Sensors in general

Tiny
PGAILEY
  • MEMBER
  • 1998 CHEVROLET SUBURBAN
  • 188,000 MILES
My question is about sensors and how they transmit data. For instance to a GPS system, or a remote diagnostic based system.

If I were to create a new sensor for my vehicle that performs a specific reading. For example a new feature that needed to be monitored. How would I be able to tap into the computer to transmit my data in real time?

For example: GM has OnStar that will transmit when there is a car crash, or some kind of an incident. How does that work?

Regards,
Peter
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Thursday, May 2nd, 2013 AT 2:34 PM

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Tiny
CARADIODOC
  • EXPERT
It sounds like you're imagining the sensors are like home computers all tied together on the internet and you can just plug in another computer. Car systems don't work that way. Every sensor is connected to just one of the many computers on the truck, and every computer requires a sensor to be connected to the right pins. Those are the inputs the computer uses to monitor operating conditions. There are no blank pins to connect additional stuff to later.

There are dozens of computer modules on your truck and they all share information over two wires called the "data buss". Typically traffic on the buss is controlled by the Body Computer. The computers take turns transmitting digital information onto the buss. It goes to all the other computers, and they decide what information to use and what to ignore.

For example, the vehicle speed sensor is an input for the Engine Computer. It modifies fuel metering based, in a small part, on its readings. The Transmission Computer, (when used), needs to know vehicle speed to calculate shift points. The Traveler Computer, (when used), uses that information to calculate miles per gallon to put on the display, and the radio has to know speed for its "speed-sensitive volume" function. There's at least four computers that need to know vehicle speed, but there are not four speed sensors on the truck. The Engine Computer will transmit speed, throttle position, coolant temperature, and dozens of other things onto the data buss. The radio doesn't care about engine temperature so it ignores that but it does look at speed. All the other computers look at that digital information and decide what to use and what to ignore.

When the Engine Computer is done transmitting its information, another computer transmits its onto the same two wires to share what it knows or to tell the other computers what it is doing. Without that data buss every computer would have to have two wires going to every other computer. The GM engineers have gotten so carried away with the use of inappropriate technology that some of their cars can have up to 47 computers! Most new cars already have over two miles of wire in them. Now imagine that multiplied many times.

OnStar is just one more computer, and a very disturbing one. It shares the same data buss so it knows your speed and location. One of its inputs is the antenna which is a direct link to the company, which can be a good thing, but it's also a direct link to the government. Now they know if the Check Engine light is on and you're polluting excessively and ignoring it. They know where you went and how fast you were going. I don't want that kind of intrusion into my privacy so I will never own any vehicle with that system.

You can add other sensors and monitoring equipment but there is no way to incorporate it into the truck's network if it wasn't designed for it. It's not as simple as plugging in a variety of items into a USB port on your home computer.

The exception is some of the "chips" people install to increase power and fuel mileage. Those do connect to the data buss and read some of the engine-related data. The more expensive ones have displays that show the ignition timing advance and a few other things, and they work by sending out digital signals that change what the computers think is coming from a sensor. Basically the sensor is still sending correct information but the chip interprets it, then sends out a lie to the other computers. The Engine Computer responds by dumping in more fuel or advancing the ignition timing more than normal. Usually the trade-off is increased emissions. What they also do not tell you is if you have a Traveler Computer that displays instant and average fuel economy, those will read too high. On my buddy's Dodge dually diesel he was getting 22 miles per gallon and the display was correct, and with the chip he gets 24 mpg but the display shows around 28 mpg. That's because the Traveler Computer is basing its calculations off false information provided by that box.

The cheaper add-on mileage and power boosters just plug into a sensor and change its reading. Most commonly it plugs into the intake air temperature sensor and makes the computer think the air is colder than it really is. As with a choke on older carbureted vehicles, the computer will command more fuel so you think you have more power.
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Thursday, May 2nd, 2013 AT 3:37 PM

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