The VDO Analog Cruise Control Module by James A. Mahaffey, Peachtree Section An analysis of what goes on right and wrong, inside the black box of your cruise control, with methods of diagnosis and repair. The Circuit The electronic module of the VDO cruise control, as first installed in 1978 in the 280, 280C and 300D, is distinguishable from other modules by the number of pins on its connector (10) and the length of its aluminum enclosure, 5-5/8 inches (14.4 cm). The 10-pin connector is actually a 12-pin connector with pins 1 and 2 blanked off. Later digital cruise control modules have 14-pin connectors, and their enclosures are longer. The speed-sensing portion of the cruise control circuit is shown in Figure 1. Test points are indicated by boxed numbers. Speed is sensed by a 16-pole magneto connected in series into the speedometer cable. A speed of 60 MPH gives a 131 Hz sine wave at about 5 volts, inserted into the cruise control module at pin 11. This sine wave, observable at point 1, is then half-wave rectified (point 2). The signal is then amplified and limited by Q5 and Q6, giving a rectangular wave of 6 volts at point 3. At point 4 the wave is a 0.1 to 0.2-volt ripple. The signal is converted to a DC voltage, varying linearly with respect to the frequency of the speed sense signal, observable at point 5. R69 trims the sensitivity. Voltage Comparison The voltage generated in the speed-sensor circuit is captured on capacitor C19 when you set the cruise control by engaging the Accelerate Set or Decelerate Set controls (see Figure 2). In the engaged condition, the output of Q9, a dual-gate FET riding C19, becomes the standard voltage against which the car's throttle is controlled through a servo-amplifier. An Accelerate Set signal latches down relay 2 (K1 in Figure 5), overriding the controller, opening the throttle and connecting C19 directly to the speed sensing-circuit. A Decelerate Set command does the same except that the output current is turned off, closing the throttle. A Cancel command disengages the controller by dropping the latch of relay 1 (K1 in figure 5), but the charge on C19 is preserved. Setting Resume relatches K1, and pressing the brake pedal unlatches KI. The standard DC voltage is amplified by a section of the quad comparator, U1, connected as an op amp. R76 trims the servo gain. Another part of U1 is connected as a multivibrator, as shown in Figure 3. The amplified DC voltage controls the pulse length of this oscillator, observable as a rectangular pulse train 7 volts high at point 6. The oscillator output is converted to a current of 100-300 mA, on pins 7 and 3, by Q2 and the final driver Q1. The current is used to drive a pilot valve controlling a vacuum-diaphragm actuator connected to the throttle. Voltage indicative of the average current produced by Q1 is observable at point 7. Q3 is the current-to-voltage converter, providing a negative feedback stabilizer to the multivibrator. R27 trims the multivibrator switching decay rate, and R11 trims the negative feedback. A third section of U1 is connected as a Schmitt trigger, as shown in Figure 4. If the car's speed drops to 10 MPH below the "stored" speed (standard voltage), the Schmitt trigger fires, giving a negative pulse observable at point 8. The pulse, amplified and inverted by the fourth section of U1 and Q4, causes K1 to unlatch, dropping control of the throttle. This condition is perfectly normal and is particularly noticeable in a diesel on a steep climb. Another normal drop-out condition is a speed below about 12 MPH. When the standard voltage (charge on C17) drops to 1.0 volt, Q8 conducts, grounding the negative input to the latchout amplifier, as would the Schmitt-triggered negative pulse. R74 trims the Schmitt trigger threshold, and R36 trims the latchout amplifier gain. Figure 5 shows the relays and the diode logic array for the four cruise control switches; all four are momentary switches. A Decelerate Set command connects pin 4 to +12 volts. Accelerate Set connects pin 9 to + 12 volts. Pin 10 is connected to +12 volts upon Resume, and pin 8 is always at +12 volts except momentarily upon the Off command. Pin 6 is connected to the brake light, with K1 grounded through the light unless the brake pedal is pressed. The wiring is diagrammed in Service Manual, Chassis and Body, Series 114/115 in section 54.2, page 320/3. (Owners of other models can likely find similar diagrams in the appropriate chassis manual for their models. Ed.) The components referred to in the schematics are indexed to a drawing of the circuit board top in Figure 6 on page 31. Variations There are at least three versions of the VDO analog cruise control module. The first is distinguishable by its gray connector and by additional circuitry shown in Figure 1. An open-collector op amp, in a single-circuit package (U2), maintains a second copy of the standard voltage on a large capacitor, C25. This auxiliary circuit stores the last set speed for an indefinite time, even with the ignition off, so a Resume command will automatically accelerate the car to that speed. In the second version, distinguishable by a black connector, this circuitry was deleted. A set speed is retained only as long as the ignition is on. The same circuit board (no. 304) was used, with the auxiliary circuit components not mounted. The same schematic and pictorial apply, but delete R79-91, D34-39, U2 and C21-25. A third version has the same circuit as version two but with a modified circuit board with no provision for mounting the long-term memory components. Although the positions of a few components were shifted in this version, the schematic and pictorial in this description are still basically applicable. Servicing Techniques The best way to service this module is on a bench, using a 12-volt power supply, a sine-wave function generator to supply the speed input, and an oscilloscope to trace through the circuit. Build a temporary array of momentary switches to simulate the actions of the cruise control stalk. The response of the cruise control circuit to a sine-wave input is linear from about 50 to 250 Hz, at 3 to 6 volts. The Hz-to-MPH conversion factor is 2.18 Hz per MPH. Use a sharp pointed probe to penetrate the varnish covering the circuit. Solvent will dissolve the varnish, but there's some danger of dissolving components. The varnish evaporates under a soldering iron. An alternative diagnosis setup (but only for cars without a limited-slip differential) is to jack up one rear wheel, firmly chock the other three wheels and trace the circuit with the driveline engaged. Extend the module to a comfortable working position by unsoldering the connector and temporarily splicing in an eight-foot length of 10-conductor, 24-gauge cable. The copper rivets which must be drilled out to remove the connector may be replaced with pop rivets. Note that acceleration rates under this condition will be unrealistic due to the no-load condition, and the rear wheel will turn at twice its normal speed. This setup won't work if your car has a limited-slip differential. Common failure modes include invisible solder joint defects, integrated circuit failures and transistor burnouts. If you replace any component (except a relay), the nearest op amp will probably have to be trimmed. The trimming resistors are mounted on standoffs. Unsolder the existing resistor and substitute a potentiometer for it. Adjust the circuit with the potentiometer, then measure the resistance and replace the potentiometer permanently with a resistor of closest value. Be sure to remove all slack from the throttle actuator by adjusting the plastic nut at the end of the Bowden cable. Parts Availability Parts are listed in Table 1. An asterisk after a resistor number indicates that this is a variable resistor, characteristic of the unit that was analyzed for this table. Your resistor is probably slightly different. For some units, R39 may be a 24K, and R40 may be a 1.6K. In some units, a Motorola M33O2 quad comparator is substituted for the LM2901 integrated circuit. A BD437 power transistor can substitute for the 2N5190, and a Motorola FE3004 dual-gate N-channel FET can substitute for the FE376. [JEC: Mine has a BSV81 FET in it. I don't think it's dual-gate at all but rather has a substrate connection. Found "BSV81 N-FET 30V 50mA 0.2W 100E", "BSV81 N-CH M-FET 30V 0.05A T0-72", and "BSV81 N-M-FET-d Chopper, sym 30V, 50mA" on-line as descriptions. 100-ohm N-channel depletion mode MOSFET? Is this also a 3N138?] The following transistor substitutions are available from Radio Shack: for BD437, substitute 276-2020; for BC557, substitute 276-2023; for BC238 or BC548, substitute 276-2009. The only part that may be impossible to buy is the TAA766 single open-collector op amp. This integrated circuit was manufactured by Thomson-CSF in the 1970's and has not appeared in IC Master for many years. Good substitutes are the TAA761 and TAA762, made by Siemens. Sincere thanks to Elmer Rhodes of Marietta, Georgia for his thorough analysis of the "black connector" unit and to Ken Redmond of The Benz Store for the loan of units for analysis. Figure 1: Speed sensing circuit. Figure 2: Servo amplifier circuit. Figure 3: Multivibrator and pilot valve driver circuit. Figure 4: Latchout circuits. Figure 5: Driver control and diode logic. Figure 6: Circuit board layout. Back to main article