Easy way to test glow plugs in car? - PeachParts Mercedes-Benz Forum

Walter K · #1 10-25-2014, 01:05 PM

Gents,

What is an easy way to test glow plugs in the car? I have found that an Ohm meter does not work reliably, since some burned out plugs still show resistance.

Thanks,

W.

SD Blue · #2 10-25-2014, 01:35 PM

It might be possible to compare the readings using a clamp-on DC ammeter.

I've never done this before as the meter is about the same cost as a set of glow plugs.

97 SL320 · #3 10-25-2014, 01:42 PM

You need to look for a specific resistance not just conductivity. Test a new one then use that % as your basis. A burned out one might conduct a bit but is will be a much higher resistance than a good one. Be sure to look at the scale on the meter if you are using auto ranging.

Also, a quick check would be to disconnect the jumper wires between the GP then take a wire attached to the + battery terminal and touch it to each GP. If there is a spark, it is working.

Also, you can check all of them at once if you know the resistance of one. ( if the jumper wires are still attached. ) If one measures 10 Ohm, 4 in parallel will read 1/4 of one 10 Ohm or 2.5 Ohm.

On a side note, I haven't done it but there was a technique to use a selector box to read glow plug resistance while the engine was running to test for a miss firing cylinder. As the glow plug heats up, it increases in resistance. if the cyl isn't firing resistance won't go up much.

mach4 · #4 10-25-2014, 01:47 PM

Pull your injectors and you can visually see into the prechamber and determine whether the glow plugs are working when you cycle the key. Just make sure you've got new heat shields for when you reinstall the injectors.

vstech · #5 10-25-2014, 01:52 PM

You can test for definitely bad plugs but no 100% test exists to show a for SURE good plug but pulling it... pulling the injectors would show you which ones are glowing at the tip... seems like a lot of work though. ..

Diesel911 · #6 10-25-2014, 02:04 PM

Using the Amp Meter in series with the Glow Plug could work. However you are going to need a DC Amp Meter that goes up to 100 amps.

When I tested a new Monark Glow Plug with an 60 amp Amp Meter it pegged the Guage at 60 amps and with in 1-2 Seconds as it got hot settled down to about 15-16 amps and held steady.

In the pic is a Glow Plug that had good resistance when checked on the Ohm Meter but still caused nailing issues and was pulled out and tested on the Battery. The Center Portion of the Plug got hottest first and was the part that stayed the Hottest. The Center parto f the Plug does not extend out into the Prechamber so it is not going to transfer Heat well to the Prechamber.

If it looks different from the Bosch Glow Plugs it is becuse it is a Monark Glow Plug.

There is a device I have seen on eBay that is called a Glow Plug Tester. It is likely that it does what an Amp Meter would do.

Skippy · #7 10-26-2014, 07:11 AM

Quote:

Originally Posted by 97 SL320

You need to look for a specific resistance not just conductivity. Test a new one then use that % as your basis. A burned out one might conduct a bit but is will be a much higher resistance than a good one. Be sure to look at the scale on the meter if you are using auto ranging.

This. I recall good ones on my 617 reading in the area of 1.6 ohms. This isn't as sure of a test as pulling and bench testing plugs, but it's been good enough to keep mine starting into the teens.

Quote:

Also, a quick check would be to disconnect the jumper wires between the GP then take a wire attached to the + battery terminal and touch it to each GP. If there is a spark, it is working.

Also, you can check all of them at once if you know the resistance of one. ( if the jumper wires are still attached. ) If one measures 10 Ohm, 4 in parallel will read 1/4 of one 10 Ohm or 2.5 Ohm.

On a side note, I haven't done it but there was a technique to use a selector box to read glow plug resistance while the engine was running to test for a miss firing cylinder. As the glow plug heats up, it increases in resistance. if the cyl isn't firing resistance won't go up much.

Edit: It might have been 0.6. It's been a few years since I've had glow plug issues. That means I'm probably due. Anyway my meter is a $2 Harbor Freight cheapie so I don't trust it to be super accurate.

sokoloff · #8 10-26-2014, 09:49 AM

Very small sample of course, but for 30 years I have always found good/bad plugs using only my ohm meter. Good ones measure about 0.7 ohms.

jay_bob · #9 10-26-2014, 10:23 AM

In Diesel911s post above he shows a picture of a classic failure mode.

The glow plug is constructed as a coil that starts at the back end of the element and proceeds to the front. Once the coils are done at the very tip the electricity has to get back out of the tip. This means there is a wire running along the length of the coils, most likely the tip end of the coil connects to the grounded shell.

If that return wire shorts to the coil somewhere along its length you will get the result as shown in the photo below.

Any standard ohmmeter is not going to resolve the difference in resistance between a full coil and half a coil. You have too many other errors in the chain (contact resistance of your probe tips on the connector can often exceed the resistance of the element itself for example).

However your prechamber will definitely know the difference.

This is why you can definitely determine you have a bad glow plug by resistance, but you can not definitely determine if the plug is good just by measuring the resistance.

If I had a clip on meter I would be looking for too much, as well as not enough, current compared to the others. A half shorted element will draw double the current of a good one.

ah-kay · #10 10-26-2014, 11:58 AM

Quote:

Originally Posted by sokoloff

Very small sample of course, but for 30 years I have always found good/bad plugs using only my ohm meter. Good ones measure about 0.7 ohms.

x 2.

I have yet to find the ohmmeter doing a bad job in my ownership of MBZ diesel for 6 years. I measure the GP resistance between the chassis and the big connector inside the GP relay. Good GP measures low resistence, the value is 3 ohm or lower. A bad GP will be infinity, or high resistence. There is NO 'shorted' GP as such. If it is 'shorted' then the battery will either blow it or the fuse will blow.

Diesel911 · #11 10-26-2014, 12:08 PM

Quote:

Originally Posted by Skippy

This. I recall good ones on my 617 reading in the area of 1.6 ohms. This isn't as sure of a test as pulling and bench testing plugs, but it's been good enough to keep mine starting into the teens.

Set your Meter on the 200 ohm setting and hold the 2 Probe tips together and see what resistance reading you get. If you have a cheap meter the resistance will not be Zero like it would be on a good quality Meter.

You should subtract the resistance of the Probes from the total resistance to get the correct resistance reading.

Diesel911 · #12 10-26-2014, 12:34 PM

I recently went through this as I was having Nailing on one Cylinder.

I checked the Glow Plugs with the Ohm Meter and they were fine.

But, I still had the Nailing till Engine warmed up symptom. So the next step was to pull them and check them on a Battery. They all passed and I reamed the Carbon out of the Holes and re-instlled them.

So in My case I confirmed the Glow Plugs were not causing the problem.

I pulled the Injectors and they were all excellent 26K after they had been rebuilt.

Did a compression check and found that the nailing Cyliner had 50 psi less than the next lowest Cylinder. So I found the cause of the Nailing but have not investigated the cause of the low compression.
The Valves had been re-adjusted one Month previous and it Nailed before the Adjustment and also after the Adjustment. But, just because it is easy to do I am going to inspect the Valve Tarin again.
Next in line is as another Member recommended a Leak Back test to see if it is a Valve seating issue or not.

So I started with the easiest to do test that as the other Member said catches most of the Glow Plug Problems. Since the symptoms were still their it required pulling the GPs to do a better check. And, so on down the line.

Walter K · #13 10-26-2014, 06:40 PM

Thanks everyone for your replies!

I have a cheap 40+year old VOM that I use. I checked two burned out plugs that I had laying around, and a new Bosch plug. The Bosch plug is off my scale. One burned out plug had no continuity, and the other one 3 ohms (iirc). Before I had just looked for any conductivity.

I pulled the rear plug, as it is harder to get to, and that the front four were clean like they had been changed more recently. The rear plug turned out to be an Autolite plug. I tested it for grins, and found that it did not glow orange with less than 14 seconds on power. The new Bosch took 7. The glow plug light stayed on for about 7 seconds also. I replaced the Autolite with a new Bosch. Why would the Autolite plug take so long to heat up? Age?

I couldn't tell that the engine started any better, so started looking at the other 4 plugs. Resistance checked like the new Bosch. I finally found what I think was the problem. The connector for the wire on #2 was sitting next to the plug, but not attached to the plug.

I bought this car from a widow whose husband was a heavy equipment mechanic. I had met him last year, and he was in his late 80's if not in his 90's. I suspect he couldn't see well enough to tell that the wire was not connected to the plug. My hats off to the fellow for still trying to maintain her car in his elder years! Hope I can do the same at that age.

Thanks again for all the replies! I have large hands, and it is a real pain to change glow plugs with the big mitts I have. I really didn't want to take all the plugs out and check them.

Regards,

W.

Skippy · #14 10-26-2014, 06:47 PM

Folks with big hands often find it's faster to remove the injector lines, do what they need to with the glow plugs, and then reinstall the injector lines and bleed the fuel system. My gloves are size S, so I don't have to mess about like that.

funola · #15 10-26-2014, 07:04 PM

The run of the mill digital ohm meter is fine for checking if a glow plug is open circuit or not, but not measure its resistance accurately. If you get a glow plug that measures 1.9 ohms instead of 1.6 or 0.7 ohms as some posters say they should measure (haha!) and throw the glow plug out, you may be replacing it unnecessarily.

The easiest way to test glow plugs is to measure the current. This test takes less than a minute. In the video is an inductive and a clamp on digital ammeter. The surge is about 103 amps for 5 Bosch Duraterms when they first power up and drops to just under 40 amps after 45 seconds. If I see a surge of 80 amps, I know 1 glow plug is out, 60 amps, 2 are out etc. The only thing this amp test won't tell you is whether the glow plug is heating at the tips or not, but such a failure is very uncommon. 99.9% of the time, they fail open circuit.

Be sure all other electrical accessories are off when doing this test at the battery cable.. E.g. dome light, blower motor, radio etc.

CLICK TO PLAY! Enjoy!

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