All Gen III vehicles were available with cruise control. Engines with a cable-actuated throttle body require an electronic cruise control module with an internal stepper motor to pull and release a throttle cable to maintain vehicle speed. Engines with an electronic throttle body rely on the TAC module to increase and decrease throttle blade angle to maintain vehicle speed.
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Cable Throttle Systems
Early cruise control systems took control of the throttle blade angle through a vacuum-operated actuator that pulled and released a throttle cable. The actuator relied on engine vacuum for operation. Early cruise control systems have a lag response to driver commands. Making cruise control performance worse is an aged diaphragm that has developed a leak and no longer operates properly.
Electronic throttle systems use a TAC module (right), while cable throttle systems use a cable-driven cruise control module (left). The TAC module is wired to a motor mounted within the throttle body assembly and uses PCM data to change throttle angle to maintain cruise control vehicle speed. The cable throttle cruise control module shown here also uses PCM data to maintain cruise control vehicle speed, but must manually pull the throttle linkage to change throttle angle.
Newer cruise control systems abandoned the vacuum actuator for an electronic module with an internal stepper motor to pull and release a throttle cable to maintain vehicle speed. These systems have a better performance feel than the old vacuum-type systems. The required components of the cruise control system, however, have not changed much.
Required Components
To install a GM cruise control module for your cable throttle system, several components are required. Most of them, such as cruise control switches, clutch switch, and brake switches, may already be in your car. Any missing components can either be sourced from a salvaged GM vehicle or through the aftermarket.
Cruise Control Module: The cruise control module for cable throttle vehicles is a relevant discussion for Gen III PCMs because of the PCM’s availability of primary and secondary VSS outputs that may be used to satisfy the cruise control module’s 4,000-pulse-per-mile input requirement.Also, the 1999–2002 Camaro/ Firebird cruise control modules require cruise inhibit and cruise enabled status communication signals with the PCM .
Early GM systems used a vacuum-controlled actuator to pull a throttle cable. These systems require a VSS (most commonly a 2,000-pulseper- mile signal) to maintain cruise control vehicle speed. Rather than using an internal motor like the newer cable-driven cruise control modules, these early units use engine vacuum to control a diaphragm that pulls a throttle cable. The result is a lazy response to the driver’s cruise control commands.
This 1996-newer GM cruise module assembly contains a circuit board for processing cruise control logic, an internal motor to operate a worm-driven gear, and a short braided strap to pull the throttle cable. This cruise control module uses PCM data to control the movement of the throttle cable when cruise control is active.
This Rostra cruise control switch (# 250-3020) may be used in a GM application for on/off and set/coast signals. The aftermarket offers several types of cruise control switches for different steering column configurations. If you cannot identify one to suit your application, you could make your own set of dash or center console mounted switches. With an on/off switch, two momentary switches, and little creativity, you can create an inexpensive cruise control switch solution.
Cruise control modules compatible with Gen III conversions are plentiful. You may choose from cruise control modules found in 1996- newer Vortec trucks and LS series vehicles. The cruise control module receives signals from several switches and, when necessary, takes control of throttle blade angle through a cable attached to the engine’s throttle body. Installation is rather simple. The most difficult part may be adapting the cruise control throttle cableto your throttle body’s linkage if the cable end does not match.
PCM: In production vehicles, the PCM is used to output a 4,000-pulseper- mile VSS signal to the cruise control module. This is a common pulse count used with many early GM ECMs and VSS buffer boxes. An optional cruise engaged status signal is available for communication between the PCM and cruise control module. For retrofits using a 1999–2002 Camaro/Firebird cruise control module, the cruise engaged status and cruise inhibit signal wires must be used.
VSS Signal: The 4,000-pulse-permile VSS input to the cruise control module is required, but the PCM is not required to provide it. Retrofit projects sometimes introduce VSS buffer modules to receive a VSS signal that is not compatible with the Gen III PCM and to provide several VSS outputs for devices such as the speedometer or PCM.
Not all aftermarket cruise control switches provide the signals required by the GM systems. This Rostra switch does not provide an independently pulsed 12V signal fo the resume/accel function. In such cases, only the on/off and set/coast switches may be used.
1996-newer GM cruise control module schematic looks like this. Notice the PCM cruise status and cruise inhibit signals are only required for 1999–2002 Camaro and Firebird cruise control modules. In general, the cruise control module requires 12V switched ignition, ground, VSS, cruise control switches, normally open brake switch (stop lamp switch), normally closed brake switch (commonly a torque converter clutch switch), and a manual transmission clutch switch (if so equipped).
This schematic represents a typical GM cruise control switch wired to a TAC module. The TAC module generally requires 12V switched ignition, ground, PCM serial data circuits, cruise control switches, normally open brake switch (stop lamp switch), normally closed brake switch (commonly a torque converter clutch switch), and a manual transmission clutch switch (if so equipped).
A VSS buffer module may also be used to provide a 4,000-pulse-permile VSS signal to the cruise control module. Remember, however, the Gen III PCM has two configurable VSS outputs to choose from that provide a 4,000-pulse-per-mile signal dedicated to the cruise control module. (See Chapter 15, Project 1, on page 112 for setting the 4,000-pulseper- mile VSS signal with EFILive.)
Cruise Control Switches: The required cruise control switch signals multifunction lever of GM steering columns. The on/off signal is normally open switched 12V, the set/ coast signal is momentary 12V, and the resume/accel signal is momentary 12V.
Rostra Precision Controls offers a variety of cruise control switches that may be used if your steering column’s multifunction lever does not have the necessary cruise control switches. (See Chapter 15, Project 1, on page 112 to see an early GM steering column with only a set/coast cruise control switch.)
Brake Switch: The cruise control module requires two brake switch signals. A normally closed 12V switch applies power to the clutch/brake switch input (pin D) and a normally open 12V switch applies stop lamp voltage to the brake input (pin G). These signals are used to disengage cruise control when the brake pedal is depressed.
Clutch Switch: If using a manual transmission, a normally closed clutch switch should be wired on the same circuit as the normally closed brake switch. Depressing the clutch switch removes 12V from the cruise control module (pin D) to disengage cruise control.
Operation Overview
With the exception of the 1999– 2002 Camaro/Firebird cruise control modules, the 1996-newer truck and 1998 Camaro/Firebird cruise control modules can be used as standalone units. These modules require 12V ignition, ground, cruise control switch signals, brake switch signal, clutch switch signal, stop lamp switch signal, and a 4,000-pulseper- mile vehicle speed signal. Once installed in a retrofit vehicle, the performance is improved compared to early cruise control systems operated by engine vacuum.
Set/Coast: To set cruise control, the cruise control module requires a 12V switched-on signal and a 12V momentary set signal from the multifunction lever. With the 12V on signal present at the cruise control module and the vehicle at a desired speed, pushing the set button adds the vehicle’s current speed to the cruise control module’s memory.
The driver may remove his or her foot from the accelerator pedal and the cruise control module overrides the throttle blade angle to increase or decrease vehicle speed. Pressing and holding the set/coast button causes the vehicle to coast to a reduced speed. Releasing the set/coast button stores the new reduced vehicle speed in the cruise control module’s memory. Quickly pressing and releasing the set/coast button subtracts 1 mph from the cruise control module’s memory.
Resume/Accelerate: Once cruise control is active and the driver depresses the clutch or brake pedal, the cruise control module no longer controls throttle blade angle. The previously set vehicle speed remains stored in the cruise control module’s memory. Sliding the resume/accel switch requests the cruise control module to take over throttle blade angle and accelerate the vehicle to the previously set vehicle speed.
Sliding and holding the resume/ accel switch increases vehicle speed until the switch is released. The new vehicle speed is stored in the cruise control module’s memory. Quickly sliding and releasing the resume/ accel switch adds 1 mph to the cruise control module’s memory.
If the cruise control on/off switch is set to off or the vehicle is shut off, the cruise control module’s memory is erased and sliding the resume/accel switch does not return the vehicle to a previously set speed.
Erasing Cruise Speed Memory: When the driver disengages cruise control by depressing the clutch or brake pedal, the cruise control module retains its memory and allows the driver to use the resume/accel switch to resume the vehicle speed stored in memory. To clear the cruise control module’s memory of vehicle speed, turn off the on/off switch or shut off the vehicle’s ignition switch. This removes 12V power from the cruise control module and clear its memory.
Disengaging Cruise Control: The cruise control module retains the set vehicle speed in its memory until it loses 12V power through the cruise control on/off switch or the vehicle’s ignition switch. To disengage (but not disable) the cruise control module’s control of throttle blade angle, gently apply the clutch or brake pedal to momentarily remove 12V power from the cruise control module’s brake/clutch signal input.
To disable cruise control, slide the cruise control on/off switch to the off position.
Inhibit Signal
Be cautious of cruise control modules from 1999–2002 Camaros/ Firebirds. These cruise control
modules are “more intelligent” than truck modules. In the 1999–2002 Camaro/Firebird, the PCM may inhibit (prevent) cruise control operation based on vehicle speed, park/neutral switch signal, engine RPM, or battery voltage. If the PCM commands the cruise inhibit signal to the cruise control module and the cruise control module returns an active cruise engaged status signal, the PCM sets a DTC related to the cruise control system.
Most people find this extra layer of protection unnecessary and prefer a cruise control module from a 1996-newer truck or 1998 Camaro/ Firebird. The 1998 Camaro/Firebird PCM cannot output a cruise inhibit signal, so the 1998 Camaro/Firebird cruise module functions the same as the 1996-newer truck modules.
Identifying a 1999–2002 Camaro/ Firebird cruise control module is rather easy. All ten harness connector cavities are used. Vehicles with the cruise inhibit signal require its use when installing the cruise control module in a retrofit vehicle. While you likely do not see a wire in connector
cavity H for any other vehicle, a quick visual inspection of the wire harness shows whether the cruise inhibit signal is required.
Electronic Throttle Systems
All Gen III vehicles with an electronic throttle body rely on the TAC module for cruise control operation. During normal engine operation, the TAC module is used to increase and decrease the throttle blade angle. By using the available cruise control switch inputs, the TAC module can respond to the driver’s cruise control commands to maintain vehicle speed.
Being fully integrated into the electronic throttle system, the driver’s cruise control commands are instantaneously applied to the throttle and the feel of acceleration and deceleration is smooth and seamless.
The switches and inputs are very similar to those of cable-operated cruise control modules.
Required Components
It’s likely that after an electronic throttle installation your vehicle will have all of the components necessary for integrated cruise control. By using the TAC module’s cruise control switch inputs and brake/clutch switches, any retrofit vehicle can be ready for cruise control. The following switch signals are required by the electronic throttle system for cruise control operation.
Cruise Control Switch: The required cruise control switch signals are no different than those for Gen III vehicles with cable throttle body and cruise control module. The on/ off signal is normally open switched 12V, the set/coast signal is momentary 12V, and the resume/accel signal is momentary 12V. If your retrofit vehicle does not have a multifunction lever on the steering column with cruise control switches, you can find them in the aftermarket.
Brake Switch: The cruise control system requires two brake switch signals. A normally closed 12V switch applies power to the TCC brake switch input of the PCM and a normally open 12V switch applies stop lamp voltage to the stop lamp input of the TAC module. These signals are used to disengage cruise control when the brake pedal is depressed.
Clutch Switch: If using a manual transmission, a normally closed 12V clutch switch must be wired to the PCM (blue connector, pin 35). Depressing the clutch switch removes 12V from the PCM’s clutch switch input to disengage cruise control.
Written by Mike Noonan and Posted with Permission of CarTechBooks
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