This was the real start, the check engine light. All vehicles sold in the US were
required to have a monitoring system which would turn on this lamp if there were an
electrical failure of any emissions related component and vehicles would need to be able
to display a ìfailure codeî which would tell the repairer which system was the culprit.
Of course with the stricter emissions standards, the manufacturers would need to switch
over to modern fuel injection anyway (as it was way more efficient) and computerized
controls were necessary to control this injection. General Motors was the forerunner in
computerized technologies with the other two domestic car manufacturers only meeting
the bare minimum to abide with the new regulations. So the modern story really begins
in 1980 with a check engine light and a primitive computer in every U.S. vehicle.
These computers were pretty primitive all right, big tin boxes with edge-board
connectors which had a tendency to oxidize and cause drivability problems. I remember
well removing the computer connectors on early 1980ís Cadillac models and rubbing the
edge-board connectors with a pencil eraser. This would clean the oxidation on the circuit
board traces and restore a poorly conducting circuit. The early automotive computers
also had mechanical devices in them such as vacuum sensors which would require a
small vacuum hose to enter the computerís external case which affected case integrity.
Eventually someone got the right idea to put the vacuum sensor external in the engine
compartment with sealed wires with weatherproof terminals entering the computer cases.
We used to have whatís called the ìtap-testî which was just what it sounded like; you
tapped lightly on the computer of a vehicle with a strange engine problem. If tapping on
the computer caused the car to react, the computer was failing. This was quite common
in the eighties and early nineties until a more robust case and circuit board was designed
in 1996 with the advent of more stringent emissions laws. The new circuit boards had a
coating on them to resist corrosion of the solder joints due to vibration and the new cases
were made of cast aluminum and sealed much better.
All modules can listen in and decode any messages, though, so they can
read certain info such as engine coolant temp data which is on the bus often. A module
isnít allowed to respond unless it is addressed directly by the BCM. The BCM canít send
any other messages until it has first finished the conversation it was just having. The
receiving device will pull the system high so it is declared idle, then it will respond by
sending its own address code, then it sends its data. It will then hold the system high so it
is once again declared idle. The BCM will then be allowed to continue talking.
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