Glowplug Testing

[Cunning Plan]

Now the weather is a bit colder, I've noticed a few times that the Cherokee has a couple of splutters within the first 2 seconds of starting, which it didn't used to.

It starts and runs, but you do feel a couple of rough judders, literally only for 1 - 2 seconds, then it runs OK.

The glow plugs are relatively new, under 25/30k miles, but I thought I would test them in-case one is not warming up properly.

I found that you can test by using a multimeter connected to the battery and the top of the glow plug, supposedly, if the multimeter reads what the battery's base voltage is, that plug is OK and you move on to test the next one.

All of them seemed okay, but I was unsure if the test actually works Has anyone done this before or had any expericence of it working? I was just thinking that the plug must be 'earthing' itself through the head and would complete the circuit to read the battery base voltage, whatever its condition is.

This is the 'How To' video I followed if anyone is interested:

[art b]

take em out and connect them to 12v ..batt or batt charger...

they glow red when workin..

[Andy]

Right, it's taken me far to long to work out what that guy is checking!

He is basically checking the return path to the battery via the glow plug tip. Which in itself is a fine check for continuity of the circuit, you really shouldn't see any volt drop.

I don't think it does prove a glow plug is working though. Like art said, pull one and carefully connect it across a battery. But ruddy careful though as they get ridiculously hot and stay that way for a while.

The reason I say that, is a glow plug is just a big resistor, and it works by having a big resistor coil getting heated up by the current going through it. Now if that were to short, or become a lot lower resistance the most probably thing is a fuse would go, or the glow plug cycle at the start wouldn't heat the cylinder up at all. Now by taking a reading from the battery and bypassing the positive side fuses, the glow plug could be shorted and the fuse has popped, or could be working. The only thing it rules out is it being open circuit. He really needed to follow that test up with an ohm meter reading and proving with a new glow plug that the resistance of the glow plug (tip to engine block) was similar or that same across the engine.

This however still doesn't prove the glow plug is work to maximum efficiency, as it could be well and truly covered in carbon that isn't dissipating the heat through the cylinder.

[Cunning Plan]

Nice answer Andy, thanks

It's okay most of the time, without upsetting anything I think I'll leave it for a bit, but if it does it again, I'll just replace all of the plugs

[Andy]

It makes more sense to do all of them at once, especially if they're a pain to do. The denso once I had lasted about 4-5 years and you really noticed them going about November time. It's one of those things that's guarenteed to fail during winter when it's too bastid cold to change them yourself!