Quote:
Originally Posted by FR-S2GT86
The issue is that the '72 doesn't have sub-5 volt circuits in it that could easily be damaged by connecting a 12-volt incandescent bulb in series with them. Modern cars do, and the possibility exists that when you are probing for the suspected correct circuit, you might accidentally probe one of those low voltage circuits. Then you'll have to explain to your customer why they need a new ECU when all they originally dropped their car off was for a non-start situation....and you'll still have troubleshoot and fix the original issue once the ECU has been replaced and reprogrammed, etc. And you may have introduced even more issues to some of the low voltage circuits that the ECU is monitoring even after all of that. That is a risk that I would not take as a technician with many years of experience diagnosing vehicle electrical issues, and just about any other kind of electrical issue that you can think of.
For all of the old-school guys that still insist on using a "test light" to troubleshoot with, I would recommend a digital version of it such as this one here,
which as you can see has sequencial LEDs to show the level of voltage in the circuit you are testing, and can conveniently allow you to have one hand free if you attach an alligator-type clamp adapter to the end of the wired probe. The low voltage electronics in the digital "test light" pose no threat to modern electronics. Using a tool like this would be insurance taken to prevent a huge headache down the road.
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Old school doesn't mean not effective, and most test lights I have seen recently are LED and have no effect on 5vdc circuits. Not to mention I hope you know what you are probing before hand, meter or test light. The most common control board we use here at work is 20 years old and is AC input, 5vdc for switching and controls. It is so sensitive if an AC unit or an air compressor is on the same breaker and kicks on, it would trigger some of the DC side signals. We now run filters on the DC side to prevent this, but a test light has never caused an issue with it. The control panel is always at least 60ft away so there is no way I am seeing that little test pen with different LED's for different voltages that far away, on top of the fact I am not looking to read voltage , I am just checking that it is switching. What Grady recommended would give me more information, but is not really necessary and for as often as I work on these, not cost effective. Now on to the OP's post.
Going back to the OP's first post, it doesn't sound like the battery to me. It sounds like something like a clutch switch as soundman98 said. Doesn't mean you shouldn't check the battery and if you are going to do that you should load test it which means the battery should be fully charged. So while you put a charger on the battery, that isn't going to stop you from checking other things, especially if the issue is indicating something else. By checking the starter terminal like RZNT4R said you are isolating where the issue is(which a test light is good for). Light comes on-weak battery, fuse, or bad starter/starter solenoid. No light- it's on the switching side. Clutch switch, starter relay.