Seeing as I get asked whats the quickest way to check a circuit on any electrical component almost daily, I thought i'd do a short how to do and why here.
What is volt drop ? its when a circuit has a high resistance on a connection point
What does volt drop tell me? it tells you by the meter reading the state of the connection, Voltage will always follow the path of least resistance so if the connection being tested is dirty then some voltage will flow through the meter and show a reading
Can I just test any connection for volt drop? Yes but the rule is you cannot test for volt drop on any circuit unless there is a load present, for example a simple 2 terminal switch when operated by switching the circuit on you can measure volt drop between the 2 terminals. You set your meter to 20v and place the red meter terminal on one switch terminal and the black on the other
The reading you get will indicate the state of the connection, ideally you would like to see a reading of 0.01-05v, if its any higher than 0.05v then strip the connection and investigate why. In extreme cases where the connection is bad then you can have battery voltage on the terminal and when put under load the voltage dies away to 1-2v in that instance the volt drop on the meter would be 10v, and thats not good for the meter to be taking 80% of the voltage if its a high current circuit, so if you tested a connection with the meter Red probe on the feed in/out and the Black probe to earth and under load it dropped away to 2v there is no need to do a volt drop test across the terminals
How To Test the Battery Connections & Charging Circuit
Set your meter to 20v
Place the Red meter lead on the + battery post, the Black lead on the actual terminal and get someone to crank the car over note the reading on the volt meter, 0.05v is the max any more you have a high resistance between terminal and connection
You can repeat this along the + battery cable all the way to the starter solenoid and onto the terminal at the back of the starter
you can repeat the test on the - battery connection to earth and all the points along the earth connections of the car
To Test The Charging Circuit For High Resistance
Set the meter to 20v
You will need a good quality extension test cable of approx 6.0mm 84/0.30mm Thin Wall Cable for testing a vehicle with the battery in the boot or a VW Beetle
Place your extension test lead onto the battery + Connection (Important Its Not On The Negative as that a dead short in this test) and connect the end of the test extension lead to the Black terminal of the voltmeter, you place the Red connection onto the B+ terminal at the back of the alternator, either in the alternator plug or the B+ stud
You run the engine at 2000rpm and turn everything on, headlights heater fan will do, now check the volt drop on the meter, any more than 0.05vmax you have a high resistance somewhere on the circuit your testing, or your battery main cable is to small.
You can test any circuit or connection like the connections into and out of the bullet connectors, switches, light bulb holders, multiplugs and main feeds and earths
As were talking classics then one place I test first where its not cranking related is the Ignition Switch itself.
Due to the age of the switches now being 20/30/40 yrs old I see a lot of switches with a volt drop of 2volts or more due to worn/dirty connections internally , to test for volt drop on a Mini Ignition Switch you locate the Brown main 12v feed into the switch, place your Red volt meter lead on this point, you place the Black volt meter lead on the White Ignition Switch Switchable 12v out terminal your coil will provide the load in the circuit (4-5amps) and take the meter reading, 0.01 is ideal, 0.05v is the maximum, if you see anything upwards of 0.05v then you have dirty/burnt Ignition switch contacts and you should replace the switch as the volt drop and resistance will only increase leading to a breakdown
The 3 places I check first are
1.The main battery connections
2.The fusebox connection across the fuse including the riveted connections on the rear
3. The ignition switch as mentioned above.
The Golden Rules
0.05v is the max volt drop
You can only check for Volt Drop on a live circuit with load present
If you check both sides of a circuit with a Bulb between there will be a volt drop as the bulb is a resistance and thats why it glows !
Edited by KernowCooper, 22 October 2015 - 10:22 AM.