Questions and Answers

VSS And Can-Bus Q. Dear Mr. Ciulla, Hi, my name is Steve and I live in the UK. I have been involved in the distribution sector for the last 25 years and it was boring me senseless, Mid life crisis, plus be true to thyself. I have always done my own auto repairs and was involved with the trade many years ago. I have recently been taken on by a company here in the UK that is marketing cruise control systems.

I find it quite easy to do the installation, it is the electrical part that I am having trouble with. I have to tap the vehicle speed signal, and I have tried to learn as much as I can about the different types of transducers. I was wondering whether you know of a resource or where I can get information on this particular area of vehicle electrics as I need to get a good understanding of the area of speed pulses?

I have also started to learn about Can-Bus . Even though new vehicles use this Can-Bus system, am I right in thinking that there still must be a transducer in the system? By the way, I notice that it is much easier to get technical information in the US.

Regards,
Steve

A. The Vehicle Speed Sensor (VSS) is a pulse counter type input that informs the PCM how fast the vehicle is being driven. The VSS system uses an inductive sensor mounted to differential housing and a toothed reluctor attached to the ring gear. As the reluctor rotates, the teeth alternately interfere with the magnetic field of the sensor creating an induced voltage pulse. The VSS produces an AC voltage signal that increases with vehicle speed. The PCM processes this signal and sends it to various components.

There are some VSS that open and close a circuit and the PCM counts these and translates it to actual vehicle speed. But I don't know of anyone who uses this type of VSS anymore. Some vehicles, Later Chrysler products for example, do not use a VSS such as we know them. On Chrysler trucks the speed signal comes from a speed sensor inside the differential. Others use transmission main shaft speed sensors. Since everyone does the same things in different ways, it's hard to generalize the VSS. The best way to locate a place to tap into the VSS signal is with a wiring diagram for that particular car. In almost all cases, even if the component is not installed in a car, the wiring for it is and that should make your job somewhat easier.

Can-Bus, CCD, K-Bus and CAN Databus are all the same. They network all the control units and modules exactly like any other computer network. In the bus system the individual control modules are interlinked and can exchange data.

The signal from a sensor is transmitted to the nearest control module, processed by it and transferred to the data bus. Each control module, which is connected to the data bus, can read the signal, calculate a manipulated variable from it and activate an actuator or device.