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Howto perform charging system voltage drop test

To anyone else reading any of my posts I usually include the line: Today's cars simply will not run right without a strong battery and charging system! Cut corners here at your own risk

I'm a big believer in starting with the basics. As such I generally advise everyone to start every electrical trouble shooting session with a thorough check of the battery and charging system. For the average DIY'er it's easier to have an auto parts store test the alternator and battery. Most auto parts stores have the specialized tools to remove any guess work. Not to mention that most will do it for free. This is especially true for the AC ripple test (bad diode). Many DIY home meters can not perform this test correctly (really need a true RMS meter).

But don't stop there. The battery and alternator work as part of a system. A weak electrical connection can make a good alternator bad.

CLEAN and check (emphasis on visual checks and clean);
  • battery post. Don't ignore those corrosion cauliflowers growing on the posts
  • battery terminals (pay special attention to the battery negative as it's bad about splitting)
  • Clean the acid migration from the top of the battery.
  • Grounds around the radiator core support.
  • Battery condition. Does it hold a charge? Is it fully charged? Many of you won't believe it but a bad battery can "cause" the alternator to drop out. No battery=no alternator. Never ever start a trouble shooting session unless the battery is at least 75% charged (>12.45 volts).
Measure the voltage at the battery terminals. Repeat the measurement at the alternator B+ and case. Perform the test with the motor off and again with the motor running. Note these numbers as values should be close to each other under the same conditions.

Note, if the alternator is not charging performing a voltage drop test doesn't really give usable information. The alternator has to at least be putting out more than base battery voltage.

Start by measuring the voltage drop between the alternator case and battery negative with the motor running and all loads on. Best to set the VOM on the millivolt range (mV). Place the VOM probe leads on the battery negative and the other on the alternator case. It should be under the 250mV limit (that's 0.250 volts). Then do the same thing to the battery positive and alternator main B+.

NEVER EVER test a modern alternator by disconnecting the battery negative with the motor running. This is an old school testing method that has no place on a modern car.

Once the charging system is known to be in tip top shape, continue the more difficult trouble shooting.

What to clean/check if voltage drop too high:
  • Check the motor grounding strap from the left hand motor mount to the frame rail. Usually visible near the oil filter.
  • Clean the alternator mounting points including the engine block where the alternator sets. Brush the block with a wire brush to remove rust. Do the same for the alternator's mounting points. A small amount of di-electric grease will help keep it clean.
  • Main alternator B+ cable. Clean both sides to shinny metal.
  • Check the small alternator connector. Look for bent pushed pins.
  • Battery terminals and connector should be clean from previous steps (right?).
  • Frame grounding points on the radiator core support near the battery (or any others).
If the alternator grounding voltage drop is still too high, try attaching a jumper cable from battery negative to the alternator case. Retest the voltage drop. Is it below 250 mV now? If so this proves there's a weak ground between the alternator case and battery negative.

Here's some information should some of you wish to rebuild your own alternator.

Rebuilding your own alternator:

Ever wonder how a professional does it?
Hopefully the information below can help anyone trying to understand HOW to trouble shoot and locate charging system problems especially problems associated with excessive voltage drop. Here's a very good video from the "South Main Auto Repair" channel. Eric O goes through the step by step process a real professional uses to track the problem down. Note how Eric O uses the voltage drop test from the alternator case to the battery negative. Also note the voltage drop test between the alternator B+ and battery positive. Most importantly note the results of the test showing excessive voltage drop (much greater than 250 mV).

Also note how Eric O used the voltage probe to further isolate exactly where the bad connection was. IMO it's important to note that none of these tests required fancy tools. The bulk of the work was done with a simple Volt-Ohm meter (VOM). Later in the video notice how Eric O cleans the electrical terminals before putting it back together. Shinny on both sides.

The final point. Note that Eric O CONFIRMED the repair was effective by re-measuring the voltage drop.

I'm a fan of the SMA channel and I think that Eric O does a lot to promote sound trouble shooting techniques. However I think that sometimes Eric O over does things for the sake of the video. I personally wouldn't test and re-test the amount that Eric O often does. But on the other hand, Eric O finds negative results (check out the bogus test result from the burnt test clip for example).

As such I would generally advise someone to "clean everything". While cleaning everything may result in cleaning things that don't need to be cleaned it is a technique that can be explained in a forum post (could you image trying to explained HOW to do all of the tests that Eric O did?).

Hope this helps someone. Remember. Today's cars simply will not run right without a strong battery and charging system!

Charging System Problem - Voltage Drops. From Eric O at SMA


Here's another SMA Youtube video showing how to use the voltage drop method to diagnose and locate a bad body ground.

Chevy Truck - No Start, Flickering Lights & Gauges - Voltage Drop Testing

Yet some more information on what can be done via voltage drop testing to locate the exact source of a poor connection.
Voltage Drop, Do We Really Need To Know It!?

Here's some information on how to make your own DIY test lights. The main advantage over a store bought unit is having varying sizes depending upon the expected test "load".

How To Make Your Own Test Lights
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