OnBoard Diagnostics

OnBoard Diagnostics: Also known as OBD. Mandatory since 1981 on U.S. cars.

There are two kinds of OBD systems. OBD1 is for cars made from 1981 to 1995. OBD2 is for 1996 and newer vehicles. Since the first one falls within the smog-era, that’s what we’re covering here, through ’83.

General speaking, OBD is a computer system that runs your engine for lower emissions. Several components entail the overall system which are…

Control Module. This is the computer brain of the system which make calculations and adjustments to the carburetor and distributor, based on what it sees in sensors. It has a full name of electronic control module (ECM), or powertrain control module (PCM). These units are mounted in a cool dry spot, such as inside fender or in an interior kick panel. The modules also store trouble codes that can be deciphered on the check engine light (see next).

Check Engine Light. Also known as the Service Engine Soon Light. When this illuminates, it means there’s a trouble code stored in the module ready to tell you what sensor or solenoid is defective.. By counting the flashes you can decode the issue. GM vehicle codes can be read by shorting two specific terminals on the OBD port under the dash. Fords back in the day were tripped by using a vacuum pump on a certain hose under the hood. Chryslers were tripped with a series of on-off motions with the ignition key.

Feedback Carburetor. These carburetors facilitate being adjusted by the module. There is a mixture control solenoid inside them that the computer can move around to make adjustments.

Feedback Distributor. Like the carburetor, this is a distributor that the module can make adjustments on, though they’re regarding spark timing here.

Throttle position Sensor. Also known as the TPS sensor. This sensor is inside the carburetor. It tells the module how much accelerator input is being requested.

Coolant Temperature Sensor. This sensor is located in the thermostat housing or one cylinder head. It tells the module how hot or cold the engine is.

Manifold Absolute Pressure Sensor. Also known as the MAP sensor. This sensor is located on top of the intake manifold usually off to one side or the other. It tells the module the amount of vacuum that the engine is producing.

Oxygen Sensor. Also known as the O2 sensor. This sensor looks for the amount of oxygen inside the vehicle’s exhaust so the module knows to richen or lean out the fuel mixture.


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