Voltage Drop Testing

Voltage drop testing is one of the most valuable testing procedures for working on any vehicle that uses electricity ;) A circuit can test fine for voltage, and ohm out properly and still be bad. Corrosion, loose crimps, poor connections might not show up with just a voltage/ohm test, and that is where voltage drop testing really shines. In order to have a voltage drop, there must be current flowing through the circuit, so lets get to some testing.

A digital volt ohm meter is pretty much a required piece of equipment when working on the electrical side of our vehicles. They are cheap and easy to use. http://amzn.to/2ongmVa

If the MFG lists an acceptable voltage drop use it, otherwise use the voltage drops below. You will notice that higher amperage circuits listed below (starter/alternator) allow for a lot higher voltage drop than the lower amperage circuits, this is not a mistake. If you are looking for manuals, see User/Service Manuals and Docs

Be careful about hot/moving parts like the exhaust/belt/fan/etc.

    1. First step is to check your battery voltage. See Batteries If your battery is below 90%, throw a battery charger on it and let it charge before continuing

    2. Disable fuel pump(pull fuse), battery negative to positive post while cranking for 5 seconds(record reading), then test the starter housing to positive post on starter while cranking for 5 seconds(record reading). If the difference between the two readings is more than 1vdc there is a problem in the starter circuit.

    3. Start the engine and let it idle, set meter to vdc(20vdc on non auto-ranging models).

      1. Negative to positive battery posts(posts, not clamp/cables) - ~14.1 to 14.8vdc

    4. If you have a non auto-ranging meter, set it to the low scale 1-2vdc

      1. Negative post to engine block - 0.04vdc or less

      2. Negative post to body - 0.02vdc or less

      3. Negative post to starter housing - 0.3vdc or less

      4. Engine block to firewall - 0.02vdc or less

      5. Positive post to under-hood fuse box, test several fuses - 0.04vdc or less

      6. Positive post to alternator stud - ~0.3vdc some as high as 0.7vdc on really long charging circuits Example: FSM for 03 Astro AWD specifies 0.5vdc drop for this test.

Fluke has a good read on their site: http://www.fluke.com/fluke/uses/comunidad/fluke-news-plus/articlecategories/electrical/diagnosevoltdrop

Also, see the Parasitic Battery Drain page and Fuse_Voltage_Drop_Chart_all.pdf

Quick and easy voltage drop testing on a complete circuit: LoadPro https://amzn.to/2w96rYs aka resistor with a switch in a set of DVOM leads. If you are using it all the time, probably worth ~$40, otherwise just put a resistor like this in line, and https://amzn.to/2NgJ5GK for testing up to a 30amp circuit.