I think its that time of year. Many threads say pull the glow plugs to test them to see if they are really getting hot. But all that work is not necessary . You just need to check if they are pulling amps. All good plugs I tested pulled 20 amps. If they do, they have to get hot right?

I was clued in on this easy way in a post somewhere on this forum. And my friend even made it simpler. I'll go through the whole glow test procedure for newbies.

First pull the glow plug wire from relay. Relay is black box on drivers side wheel fender under hood. When you turn the key on you should have voltage at those pins on the relay. If you are getting voltage at all the pins on the relay, good. If not, check the metal fuse at top of relay if you have one or you may need a new relay.

Next the book procedure says to check resistance with an ohm meter. However this is not always accurate although most times it will show bad plugs. So if you have only an multi meter without a big ammeter, go ahead and check resistance on pins of plug. Put your ohm meter in each pin in the harness plug you unplugged from the relay. Resistance Should be around .6 - .8 ohms , no more than 1.2 or 1.6 or whatever the manual says. My experience is a blown one will be infinite or way above 2 ohms. But regardless, this test is not always one hundred percent accurate so go on to the next test. Its better than nothing if all you have is a ohm meter.

Next *Instead of pulling glow plugs and watching them get hot* , put a jumper wire from battery and touch it to each pin on plug. See if you get a good spark. If it does, its drawing current good and hard which means it should get hot. That's my friend's dead simple way to test glow plugs. He's an electrical engineer. You can also use a test light. If it lights, the plug is drawing current.

If you want to get fancy , buy a cheap ammeter that measures 25 amps at least. I got a meter that is for dash mounting at Harbor Freight for like $5.00 or something. I put a long wire with clip that goes to the battery on the B+ term and a wire on the other term. Strip the end of the other wire and stick it in a glow plug pin on the plug harness. A good glow plug draws 20 to 25 amps.

Thats it. No need to pull glow plugs. My understanding is it is physically impossible for plugs to not get hot if they are drawing the right amount of amps. I'm interested in the physics behind this and why the resistance check is not always accurate. With Ohms law -- V(oltage) = I(amps)R(esistance) or I = V/R means 12.5V / .8 ohms =15.7 amps a glow plug draws. If you measure a glow plugs R and it is within range and you have V , why would you not get Amps?

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What Would Rudolph Do?
1975 300D, 1975 240D, 1985 300SD, 1997 300D, 2005 E320 , 2006 Toyota Prius