Electrical load test method for 12v circuits.

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Tirediron

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I mentioned electrical circuit load testing in another thread, the reason for load testing is that a corroded wire will show power with a normal test light and continuity with a multi meter, but will often drop off or not carry enough wattage under load. This test is for low load circuits, for starting circuits see SheepDog's post below
the test unit is a 7 inch round 3 prong headlight bulb, most 1970 vehicles and some newer heavy trucks other 35/65 watt lights will also work, If the circuit in question is power or ground, simply connect the low beam terminal of the light to the circuit at the suspect end and the ground terminal to the system ground,(power if testing ground circuit) low beam provided s around a 5 amp load, high beam about 10, combined 15 but don't load both for long. Testing wires requires a 12v power source, usually a battery and the procedure is about the same, battery + to one end of circuit, headlight to other end as described above, and a good 14 gage wire to go from the battery - to the headlight - if the circuit is remote. this same tester is handy for finding low load shorts in powered circuits, as long as the load is less than the 10 or 15 amp capacity, if the light shines bright you probably found your short. . the parts store should have a replacement plug for the headlight that you can solder some terminals or gator clamps too to make testing quick and clean cotter pins make good probes for connector blocks etc, and they com in all kinds of sizes
 
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SheepDog

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Tirediron,
If you measure the voltage drop across the conductor you get good results and it only takes a volt meter. Wires and connection points should have less than 1/2 volt from one end to the other. If you find a wire that has more voltage between the two ends it is bad. If there is more than 1/2 volt between a connector and the terminal the connection is bad. I did this for a living and here is an example of how to do it:

Checking the Starter and starting circuit power components:
Using a volt meter (multimeter on volts) place the red lead on the Battery positive post (not the clamps) and the black lead on the negative post. While cranking (or trying to) you should read more than 9.5 volts. If it is less then you have a bad battery - charge or replace it. Next put the red lead on the battery positive post and the negative lead on the positive battery cable clamp. While cranking the engine (or trying to) you should have less than 0.5 volts showing on the meter - more than this means the connection is bad clean and retry the test. If you get less than 0.5 volts then move the black lead to the the end of the cable at the relay (solenoid) and measure the voltage again. It should be less than 0.5 volts. If it is more then the cable is bad - replace it. If it is less than 0.5 volts then move the black lead to the large post the cable is connected to. test the volts again (always while cranking the engine or trying to) . It should be less than 0.5 volts. the next check is to move the black lead to the other large post on the relay and repeat test. Then to the cable end connected to the relay, then to the cable end at the starter then the post on the starter. These tests should all show less than 0.5 volts. Any component that is over 0.5 volts is bad or corroded. If it can be cleaned then clean it and if it is a wire then replace it.
From the negative side place the black lead on the negative post and the red lead on the clamp. Crank the engine (or attempt to) and check the voltage. As before it should always read 0.5 volts or less. for each successive test move the red lead to the next connection and retest. Keep going until you get to the engine block.
These tests are called "Voltage drop tests" and are used to identify any open, resistive, or shorted connections. They are the easiest way to determine bad components without random replacement of components which is time consuming and expensive.
 

Tirediron

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SheepDog,
I agree with the voltage drop test, on a higher load circuit, but unless the voltage tester is very accurate and operated by experienced "eyes" finding bad circuits is questionable, especially for a DIY person, And as you know Electrical troubleshooting is "easy" if you have the skill. and black magic if you don't. thanks for posting your method.
The "headlight load tester is more suited to low load and automotive electronic circuits. (please not that the sensor or actuator should not be part of the test)
I always load test both ECM power and ground on roadside rescues before I drop my scissor neck to winch a dead electronic Highway tractor on to the trailer.
 

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