Curbside Tech: Factory Cruise Control Part 3

I feel the need, the need for speed (input)

Yep, more on cruise control design, theory, and installation. I touched on speed inputs in Part 1, but today I’ll dig further down, with a focus on installing a speed control system on an older vehicle that never offered factory cruise.

A salvaged Cruise Control System, mostly ready to plug and play in an ’89 Camaro

As I discussed in last week’s post, if you’re salvaging a stock factory system out of the same model as your car, the speed input is defined by design, and your solution is a simple exercise in plug and play.

Roader’s aftermarket installation- Nice work, but spinning magnets on a driveshaft makes me nervous…

However, if you’re installing a speed control system into an older vehicle that never came with cruise, or modifying a system due to drive train changes you’ll need to get creative. There are aftermarket pick up systems that add magnets to the driveshaft and use a pick up on a bracket to track speed, but they have the potential to negatively affect drive line balance and driveshaft movement can affect the speed reading (although CC reader “Roader” successfully used the setup shown here).

An inline sensor- The drive cable functions normally, and the wire provides a speed signal

Almost all older vehicles used a cable driven speedometer, so a nice clean option is this aftermarket speed sensor mounted inline with the drive cable.

Several mounting location options- Red by the transmission, Green behind the speedometer, Blue behind a RHD speedometer

When using this option, I prefer to install the sensor near the speedometer, to isolate the sensor from under-car dirt and avoid clearance issues with the transmission tunnel.

Early Ford Vehicle Speed Sensors (VSS) included a mechanical speedometer drive. GM, not so much…

In some cases, the manufacturer built a speed sensor into the drive cable output at the transmission. This gives you OEM fit, quality and reliability, but this type of combination sensor was only used for about ten years during the mid eighties.

Analog vs Digital Signals

When choosing an inline speed sensor, you definitely need to match up the sensor output and speed pulses with the controller you’re using. All sensors create pulses to track the vehicle speed, but some controllers look for an A/C or sine wave to create the pulses, while others want a square form digital signal. I’m not going to delve into electrical theory here, so you may need to do further research to better understand the differences between these two electrical signals.

In addition to differences in wave forms, the number of signal pulses per mile vary from system to system. The two most common pulse counts are 4000 pulses per mile, or 8,000 pulses per mile. If you use the wrong pulse count, the cruise control system may not work as desired.

A VSS output on an oscilloscope display- In this case, a digital pulse

Unfortunately, it’s very difficult determine wave form and pulse count without an oscilloscope, but certain sensor design factors can help you determine the correct wave form and pulse count.

Typical three wire sensor- The sensing element can be optical instead of Hall Effect

If the sensor uses three wires (power, ground, and signal), you’ve got a digital signal VSS. Two wire sensors USUALLY make an analog signal, but some two wire sensors generate a digital signal using a magnetic reed switch.

Typical reed switch- Mechanical contacts moved by a magnet (not shown)

To identify a reed switch sensor, measure the resistance between the two wires while slowly turning the sensor drive shaft. If the resistance flops back and forth between open and closed (continuity and no continuity), it’s a reed switch. If not, assume it’s an analog VSS.

You can also roughly determine pulse count by measuring the sensor output. If a DMM shows 4 pulses per revolution, it’s PROBABLY a 4,000 pulse per mile sensor. Therefore, 8 pulses per rotation, tells you it’s an 8,000 pulse per mile sensor.

Lots of DRAC articles on the interwebs…

There are also black boxes available that will modify speed pulses, including a GM device called a “DRAC” or Digital Ratio Adapter Controller. I may touch on these in future postings, but they’re rather advanced for this discussion.

OK, we’ve covered the major issues related to installing cruise control in any car- Speed sensor location, waveform type, and pulse counts per mile. However, we really ought to include electric speedometers and engine swaps in this discussion, since they go hand in hand with cruise control speed inputs.

Mechanical speedometer on the left, Electronic on the right (Image thanks to

Electronic speedometers use a speed signal instead of a cable, and became common in the 1980’s for two reasons. First they eliminated the cable, which wears out, squeals, and cause “jumpy” speedometers. Second, most fuel injected engines  needed a vehicle speed sensor, so the speed signal was now present somewhere on the car. Because the electronic speedometer became the go to option, most modern transmissions don’t include a connection or drive gear for a speedometer cable.

Modern VSS- No gears, no cable drive…

There’s a couple of solutions if you install a newer drive line and the transmission does not have a cable drive. First, you can buy an aftermarket electronic speedometer and mount it in place of the mechanical speedometer. This option assumes you can find a speedometer with the same general shape as the factory speedometer, which can be a problem.

Proof of concept- One of my G-body brethren modified a mechanical Monte Carlo speedometer to use an electronic drive (the green circuit board in the right hand image)

Second you can replace the mechanical drive of the OEM speedometer with an electronic drive mechanism. This maintains the factory appearance, but requires serious fabrication skills. It may be fairly simple, since electronic speedometers in the eighties were an evolution of the mechanical units, so you may find a mechanism pretty close the cable drive mechanism in your original speedometer. But once again, matching the pieces can be a problem.

The magic box- Not a cheap solution, but very elegant

The third option is a fairly recent innovation, and a pretty cool solution. Several companies now offer a black box containing a motor and a speedometer cable connection. Based on a speed sensor input, the box drives a speedometer cable connected to the mechanical speedometer. Through careful calibration, this magic box can display the correct speed in the original (cable driven) speedometer.

Well, that wraps up today’s post. I’m headed to Lake Powell this afternoon for a week on the water, so I’ll check back with you in two weeks.

Thanks, D/S