. I appreciate all improvements anyone can offer to my input, and hope the audiworld moderators will help us all produce an excellent document. First, I'd like to clarify a couple of items in a-aranda's FAQ:

A. "Control Unit: This piece rarely goes bad...." My malfunction turned out to be the Control Unit, and my dismantler says they sell quite a few of them.
B. a-aranda says "the bellows should move the throttle a bit (nowhere near WOT)." I don't know what a "Mity-vac" is, but my vacuum bellows test using the Audi components sucked the diaphram in all the way.
C. "Part number for the control unit is 4B0 907 305." Audi uses the same Control Unit for 4, 6, and 8 models up until 2002, with the earlier cars having part number 4D0 907 305. Part 4B0 907 305 is listed as superceding 4D0 907 305. The pinout is the same, and probably the only difference is in the generation of electronics inside the module.
D. a-aranda's test of the control unit came up with some values corresponding to testing with the controller connected, and some with it disconnected.
E. While he seemed to detest working on the Control Unit, feeling it could only be done upside down in the passenger footwell, I was able to dangle the pack, push back the passenger seat, and work bent at the waist. Your mileage may vary.

The Control Unit uses a combination of passive and active circuits. The circuits are designed, ingeneously, to fail safe, and fail to ground.

Explanation of the Control Unit pinout, followed by detailed diagnostics:

1. Ground - not present in all models
2. Not used
3. Not used
4. Signal from the Set/Decelerate button. With the system powered, pressing the Set/Decel button should cause pin 4 to have a +12 volt event.
5. Ground
6. Return from Vacuum Pump. When accelerating, this wire should have 0 resistance to ground. Otherwise, it should have resistance to ground. The Control Unit works with dual redundancy by supplying +12 volts on pin 7, and connecting pin 6 to ground to accelerate. If either the +12 quits, or there is no longer a connection to ground, the Cruise Control deactivates.
7. Power to Vacuum Pump and Vacuum Hold Valve. When activated, this pin should have +12 volts. Instead of testing the vacuum system piece-by-piece, I connected pin 7 of the Cruise Unit's connector to +12, and connected pins 6 and 12 to ground. The vacuum pump sucked the bellows in all the way. Additionally, I was able to see the brake valve dump vacuum, and - I have an automatic - assume a working clutch vacuum dump valve would do the same.
8. Power supply, from pin 4 of the Stalk Switch. On the Stalk Switch, the connection between pins 4 and 5 is the Control Unit's On/Off. On a manual transmission car, pin 5 of the Stalk Switch is powered by fuse 31. On automatic transmission cars, pin 5 of the Stalk Switch is powered by pin 9 of the Transmission Control Module, modulated to ground by pin 5 of the Multi-Function Transmission Range (TR) Switch. In short, on an automatic, the Cruise Control Unit is only powered when the car is in a Drive gear.
9. Vehicle speed input from pin 6 of the yellow 26-pin Instrument Cluster connector. Probably a square wave, with a low of 0 volts and a high of +12 volts, ranging from 10% to 90%, depending on vehicle speed.
10. Resume/Accelerate momentary switch. When the Stalk Switch Res/Accel is pressed, this pin should shift from 0 volts to +12 volts.
11. Brake light circuit. As opposed to all the other passive circuits, this circuit is active, sending +12 volts to ground through a normally-closed brake switch, and then through the cold brake light filaments. When the brake is pressed, this circuit from pin 11 detects the connection to ground is broken, and the Control Unit disengages. Having both brake lights out can also cause the system to malfunction (check 'em!). The Momentary Off switch on the Stalk Switch is in series with the brake switch; the Control Unit detects pressing the Stalk Switch Momentary Off as the same event as pressing the brakes.
12. Return from Vacuum Hold Valve. When tracking a speed, the Control Unit activates the Vacuum Hold Valve to maintain the vacuum. In the event of power failure, the Hold Valve releases. Like the Vacuum Pump, the Control Unit manages the Vacuum Hold Valve by controlling the routing of the return wire to ground. when activated, there is no resistance to ground. When dumped, there is resistance to ground.


A detailed breakdown of values at the Cruise Control Connector. The first five are with the Control Unit removed from the harness:

Controller removed, power off; resistance-to-ground values:
1. 0.
4. Infinity, going to 0 when the Set/Decel switch is pressed; if less, dirty stalk switch.
5. 0; if less, dirty stalk switch.
6. Infinity.
7. Infinity.
8. Infinity when Stalk Switch is off, 1 ohm when Stalk Switch is on, as voltage bleeds across the brake lights.
9. 3 ohms resistance as the ohmmeter charges an output-filter capacitor on the instrument panel, followed by a brief period of 0 ohms as the capacitor outputs, cycle repeating.
10. Infinity when Stalk Switch is off; when the Stalk Switch is on and the Res/Accel button is pushed, 1 ohm through a backtrack to the brake lights.
11. Infinity when Stalk Switch is off; when the Stalk Switch is on and either the Momentary Off or the brake is pressed, 0 ohm. If less than 0, dirty Stalk Switch.
12. Infinity.

The next four are with the Control Unit removed, but with the system powered (Manual: key turned to ignition; Auto: key to ignition, transmission in drive):
Controller removed, system powered, Stalk Switch OFF, voltage:
1. 0.
4. 0.
5. 0.
6. 0.
7. 0.
8. 0.
9. Depending on your meter, appears to be 0, but is actually a few millivolts positive power from the instrument panel.
10. 0.
11. 0.
12. 0.

Controller removed, system powered, Stalk Switch ON, voltage:
1. 0.
4. 0; when Set/Decel is pressed, +12 volts.
5. 0.
6. 0.
7. 0.
8. 0.
9. Depending on your meter, appears to be 0, but is actually a few millivolts positive power from the instrument panel.
10. 0; when Res/Accel is pressed, +12 volts.
11. 0.
12. 0.

Controller removed, system powered, Stalk Switch OFF, resistance to ground:
1. 0.
4. Infinity.
5. 0.
6. Infinity.
7. Infinity.
8. Infinity.
9. Depending on your meter, appears to be 0, but is actually a few millivolts positive power from the instrument panel.
10. Infinity.
11. Infinity.
12. Infinity.

Controller removed, system powered, Stalk Switch ON, resistance to ground:
1. 0.
4. Infinity, going to 0 when Set/Decel is pushed.
5. 0.
6. Infinity.
7. Infinity.
8. 0.
9. 3 ohms resistance as the ohmmeter charges an output-filter capacitor on the instrument panel, followed by a brief period of 0 ohms as the capacitor outputs, cycle repeating.
10. Infinity, going to 0 when Res/Accel is pushed.
11. 0.
12. Infinity.


The next five are with the Control Unit installed in the harness:

Controller installed, power off; resistance-to-ground values:
1. 0.
4. 1 ohm.
5. 0.
6. 1 ohm.
7. 1 ohm.
8. Infinity when the Stalk Switch is off. When the Stalk Switch is on, 1 ohm bleeding through the brake lights. While Stalk Switch is on:
- Set/Decel should change the resistance to .5 ohm.
- Res/Accel should change it to .5 ohm.
- Momentary Off should change it to 1.5 ohm.
9. 3 ohms resistance as the ohmmeter charges an output-filter capacitor on the instrument panel, followed by a brief period of 0 ohms as the capacitor outputs, cycle repeating.
10. 1 ohm, going to .5 ohm when Res/Accel is pressed.
11. Infinity when the Stalk Switch is off. When the Stalk Switch is on, 0 ohms resistance, changing to 3 ohm when either the momentary off or the brake is pressed.
12. 1 ohm.

The next four are with the Control Unit installed, and the system powered (Manual: key turned to ignition; Auto: key to ignition, transmission in drive):

Controller installed, system powered, Stalk Switch OFF, voltage:
1. 0.
4. 0.
5. 0.
6. 0.
7. 0.
8. 0.
9. Depending on your meter, appears to be 0, but is actually a few millivolts positive power from the instrument panel.
10. 0.
11. 0.
12. 0.

Controller installed, system powered, Stalk Switch ON, voltage:
1. 0.
4. 0; when Set/Decel is pushed, +12 volts
5. 0.
6. 0.
7. 0.
8. +12 volts.
9. Depending on your meter, appears to be 0, but is actually a few millivolts positive power from the instrument panel.
10. 0; when Res/Accel is pushed, +12 volts.
11. Appears to be 0, but is actually a +12 volt, low amperage, active sensor signal sent to detect continuity to ground through the cold brake lights. Because this signal is soaked up by the gound, pressing the Momentary Off or the brakes will make it appear voltage is being passed to pin 11, but what is really happening is the ground is being removed, and the meter goes from not detecting, to becoming the best path to ground and detecting +12 volts.
12. 0.

Controller installed, system powered, Stalk Switch OFF, resistance to ground:
1. 0.
4. 1 ohm
5. 0.
6. 1 ohm.
7. 1 ohm.
8. Infinity; pressing Set/Decel should show 1 ohm on the backtrack through the brakelights.
9. Depending on your meter, appears to be 0, but is actually a few millivolts positive power from the instrument panel.
10. 1 ohm.
11. Infinity.
12. 1 ohm.

Controller installed, system powered, Stalk Switch ON, resistance to ground:
1. 0.
4. 1 ohm; Don't press Set/Decel! +12 volts! It might fry your meter input!
5. 0.
6. 1 ohm.
7. 1 ohm.
8. Don't touch a resistance probe to pin 8 with the Control Unit powered! +12 volts! It might fry your meter input!
9. Depending on your meter, appears to be 0, but is actually a few millivolts positive power from the instrument panel.
10. 1 ohm. Don't press Res/Accel! +12 volts! It might fry your meter input!
11. 0, Don't press Momentary Off or brakes! +12 volts! It might fry your meter input!
12. 1 ohm.


Stalk Switch, as events:

Turning the unit on:
Stalk Switch pins 4 and 5 become connected. When off, no voltage should be detected at the Control Unit. When on, and the ignition on, and automatic cars in Drive, +12 volts should appear at pin 8 of the Control Unit.

Pressing the Set/Decel button:
Stalk Switch pins 3 and 4 become connected. At the Control Unit, pin 4 becomes powered.

Pressing the Res/Accel button:
Stalk Switch pins 2 and 4 become connected. At the Control Unit, pin 10 becomes powered.

Pressing the Momentary off:
Stalk Switch pins 1 and 6 become disconnected. The Momentary Off switch on the Stalk Switch is in series with the brake switch; the Control Unit detects pressing the Stalk Switch Momentary Off as the same event as pressing the brakes. At the Control Unit, pin 11 should detect the events.


Tidbits:

Testing the vacuum system:
The vacuum pump itself is under the ABS controller, and hard to get to. You can test the entire vacuum system from the controller.
First, you'll have to have the engine cover off, so you can see the CC throttle bellows. Connect pin 7 of the Cruise Unit's connector to +12, and connected pins 6 and 12 to ground. Walk around to the engine bay, and if the vacuum pump has sucked the bellows in all the way, you've proven the system has sufficient suction. If you have someone with you, have them press the brake pedal while you watch the bellows. When they press the pedal, the bellows should open. When they release the pedal, the bellows should close back down.
This test is now over, and you can either disregard the vacuum system, or know where to fix it.

Testing the electrical system:
In time, the stalk switch connections get tarnished. You can probably count on needing to take it apart and clean it every five years or so.
If you've cleaned it, and the module still won't engage, you can test the wiring and switches from the connector of the cruise control module.
If you get weak events, where you press a switch and get 9 volts, or instead of continuity you get a few ohms of resistance, then the stalk switch has dirty contacts.
If you get solid events, where you press a stalk switch and the voltage swings from 0 to 12, or the resistance swings from 0 to continuity, then the switch is fine - most likely, the relay in the controller has failed. The fix is replacement.


In the Bentley manual, the circuit diagram for the Cruise Control on a car equipped with automatic transmission lists a connector, T4b, as being black, and at the center console rear. I was unable to find this connector in the manual transmission diagrams. The four wires are:
1. Power, +12 volts from fuse 5
2. Vehicle speed from pin 6 of the yellow 26-pin instrument connector
3. Ground
4. Drive indication from pin 9 of the Transmission Controller, modulated to ground by pin 5 of the Transmission Range Switch, through the on/off of the Stalk Switch. This signal is the same as pin 8 of the Control Unit, and is only powered when the car is in drive, and the Cruise Control is On (not necessarily engaged).

This connector seems to be positioned for an optional rear differential lockup. However, on my car (a '96), it is not in the rear of the Center Console, but in the right end panel of the dashboard, beneath the end cover. This placement makes its usefulness for a rear differential questionable, however, it might be very useful if you need to know whether your Control Unit is being powered on pin 8, need a convenient ground or +12 volts for testing, or want to hook up an oscilliscope to read the speedometer square wave.