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Old 04-30-2002, 12:14 AM
Mike Murrell's Avatar
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Join Date: Feb 2000
Posts: 2,359
Automotive Electricity 101 Question

Voltage - electrical pressure
Current - flow
Resistance - created by a consumer, break in path; resistor.

Resistance is fairly straightforward. It would seem that there's a relationship between voltage and current. One volt pushes one amp thru one ohm.

Familiar with the "water pipe" scenario used to explain the difference between voltage and amperage(current).

In automotive diagnosis, one sees that tests are required for any one of the aforementioned, depending on the device/situation. Again, an ohm(resistance) test would be carried out to find a break in a circuit. Easy enough.

I'm trying to get a better grasp on voltage vs. current. Some auto tests say to test for a given voltage. Others say test for a given amt. of current. Is there a fundamental concept/rule that decides which of the two one tests for?

Thanks for your time.
Mike Murrell
1991 300-SEL - Model 126
M103 - SOHC
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Old 04-30-2002, 12:54 AM
Join Date: Nov 2001
Location: Cape Cod Massachusetts
Posts: 1,427
Lightbulb Well!

A test for voltage would be a test for electrical potential similar to a test for fluid pressure, it is a measurement that is a difference between two points, Ground and Negative or Positive measured in Volts. This measurement is made as a "Tap' into the system with one probe with the other at the ground usually the negative battery teminal/engine/chassis ground.

A test for amperage is a test that is used to determine the quantity of electrical current flowing through a circuit. This is a function of Voltage divided by Resistance or pressure and friction.

Current measurement is taken in series with the load, you usually have to disconnect a circut and place your measuring device /meter by connecting the probes to either side of the broken circuit, the current flowing thruogh this arrangement is mesured by your meter and you get a corresponding readout. There are inductive coil sensors that can be clamped around a conductor carrying a flow of current, the current flowing through the conductor will induce a proportionate current into the coils of the sensor which is either connected to a meter or may have a meter built into it. Generally clamp-on or pass-through inductive coil sensors are less accurate the cheaper they are, accurate devices are expensive and smaller currents are more diffucult to detetct accuratly then large. Most Multi-Meters or DMM have the capacity to measure 10-20 amps directly probe to probe, higher current readings usually require the use of an external add-on shunt resistor or inductive sensor.

Generally in a diagnostic scheme you will test for known values first, the expected resistance of a device or the expected value of the electrical potential that is to normally applied to any given device. If the measured value or values are within expected specs then the net step would be to accertain whether the expected current was actually measurably flowing through any given circuit. This measure of current flow is a dynamic value that changes depending on the function of the electrical device and the work it is doing.

This is my own simplistic explaination that might help. Good Luck! It's really quite simple, until it gets complicated!!!!LOL
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Old 04-30-2002, 02:25 AM
Posts: n/a
test type depends on function

try this for size:

you will test for voltage (or lack thereof) if the function under test returns a voltage to indicate a certain state


you will test for current if the function under test returns a current value to indicate a certain state.

Normally, a motor (like the window winders, aux pump, blower motor) will show a fault if there is excessive current flow (pops the fuse) whereas a voltage test will probably not show a lot (OK, I know a short on the motor winding will show 0V across the motor... but then the wiring would have smoked long time ago ...)

Anyway, remember to disconnect the device under test (DUT !) from the circuit it functions in when resistance measurements are taken.

A rule of thumb to consider:
when there is a relay or a switch or a fuse in a circuit, there should be *no* (OK, very little) voltage drop across the switching elements. A high voltage drop will indicate a high switch resistance. This will lead to less than optimum voltage to get to the part that needs it. If you end up with a voltage drop approaching 12 or 13V you have an open circuit (i.e. you test across a popped fuse).

All motors should draw less current as per fuse rating for the circuit and the voltage drop should be close to 12V across the motor (unless you are losing voltage across a bad switch, too thin wiring or a burnt relay contact).

Relays should draw little current when energised and the resistance across the coil should be rather low (couple hunderd to 3000 ohm). As said, voltage drop across a relay's contact should be minimal.

The speed sensor on some AC compressors delivers an alternating voltage that can only be measure with a scope. Testing for current on this device will get u nowhere fast.

hope 101 made some sense. I have some test methodology on soft copy if u need more info

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