How to test for defective glow plugs?

Everything S_Niles says is correct, but you can save yourself some hassle by checking the resistance before you remove the glowplug(s) from the engine, by touching one end of your ohm meter to the end of the glowplug (after removing the wire/harness from the glowplug), and the other to your engine block. Do this to all of your glowplugs. The glowplug(s) that show greater resistance are suspect. Typically, 3 will read (nearly) identical resistance, and the 4th will show much greater resistance (on a 4-cyl engine, of course), assuming one of the plugs is bad.

Of course this method is far less precise than that described by S_Niles--you could well get higher resistance readings this way. But they key is to look for one plug that has higher resistance than the others.

This is the method I have used on my 4-cyl TDI engine.

I have always replaced all 4 plugs at once, as (at least for my engine), the parts are inexpensive.

Glow plugs need to get hot. So the wattage needs to be quite high. From a 12 battery we need a pretty high current then, If the resistance of a glow plug is say 0.6 ohms then fed at 12 volts the current flowing is I=V/r I =12/0.6 which is 20 amps. The gives a power in watts of 12v x 20 amps= 240 watts. That is for one glow plug.

However the resistance read when cold and then when hot is very different. As elements ( inside the glow plug ) heat up the resistance goes up a lot. So it takes less current then it appears it will by the resistance reading we got. A healthy battery is therefore a good thing. Lets say the power is about 120 Watts per glow plug that is still 3 x 120 Watts or 360 watts. That will draw 360/12 volts from the battery - 30 amps. And the starter will won't a big big share of what the battery can supply!

You can test these in place. Remove the connecting supply lead then connect one terminal (any) to the metal nearby and the other to the top of each glow plug in turn. Short the test leads together first as this amount in ohms needs be taken away from the test results. Use the lowest range of resistance the meter reads

The other way to confirm the glow plugs are working is to take them out and connect the 12 battery to each in turn, Connect one lead (either) to the body metalwork of the glow plug. Then the other battery terminal to the cap of the glow plug. It will do what glow plugs do if OK- glow! Watch your leads don't touch or there will be a big spark and keep the leads away from the car body if the battery is still in place and connected in the car.

You can use a clamp amp meter and check current draw of each plug when setting ignition on. Just remember that you must let all plugs cool down before you test next one since the first one is tested cold, and next one may be a little bit warmer due to first test, and so on with third, forth plug etc.

Target current draw should be around 20A for each plug. Higher current draw means less ohm, ie short in the plug. Less current means higher ohm or bad connection/break in plug.