An electrical problem can be a major pain to overcome. So, I
thought it would be nice to provide some tips in a series of articles to help
you achieve success. Some of these tips may already be in your toolbox, a few
new and maybe a few long forgotten. In any event, I’m confident they will help
you navigate through the spaghetti of wiring in today’s vehicles.
Of course, we’ll start with the basic checks of the
electrical system. Every circuit begins and ends with the battery, so battery
open circuit voltage is a great place to begin. At 80 degrees look for 12.66
volts measured across the terminals with all loads turned off. A voltage of
12.45 volts or lower indicates a battery with only a 75% or lower state of
charge. Charge a low battery early in your troubleshooting process. Combine capacitance testing with conventional
load testing as capacitance testing correctly identifies many faulty batteries.
Now, let’s perform a series of quick test. Attach the red
lead of your DMM to the positive battery post. Connect the black lead to the B+
stud on the starter. Select DC volts and RECORD. Crank the engine and read the
MAX voltage on the DMM. This will measure the voltage drop for the entire
positive cable, including the terminals at each end. Next, reposition your
leads so that the black lead is on the battery negative post and the red lead
is on an unpainted clean bolt screwed into the engine block. Set your DMM to
RECORD once again, and crank the engine. Now go to your MAX reading and you
will see the voltage drop on the ground (negative) cable. Generally you want to
look for voltage drops of less then .1 volt per connection in power circuits.
Our next test will require we move the positive lead of our
DMM to the alternator frame (away from belt and pulley) and leave the black
lead connected to the negative battery terminal. Start the car and turn on all electrical loads.
Your DMM will now show the voltage drop on the ground side of the charging
system. Keep in mind that if the alternator is charging then this will always
be a negative number. Now, move the leads to the alternator B+ stud and the
battery positive post to read the voltage drop on the positive side of the
charging system.
Then let’s take the positive lead and move it to the
positive post while moving the negative lead to the ground side post. With the
engine running off idle and all loads still on, read the loaded charging
voltage. Now turn your DMM to AC volts and read the AC ripple. Most experts
agree anything above 200 millivolts is cause for concern but some vehicles are
good with ranges as high as 450mv AC. Also, readings taken between the
alternator B+ stud and the alternator frame tend to run higher. Good practice
is to check known good cars so that you know normal when you see it.
Finally, let’s switch back to DC volts and turn all
electrical loads off to read nominal charging voltage. Now, a word of caution,
some computer controlled charging systems may not charge at all under no-load
conditions. Flush the target out by turning on the high beams and raising the
engine speed just above idle. If you encounter high AC ripple voltages, install
a jump box on the battery and retest. Since batteries do function as capacitors
adding a jump box is a good way to reduce ripple voltages. If so, charge the
battery and retest. If the high ripple voltage continues replace the battery.
Feel free to contact me through my website at
http://www.raymondsautoservice.com.
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