Best/Easiest way to test glow plugs - PeachParts Mercedes-Benz Forum

biopete · #1 11-23-2010, 09:26 AM

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?

#2 11-23-2010, 09:36 AM

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

tangofox007 · #3 11-23-2010, 09:39 AM

Quote:

Originally Posted by biopete

Relay is black box on drivers side wheel fender under hood.

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

funola · #4 11-23-2010, 09:39 AM

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.

vstech · #5 11-23-2010, 09:57 AM

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...

funola · #6 11-23-2010, 10:32 AM

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?

#7 11-23-2010, 10:35 AM

Quote:

Originally Posted by funola

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?

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

biopete · #8 11-23-2010, 01:36 PM

Quote:

Originally Posted by vstech

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...

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?

JHZR2 · #9 11-23-2010, 03:10 PM

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...

biopete · #10 11-23-2010, 03:47 PM

Quote:

Originally Posted by JHZR2

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...

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.

Jeremy5848 · #11 11-23-2010, 04:26 PM

DC current clamps are rare but available. I have an ancient HP-428B (ancient = uses vacuum tubes) that works great but its range tops out at 10 Amps. Modern solid-state versions are also available but at much greater cost than AC clamp-on ammeters.

Inductive DC ammeters (hold meter next to wire) are perhaps the best compromise. They have relatively poor resolution but are easy to use and not expensive. Available in various ranges, I don't remember whether the smallest would work for one glow plug.

Here's a link to a 30-0-30 Amp meter that would work.

Jeremy

JHZR2 · #12 12-19-2010, 08:52 PM

Quote:

Originally Posted by biopete

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.

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.

kerry · #13 12-19-2010, 09:25 PM

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?

layback40 · #14 12-19-2010, 09:37 PM

Quote:

Originally Posted by biopete

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?

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!!

JHZR2 · #15 12-19-2010, 09:47 PM

Quote:

Originally Posted by kerry

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?

It is a strip fuse, not necessarily a fast-acting device. Look at any one that has been in service and it will show the signs of heating by its droopy profile.

Remember, it started and peaked at 99, but started to drop fast. It was down below 80 before too long.

A fuse is rated by its I^2 *t profile, i.e. some element of the current flow and the time it is subjected. The fuse should be permanently capable of passing 80A through. Pass 81A and at some very long time it may open. Pass 90 through, and it would be a bit shorter time. Pass 100 and even shorter... Pass 1000A and it would pop. Short circuit it and it would pop.

In the big scheme of things 99A for a few seconds and dropping fast isnt far from 80A. Breakers and fuses have to be able to support some overage for a short while in order to allow for inrush currents during motor starts, fault isolation and clearance, etc.m depending upon the system. The glow system is definitely very simple, but it still must do 80A continuous and thus 99A for a spurt is no big deal.

Other systems can be far more sensitive, and the protection devices can be designed accordingly.