PeachParts Mercedes-Benz Forum - View Single Post

jay_bob · #9 10-26-2014, 10:23 AM

In Diesel911s post above he shows a picture of a classic failure mode.

The glow plug is constructed as a coil that starts at the back end of the element and proceeds to the front. Once the coils are done at the very tip the electricity has to get back out of the tip. This means there is a wire running along the length of the coils, most likely the tip end of the coil connects to the grounded shell.

If that return wire shorts to the coil somewhere along its length you will get the result as shown in the photo below.

Any standard ohmmeter is not going to resolve the difference in resistance between a full coil and half a coil. You have too many other errors in the chain (contact resistance of your probe tips on the connector can often exceed the resistance of the element itself for example).

However your prechamber will definitely know the difference.

This is why you can definitely determine you have a bad glow plug by resistance, but you can not definitely determine if the plug is good just by measuring the resistance.

If I had a clip on meter I would be looking for too much, as well as not enough, current compared to the others. A half shorted element will draw double the current of a good one.