no brake lights
06-30-2019, 06:27 AM
#16
CF Veteran
Under load meaning the circuit has a power draw and is under load. When checking for voltages at a connector you can easily find +12v but if you pull a current draw through it, it can drop to 0V.
I recently traced fault with tow wiring which had me stumped. At the relay I was reading +12v so at the time I thought the power source was good. It turned out that when you tried to pull power through the relay to power the tow lighting, everything died. In the end there was a bad connector under the van which had corroded causing the issues. The point being, under no load you can get false readings with a multimeter as they can't always show a true reading of a circuit under load. Had I of used my test light, I would have found the issue much quicker.
Hopefully that makes sense, it's 1:26am and I'm off.
I recently traced fault with tow wiring which had me stumped. At the relay I was reading +12v so at the time I thought the power source was good. It turned out that when you tried to pull power through the relay to power the tow lighting, everything died. In the end there was a bad connector under the van which had corroded causing the issues. The point being, under no load you can get false readings with a multimeter as they can't always show a true reading of a circuit under load. Had I of used my test light, I would have found the issue much quicker.
Hopefully that makes sense, it's 1:26am and I'm off.
I do not understand how a test lamp would be preferred over a multimeter when testing under load. For instance, when looking at my headlight circuit to see if I would benefit by hooking them up to B+ I found a 2.4V drop "under load". That seemed more diagnostic to me, and just as quick as using a test light, and saying "yeah the lamp looks a little dimmer so that must be it".
06-30-2019, 06:31 AM
#17
CF Veteran
06-30-2019, 12:47 PM
#18
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Year: 1995 RHD
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If you had +12V at the power source then isn't the power source, by definition good (unless you mean your B+ is about 13.8V so then yeah you have a drain at source).And if everything dies when you apply power, the focus isn't looking for behavior "under load", it's looking for "shorts", and you shouldn't be looking for, or testing with, B+ power, but rather resistance (with a multimeter).
I do not understand how a test lamp would be preferred over a multimeter when testing under load. For instance, when looking at my headlight circuit to see if I would benefit by hooking them up to B+ I found a 2.4V drop "under load". That seemed more diagnostic to me, and just as quick as using a test light, and saying "yeah the lamp looks a little dimmer so that must be it".
I do not understand how a test lamp would be preferred over a multimeter when testing under load. For instance, when looking at my headlight circuit to see if I would benefit by hooking them up to B+ I found a 2.4V drop "under load". That seemed more diagnostic to me, and just as quick as using a test light, and saying "yeah the lamp looks a little dimmer so that must be it".
Really it's just another testing method that can help towards the common goal of diagnosing an issue.
06-30-2019, 02:48 PM
#19
CF Veteran
In the scenario you described of finding a 2.4v drop, that is the perfect place to use a multimeter as the bulbs work to begin with and you are trying to measure the drop specifically. If the bulb isn't working to begin with due to a very high resistance on say a corroded connector, a test lamp would not illuminate whereas a multimeter could still read +12v. In my previous example, the corroded connector had nearly 1M Ohms resistance but when under no load, the other end to the relay read +12v. Had I used the test lamp, it would have stayed off.
That said, your example is the perfect indication for load testing and failure of the test lamp. If you put a test lamp there and get no power you are immediately sent down the wrong path (there's no power). If you measure B+ but a known good bulb (which you can confirm with a... multimeter) will not light, go to load testing and you'll start at the right end (connector, then wire).
But again, if you're looking at a corroded connector, you just gotta change that out.
06-30-2019, 03:20 PM
#20
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Year: 1995 RHD
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I like to think it's worth using both bits of kit in certain situations but that's just me. It turned out that the connector in question shouldn't have even been there, another reason why it took a while to find. It should have been a straight wire with fuse from the front of the van to the rear but instead, there was a scotchlok connector joining 2 wires above the fuel tank giving the resistance. The end result was just a new wire front to back and problem solved.
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