have you run a voltage drop test on your street car?

[wayne petty]

i posted this over at circle track and stock car racing a few weeks ago..


so have you ever performed a voltage drop test to verify the charging system and the rest of the electrical system is working PROPERLY...

only takes a few minutes with a digital volt meter...


get the engine running.. turn on the headlights  and several other electrical devices.. but headlights work on a street car.. track cars.. everything should be on..


set digital volt meter to 20 volts DC scale..

test 1.   positive  battery post to the negative battery post...  14.1 to 14.8 volts for a 12 volt system..   voltage depends on your charging system design voltage...   this first test proves that the charging system is working at least partially..

i am aware that some of you run higher voltage charging systems..  so you may have to use different numbers for yours..


test 2.   Negative battery post to the engine block...  0.04 volts DC maximum is expected...    since the alternator puts out current thru the negative side... its all coming thru your block and up the negative cable to the battery negative...

this second test proves that the negative circuit from the engine block to the negative battery Post is properly connected...
why expect such a small voltage... you are actually measuring the resistance of the copper wire in the circuit.. if you get a higher voltage.. you will have to examine the actual connections for issues... thats 4/100 of a volt..

test 3.   Negative battery post to the body/chassis of the car...  0.02 volts DC MAX...  this is 2/100th of a volt..

why... where do you ground everything???  do you run individual cables directly to the negative battery... or do you ground stuff to the frame/chassis/body..  don't forget... electrons flow from negative to positive... so the entire frame/chassis/body should be grounded... so you can attach ground connections anywhere and just need to run a positive side...
this test verifies that you have a proper Negative side connection..

test 4.   engine block to the body/chassis/frame..    again... this is 0.02 volts DC max...

again .. WHY?? during a race.. or even daily driving.. most of the current the various devices use come from the alternator as its putting out almost 1.5 volts above battery voltage..   this verifies that the engine to body connection is properly wired and connected...  because of the alternator mounted to the engine.. when the engine to chassis cables come loose.. you can have significantly more voltage on the engine than on the chassis...  this really screws with digital electronics..   its even harder on your alternator..    guess what... its also hard on your Ignition boxes...


where do you ground your ignition boxes at ??

if you are really smart...  you will have a 3 or 4 ring terminal engine ground harness..

what... read on..

one harness... ring terminal attached to the back of each cylinder head...  another ring terminal attached to the distributer housing...  and the 4th ring terminal.. attached to a  STUD thats properly attached to the firewall...   this stud should also be the ground location for the IGNITION BOX...

now the reason why...

spark is created in the ignition coil...  out the coil wire.. thru the cap... rotor.. jumps the gap to the cap again.. down the spark plug wire... thru the spark plug.. across the spark plug gap and into the ground side...

oh.. for those of you in the KNOW.. yes i described it totally backwards...

now that the spark is in the heads... actually all the sparks are in the heads.. where do they go...    depending on where the ignition module is...  HEI is in the distributor housing.. but the actual HEI spark return path is all the way to the black wire at the coil ground..    with MSD ready to run.. its in the distributor..   with remote MSD boxes .. the spark return path is thru the big black wire that is the ground connection...

think of the spark path..   do you really want the spark needing to go all the way thru the battery cables and then work its way to the module...  or straight thru the heads, thru your new ground harness and right back to the ignition box...  hmm.. you tell me..


ok... now that you have tested your hot rod.. and improved it in just a few minutes.. or a short while..

run the same 4 part test on your tow vehicle.. and you wife's car..

this should be printable...