difference between voltage drop and bias on control wire

Hello everyone .

i am still in section 3 materials and found some confusion to distinguish between voltage drop and bias on control wire . when i make my test when i can say its voltage drop not a bias ?

Thanks .
Ali

A voltage drop is when the voltage that reaches the load is less than the source voltage. This can happen due to bad connections and corroded wires. The bad connection, or corrosion, acts like a resistor.



Bias voltage is a voltage sent out on a circuit, for fault detection purposes. It is passed through a very high resistor. It is, for lack of better wording, an INTENTIONAL voltage drop that the computer uses. When a circuit is OPEN (disconnected/broken wire), the voltage will be the same as the source of the bias voltage. But, when the circuit is CLOSED (connected/circuitry intact) the voltage will be pulled up, or down.

I have a question on this topic. If the control module uses the bias voltage to detect an open circuit on the coil circuit and possibly illuminate a CEL if it is open how does it see any difference between an open circuit or a completed circuit that is energized since it would see 12v in both cases.

In most cases it sends a pulsed signal down the wire,if the wire is broken the return signal is not what it wants to see,it also knows if it is pulled to ground or shorted to voltage,
I maybe wrong, someone will correct me no doubt,

Thomas Klein wrote: how does it see any difference between an open circuit or a completed circuit that is energized since it would see 12v in both cases.

Generally, the control module does not supply power to the coils. Power comes from a fuse, or relay, and there is not a bias on the POWER side. The bias is on the CONTROL side. When energized, the control side should be, nearly, 0v. When not energized, the control side will see the 12v POWER, coming through the coil windings, unless there is an OPEN in the circuit. With an OPEN, the module will see bias voltage. With a short to GROUND, the bias will be pulled to ground.

I'd just like to add my perception to bias voltage. I view it as the computer's internal ohm meter in action.
If you have two multi meters, set one to ohms and one to volts.
Measure the voltage of the ohm meter. This voltage (which varies from meter to meter) is essentially your bias voltage.