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

Your method is effectively the same as measuring the resistance, which works most of the time.

That may or may not be the case, depending on model.

Because the glow plug is intermittent in the contact from the electrode to the resistance wire that actually does the heating. When cold, you measure the 0.3 ohms or whatever. When voltage is applied, it gets hot a little and expands and loses contact and does not get hot anymore. This cycle can continue and fool the unsuspecting into thinking they have a good glow plug because their handy dandy ohmeter says so.

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85 300D turbo pristine w 157k when purchased 167,870 July 2025
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked

another failure on the glow plugs is the range of heat drops, the plugs need to get hot from the tip to the stern. WHITE hot.
amps will measure from moderate heat, to white hot. only white/bright orange fully hot to the tip glow plugs will effectively start the car...

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John HAUL AWAY, OR CRUSHED CARS!!! HELP ME keep the cars out of the crusher! A/C Thread
"as I ride with my a/c on... I have fond memories of sweaty oily saturdays and spewing R12 into the air. THANKS for all you do!

My drivers:
1987 190D 2.5Turbo
1987 560SL convertible
1987 190D 2.5-5SPEED!!!

1987 300TD
2005 Dodge Sprinter 2500 158"WB
1994GMC 2500 6.5Turbo truck... I had to put the ladder somewhere!

Right. Essentially this is what i'm trying to measure with amps rather than pulling the plugs and watching them get hot. I've never seen them white hot. Just orange. I think i should add hold jumper wire and measure amps for 5 to 10 seconds to ensure it continually draws amps. Do you guys agree no need to pull the plug then and if it is drawing 20 amps it has to be getting hot?

Where could that energy be going if not into heating the tip?

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

If the GP is faulty it can draw the correct current & all the heating is occurring it the part under the bolt fitting. I have seen this.

I cant understand why so many are pre-occupied in using a test meter, you are trying to see if the GP gets a proper glow pattern, you need to see it going red. You also need to make sure the GP hole is clean & doesnt require reaming. No test meter can determine this. Its time to get your fingers dirty!!

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Grumpy Old Diesel Owners Club group

I no longer question authority, I annoy authority. More effect, less effort....

1967 230-6 auto parts car. rust bucket.
1980 300D now parts car 800k miles
1984 300D 500k miles
1987 250td 160k miles English import
2001 jeep turbo diesel 130k miles
1998 jeep tdi ~ followed me home. Needs a turbo.
1968 Ford F750 truck. 6-354 diesel conversion.
Other toys ~J.D.,Cat & GM ~ mainly earth moving

perhaps we should add to the service interval a series of glow plug service to the valve adjustment plan.
at 15K, the GP's should be pulled, and the holes reamed. also, the plugs themselves should have power applied and verify the TIP gets hot bright orange at a minimum.

__________________
John HAUL AWAY, OR CRUSHED CARS!!! HELP ME keep the cars out of the crusher! A/C Thread
"as I ride with my a/c on... I have fond memories of sweaty oily saturdays and spewing R12 into the air. THANKS for all you do!

My drivers:
1987 190D 2.5Turbo
1987 560SL convertible
1987 190D 2.5-5SPEED!!!

1987 300TD
2005 Dodge Sprinter 2500 158"WB
1994GMC 2500 6.5Turbo truck... I had to put the ladder somewhere!

You know vstech, if that was done, ~80% of the no start threads would vanish.
As I have said before, testing with a meter will tell you that you have a problem, it will not tell you that the GP's are good as you can have perfect meter readings yet not a proper glow on the GP !! If one tests bad on the meter, then any one with any intelligence would know to test them all on a bench.
Relying on meter readings is just a case of artificial intelligence ( the meter) being no match for natural stupidity (they guy who rely's on the reading)!!
Many of us are rightfully critical of the limitations of the electronics that now 'controls' many cars, yet we jump to use an electronic device to test GP's.
If your brushes are bad in your alt, you visually inspect them, same goes for GP's !!

__________________
Grumpy Old Diesel Owners Club group

I no longer question authority, I annoy authority. More effect, less effort....

1967 230-6 auto parts car. rust bucket.
1980 300D now parts car 800k miles
1984 300D 500k miles
1987 250td 160k miles English import
2001 jeep turbo diesel 130k miles
1998 jeep tdi ~ followed me home. Needs a turbo.
1968 Ford F750 truck. 6-354 diesel conversion.
Other toys ~J.D.,Cat & GM ~ mainly earth moving

I don't think glow plugs get white hot, at least not any of the ones I've looked at. Maybe the newest types do get white hot?

__________________
85 300D turbo pristine w 157k when purchased 167,870 July 2025
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked

I have seen mine get very light orange/yellow, but not white. White is very, very hot.

I have a current clamp. I think what may be easier for me at least is to just use a jumper cable from + battery to the GP with the clamp around it. It should read it...

__________________
Current Diesels:
1981 240D (73K)
1982 300CD (169k)
1985 190D (169k)
1991 350SD (116k)
1991 350SD (206k)
1991 300D (228k)
2008 ML320 CDI (199k)
1996 Dodge Ram CTD (442k)
1996 Dodge Ram CTD (267k)

Past Diesels:
1983 300D (228K), 1985 300D (233K), 1993 300D 2.5T (338k), 1993 300SD (291k)

Let me know how that works. That could be ideal. I looked for one that would measure DC and never found it.

What does it read when you wrap it around the all the glow plug wires in the sheathing? What about when you disconnect one plug? Does it drop? I think you see what i'm getting at?

Doesn't the glow plug light come on when you have a glow plug out ? Or isnt't it supposed to? Or do you have to have a couple out before it triggers it? I'd like an idiot light any time a single glow plug goes out. I'm just not sure how to build the circuit or if its worth the troubel when i can just pull relay plug and test amps really easy.

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

Well I finally did it... Was about 29 out and I needed to move my car, so I took out my Fluke 336 set to DC Amps (600ADC rated). Behavior suprised me a bit... This is on brand new Bosch GPs...

Clicked relay via turning the key, instantly read 99.6A. Continually dropped until the relay clicked at which point they were pullling about 43.8A.

43.8A is not steady state draw at temperature, they were still dropping in current draw at that point, though I did not go through another cycle.

Now, I didnt check my battery voltage before doing this, but I assume it was full. What suprised me is that a full glow cycle is not sufficient to get the GPs up to full temperature, which, right or wrong Id define as when the current stops dropping.

__________________
Current Diesels:
1981 240D (73K)
1982 300CD (169k)
1985 190D (169k)
1991 350SD (116k)
1991 350SD (206k)
1991 300D (228k)
2008 ML320 CDI (199k)
1996 Dodge Ram CTD (442k)
1996 Dodge Ram CTD (267k)

Past Diesels:
1983 300D (228K), 1985 300D (233K), 1993 300D 2.5T (338k), 1993 300SD (291k)

As cheap as ammeter's are, it might be worth starting a thread in which we wire an ammeter in place of the strip fuse and post our readings.

How can the ammeter read 90+ amps when there's an 80amp strip fuse in the circuit?

__________________
1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13