Mercedes W123 Glow Plug Test Technique

To perform this method of testing, you will need a reliable 12v power source. You could use a spare 12v battery and jumper cables. In this case, a battery "jumper" box was used.

If you are testing a glow plug that is new from the box, you can begin here.

If you are testing a plug that is currently in your engine to see if it is the bad plug, you will need to first remove it; see the glow plug replacement guide for advice on this process.

Start the test by gripping the body of the glow plug with the negative cable clamp.

Then attach the positive clamp to the threaded end of the glow plug. Be careful to not let the positive and negative clamps touch.

Pictured is a good, brand new Bosch glow plug lighting up nice and hot at the very tip. This is how the plugs should all perform.

Be extremely careful during this process! Never ever touch the tip, or place it on a flammable surface just after testing. The tip can reach temperatures over 1000F!

Shown here is a used glow plug that is not working quite right, despite glowing at the tip. The tip was slow to light compared to the new plug. You can also see that it is covered in carbon; it's a reminder that you'll want to ream the glow plug hole in the pre-chamber before installing a new one.

Shown here is a bad glow plug. It did not glow at all despite 12v being directly applied. This was the main culprit in this particular car's rough cold starts.