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Technical Mystified By Electrical Troubleshooting? I Know I Am!

Discussion in 'The Hokey Ass Message Board' started by ClayMart, Dec 6, 2023.

  1. Here's another handy guide you might want to download if you get stumped when trying to diagnose and troubleshoot electrical problems. Things can get a bit confusing when trying to explain some of these test procedures in an online forum using text only. And this guide is from Delco Remy so I figure they've got a pretty good handle on this stuff. :D

    It covers most basic and some more advanced testing procedures. Most importantly there's lots of clear graphics showing how and where to correctly connect voltmeters, ohmmeters and ammeters to get useful, correct readings. And to help keep you from frying your test equipment. :eek:
    Jalopy Joker and Sporty45 like this.
  2. jaracer
    Joined: Oct 4, 2008
    Posts: 2,377


    I'm a big advocate of voltage drop testing. I had more students grasp this technique than all the electrical theory we shoved down their throat (I taught for many years).

    To use this technique you need all wiring connected, and the device that isn't working turned on. You must also have a good battery. If open circuit battery voltage isn't above 12.4 volts, you need to charge the battery. I also recommend that you connect the voltmeter negative lead directly to the battery negative and leave it there. You may need a long jumper wire to accomplish this.

    First, check the battery voltage with the component turned on and note the reading. Get access to a connection as close to the non-working component as possible. Touch the voltmeter positive lead to the power connection going to the component and note the reading. Now touch the voltmeter positive lead to the ground for the component and note the reading. These reading will tell you what to do next.

    Power connection - battery voltage 0.2 volts of battery voltage or less
    Ground connection - 0.1 volt or less
    Conclusion - component is bad

    Power connection - more than 0.2 volts less than battery voltage
    Ground connection - 0.1 volt or less
    Conclusion - excessive resistance on the power side

    Power connection - battery voltage within 0.2 volts of battery voltage
    Ground connection - more than 0.1 volt
    Conclusion - excessive resistance on the ground side

    You can find the excessive resistance on the power side by moving your positive lead closer and closer to the battery. So if I see 7 volts at the component, I would move my lead to the the switch for the component and see what the reading was there. If it is still 7 volts, I would move the lead to the power feed for the switch. If I now see battery voltage, it means the switch is bad.

    You can do the same thing on the ground side by moving the lead closer and closer to battery negative. Once the reading drops to 0.1 volt or less, the problem is between the last bad reading and the first good reading.

    This also works for a positve ground system, but your leads are reversed.

    If you use this technique for starting systems, the acceptable voltage drops are higher because of the high current. Typically you are allowed 0.4 volts on the positive side and 0.3 volts on the negative.
  3. squirrel
    Joined: Sep 23, 2004
    Posts: 55,644


    I like a simplified version of voltage drop testing. Turn on the headlights, try to start the car, if the lights go out, the battery is dead (or the wiring to it is flaky). If they stay on, then there's something wrong with the starter or the wiring to it.
  4. A short circuit is just an out-of-control voltage drop....
    clem and firstinsteele like this.

  5. Jalopy Joker
    Joined: Sep 3, 2006
    Posts: 31,063

    Jalopy Joker

    Thanks for posting
  6. Speccie
    Joined: May 22, 2021
    Posts: 95


  7. deucemac
    Joined: Aug 31, 2008
    Posts: 1,478


    I taught auto mechanics for 15 years and always got a "deer in the headlights stare" when we started. I used a simple explanation for all electrical circuits. ALL CIRCUITS, complex or simple, require 4 things. Power, ground, load and conductors ( not connectors because they are only for convenience of assembly). I would explain that you go to the power side of your load and see if you have full voltage. If not, then troubleshoot the supply side of the circuit. If there is good voltage at the load, then it is your load or the ground side of the circuit. It was simple and could easilybuilt upon once they understood those 4 things and their relationship to each other. I also would have the students wire a car on the chalkboard. I would draw the components on the board, the three busses al cars have, battery, ignition, and accessories. They would draw lines representing power and ground wires. After a few days of that, they were much more comfortable with electrical systems.
    AVater, ClayMart, Davesblue50 and 2 others like this.

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