2 minute VOLTAGE DROP tests EVERYBODY should KNOW

[wayne petty]

i would like you to drag out your handy digital volt meter...

set it to 20 volts DC.... get the hood open...

get the car started... turn on the headlights.. . you should be able to do this in under 2 minutes...

copy and paste this and print it out large,  paste it to a wall in the shop..

1. test between the positive and negative battery POSTS... 14.1 to 14.8 volts is expected...

2. test between the Negative battery POST and the engine block.. 0.04 volts is expected..

3. test between the Negative battery POST and the body .. 0.02 volts DC is expected..

4. test between the engine block and the body... 0.02 volts is expected..

if you get 0.00 on tests 2, 3, 4,... change the meter setting to 2 volts DC.. and do test 2, 3, and 4 again... looking for the small voltages posted..

the 0.04 and 0.02 voltages are correct for ground to ground with the engine running and the headlights on..

if you get much more than the tiny readings.. you have a bad ground... and that needs to be fixed first...


this test can be done in under 2 minutes... has to be done with the engine running.. and the headlights on...

will find all kinds of gremlins that will trip you up....

remember... electrons flow from Negative to positive... if you have a bad ground between parts.. you may not have enough electrons to do the job... if you have a bad ground... when the alternator is charging.. there might be way too many electrons trapped on the engine... causing weird voltage problems.... like you are having..

this is really the first test i do on EVERY CAR that comes in.. and usually i do it before it leaves just to verify...

[wayne petty]

there are additional tests...

the 4 above find most of the basic problems...


you can do test 2.. battery negative post to the engine block while somebody attempts to crank the engine...  this will show either a good ground between the battery and the engine block  with less than 1/2 a volt reading usually..

or a bad ground.. over 1 volt..

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you can test the positive side in a similar way... this gets complicated sometimes...

battery positive to the starter top terminal..    again.. less than 1/2 volt while cranking would be good..   more than 1/2 a volt..  you are going to need to clean terminals or go to a thicker battery cable..

now.. continuing its really hard to reach the starter top post sometimes...

take a short cut..   go between the battery top post and the alternator output terminal if its connected to the top post of the starter.. many are..  some are not..  this does not work on every.. but again.. less than 1/2 a volt is expected..  more than half a volt while cranking..   clean or go bigger ...


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digital volt meters measure the voltage difference between any two parts you touch...  

when you have current flowing through the wires.. do to the resistance in the copper wire.. there will be slight differences in voltage...  with the digital volt meter you can measure this tiny difference to prove the circuit works on does not work...

this is usually taught on the first few days of autoshop.. but nobody really understands its importance...

please...    open the hood of your daily driver...   do the tests...

open the hood of your other cars.. do the test...

you will see  that this does work...

if you have more questions..   be sure to post them or drop me a Private message..

post when you have tried this.. and have found it works...

[Bruce Dorsi]

Thanks, Wayne !!

[jaybee]

Good stuff, Wayne.  I had an LTD once that would start eating electrical parts every few months...almost anything electrical at random...then the problem would just go away for a few months.  Even an auto electrical shop couldn't find the problem, but I've long thought it was an intermittent grounding problem.

[1800guy]

Nice presentation, well put.  I've done this in front of the customer a few times and people are amazed - esp. the importance of the ground circuit.  

Another fun test for the folks with a filthy under-hood enviroment:  Set your meter to Volts and probe from the battery terminal to the opposite side of the dirty battery top.