Over the span of time, I’ve added speed control to a number of vehicles, including the following cars:
The Soul Survivor (1974 Mustang II)
A 1985 Mazda RX-7
The El Kylemino (1987 El Camino)
Our 1991 Mazda Miata
A 1992 Isuzu Rodeo
In every case, I installed the factory system using recycled parts. This requires an understanding of cruise control systems so I could gather the needed parts, and then troubleshoot the system after installation (when needed). Therefore, let’s look at cruise control system designs and review how the technology has evolved over the years.
To add cruise control to any car, you really only need three new parts- A vacuum actuator, a control box, and an ON/OFF switch for driver control. The system needs additional switches and sensors, but in many cases these parts are already present in the car, and the cruise control system just “piggybacks” on their existing functions.
During operation, the control box monitors vehicle speed and commands the actuator to open and close the throttle to match the desired speed.
It also monitors the brake system, usually the brake light circuit, a speed input, and a redundant safety system connected to the brake system to ensure the system shuts down anytime the brakes are applied.
More sophisticated systems add connections to the neutral switch on automatics or the clutch pedal position switch on manual transmissions. This feature keeps the engine from over-revving should the driver disengage the clutch or move the transmission selector to neutral.
Not all manufacturers ponied up for the switches and wiring needed track the transmission status on early systems. I remember my knee knocking the transmission selector lever on my Dad’s ’76 Mercury Monarch into neutral (at speed on I-80), and discovering the engine revved pretty darn high with an open throttle and no load… Nowadays, due to both safety concerns and available computerized data, pretty much all systems monitor these inputs.
Because engine control systems in the seventies did not monitor vehicle speed (or much of anything…), early cruise systems also included a built in speed sensor. The electronics were also less sophisticated, so these systems often used mechanical safety systems rather than additional electrical inputs to the control box.
This picture shows a typical early control unit as used by GM. Mounted under-hood, it connected to the speedometer cable for the speed input. To run the speedometer, a second cable ran from from the control unit to the gauge assembly. The control unit also included electrical connections, a hose to an engine vacuum source, and a vacuum line to the throttle actuator. Another vacuum line connected to a brake pedal dump valve, which bled off vacuum anytime the driver stepped on the brakes.
Over time, Cruise Control systems have evolved to make greater use of electronic inputs and sensors. The control box and actuator may be in the same location, or may be separate. Starting in the mid eighties, vacuum actuators started to disappear, replaced by motor driven actuators. But the systems still used the same three basic components, until drive by wire throttles became the dominant under-hood technology.
With the advent of drive by wire systems, the Powertrain Control Module (PCM) now monitors vehicle speed and brake inputs, and uses an actuator to control throttle position. Because of this, a drive by wire system only needs a Cruise Control switch connected to the PCM. At least in theory. It’s not always that simple, but I’ll cover things in detail when we get there.
Over the next few posts, I’ll review what it took to install cruise control in my cars, starting with the Japanese models. In all my cars, I installed the complete factory system, including all needed safety systems. From there, we’ll talk about the domestic systems I’ve installed, and wrap things up by looking at an aftermarket cruise system that piggybacks onto the drive by wire system in my 2009 Honda Fit.
I doubt many people will follow my lead and install cruise control systems on their personal cars, but you may find the specifics on each system interesting, and could pick up a few tips that will come in handy down the road.