Difficulty

Very easy

Steps

4

Time Required

                          5 minutes

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1

  • Mercedes W123 Basic Battery and Alternator Test Technique
  • 4 steps

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Introduction

If the car will start after you charge up your battery, but not the next time you drive, your alternator may not be properly re-charging it. Same for if your lights go dim while driving, or your radio acts up, etc.

It’s very easy to do some basic testing to see if your battery and/or alternator are working properly. This can help you begin to narrow down your problem.

What you need

Step 1

              Mercedes W123 Basic Battery and Alternator Test Technique
  • Open the hood.
  • Turn on your multimeter. Set it to the setting shown, which is for measuring 20 volts and under.

Open the hood.

Turn on your multimeter. Set it to the setting shown, which is for measuring 20 volts and under.

1024

Step 2

  • Connect the tester leads on your meter to the terminals on your battery. The red lead goes on the positive (+) terminal on your battery, and the black lead goes on the negative (-) terminal.

Connect the tester leads on your meter to the terminals on your battery. The red lead goes on the positive (+) terminal on your battery, and the black lead goes on the negative (-) terminal.

Step 3

  • Check your gauge. A healthy battery should read about 12.5 volts when the car is off. This battery is only a few weeks old and is testing well as expected.
  • If the battery is reading lower, especially if it’s reading lower than 12 volts, consider taking it to an auto store for charge and load testing.
  • Disconnect the meter from the battery.

Check your gauge. A healthy battery should read about 12.5 volts when the car is off. This battery is only a few weeks old and is testing well as expected.

If the battery is reading lower, especially if it’s reading lower than 12 volts, consider taking it to an auto store for charge and load testing.

Disconnect the meter from the battery.

Step 4

  • Start the car.
  • Reconnect your leads to the battery as shown in step 2.
  • The meter will now show the volts being produced by the alternator, via the battery. This is the charging voltage of the system.
  • A healthy alternator generally will produce 14+ volts. Anything under and the alternator should be tested further. Most automotive stores will test the alternator further for free.

Start the car.

Reconnect your leads to the battery as shown in step 2.

The meter will now show the volts being produced by the alternator, via the battery. This is the charging voltage of the system.

A healthy alternator generally will produce 14+ volts. Anything under and the alternator should be tested further. Most automotive stores will test the alternator further for free.

When you’ve finished testing if everything looks good and you’re still having electrical issues you’ll need to diagnose further.

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                    Nicolas Siemsen

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Mark Puckett - Oct 11, 2014

Reply

Step 4 - After you start the car, wait about 3 to 4 minutes for the glow plugs to cut off to get an accurate voltage reading. Glow Plugs draw a lot of current and lower the voltage reading.

analogcd - Mar 16, 2018

Reply

Idling voltage test should be done under full-load condition: turn on headlamps and set blower motor to high speed, (blower switch fully clockwise for Climate Control I - three knob system; Defrost setting for Climate Control II and III). Testing must validate voltage and current values, the latter is greater with lighting and motor loads. Comparing unloaded and fully loaded voltages can illuminate weak alternator / regulator output.

Amjad Hammour - Mar 25, 2018

Reply

If the battery is new only 2 month and the problem is with the starter please feedback