Easy way to test glow plugs in car? - Page 2 - PeachParts Mercedes-Benz Forum

ah-kay · #16 10-26-2014, 08:03 PM

Quote:

Originally Posted by funola

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!

With no disrepect. Why go through such elaborate set up to deterimine 1 or 2 GP is out??? The GP light would have told you something you wanted to know. Lot to do about nothing. The ammeter will NOT tell you which one in any event.

An ohm meter is your best friend. Does not matter it is state-of-the-art or just a HF $5 unit. That is all you need. You do not need a clip on ammeter. You do not need to take the injector out unless you want to do a visual. Finding out which GP is not working is a walk in the park. To me any way. Yours experience may differ.

sloride · #17 10-26-2014, 09:10 PM

Quote:

Originally Posted by funola

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!

Do you have an idea how that would work on an 85 300D that GP relay times out at around 25 seconds? I did not see on Walts vehicle ID what this was in response to.

Partscarver · #18 10-27-2014, 01:43 AM

84 300d

funola · #19 10-27-2014, 09:02 AM

Quote:

Originally Posted by ah-kay

With no disrepect. Why go through such elaborate set up to deterimine 1 or 2 GP is out??? The GP light would have told you something you wanted to know. Lot to do about nothing. The ammeter will NOT tell you which one in any event.

An ohm meter is your best friend. Does not matter it is state-of-the-art or just a HF $5 unit. That is all you need. You do not need a clip on ammeter. You do not need to take the injector out unless you want to do a visual. Finding out which GP is not working is a walk in the park. To me any way. Yours experience may differ.

You think clamping an ammeter around a wire is elaborate? Maybe I should have made it clear, only one of the two ammeters in my video is necessary- I used both just to show the different types.

I love that digital clamp ammeter, it's been extremely useful in diagnosing electrical problems in starters, alternators, glow plugs- just about anything electrical. It also has a built in volt and ohm function. I did not say an ohm meter is not useful in diagnosing glow plug problems, just that it cannot measure low resistance accurately.

The OP asked for an easy way to test glow plugs in car. The clamp ammeter will tell you in less than 30 seconds if any glow plugs are out, can't get any easier than that. If any are out, then you find out which one with an ohm meter. True, the glow plug relay pre-glow indicator is supposed to tell you if one or more glow plugs are out (if the relay is still fully functional) and if you know how to interpret the light. But these things are 30 years old and most of them do not work as designed anymore.

funola · #20 10-27-2014, 09:06 AM

Quote:

Originally Posted by sloride

Do you have an idea how that would work on an 85 300D that GP relay times out at around 25 seconds? I did not see on Walts vehicle ID what this was in response to.

Should work on any car as far as current measurement. GP relay time out duration is temperature dependent. What ambient/ engine temp was the 25 seconds at?

97 SL320 · #21 10-27-2014, 06:56 PM

Quote:

Originally Posted by ah-kay

With no disrepect. Why go through such elaborate set up to deterimine 1 or 2 GP is out??? The GP light would have told you something you wanted to know. Lot to do about nothing. The ammeter will NOT tell you which one in any event.

Actually it will if you clamp to the wires between the glow plugs.

If the power feed is at cyl 4 , clamping:

before the plugs will give you 4 * current of a single plug,

between 3 and 4 __ 3 * current

between 2 and 3 __ 2 * current

Between 1 and 2 __ 1 * current

So, if current before 4 and between 3 / 4 is the same, # 4 is open.

if current between 3 / 4 and 2 / 3 is the same, # 3 is open.

if the current between 2 / 3 and 1 / 2 is the same , # 2 is open.

If current between 1 / 2 is zero , # 1 is open

ah-kay · #22 10-27-2014, 07:23 PM

Quote:

Originally Posted by 97 SL320

Actually it will if you clamp to the wires between the glow plugs.

If the power feed is at cyl 4 , clamping:

before the plugs will give you 4 * current of a single plug,

between 3 and 4 __ 3 * current

between 2 and 3 __ 2 * current

Between 1 and 2 __ 1 * current

So, if current before 4 and between 3 / 4 is the same, # 4 is open.

if current between 3 / 4 and 2 / 3 is the same, # 3 is open.

if the current between 2 / 3 and 1 / 2 is the same , # 2 is open.

If current between 1 / 2 is zero , # 1 is open

is this a logic lesson or deduction reasoning. I think I will stay with a multimeter. Thanks.

97 SL320 · #23 10-28-2014, 07:15 PM

Quote:

Originally Posted by ah-kay

is this a logic lesson or deduction reasoning. I think I will stay with a multimeter. Thanks.

It is a lesson in both as effective trouble shooting requires using logic and deductive reasoning. Doing it any other way is guessing right from the start.

On a V8 diesel with glow plugs, individual wires are used for connection. An amp clamp can clip on and test each one in the span of a few minutes where a ohm meter takes much longer as it requires wires to be removed.

With an inline engine / buss bar hook up, access is a bit tighter so a hold against the bus bar meter is just fine as we are looking for big changes in current flow.

ROLLGUY · #24 10-28-2014, 07:36 PM

Quote:

Originally Posted by funola

The OP asked for an easy way to test glow plugs in car. The clamp ammeter will tell you in less than 30 seconds if any glow plugs are out, can't get any easier than that. If any are out, then you find out which one with an ohm meter. True, the glow plug relay pre-glow indicator is supposed to tell you if one or more glow plugs are out (if the relay is still fully functional) and if you know how to interpret the light. But these things are 30 years old and most of them do not work as designed anymore.

If the person testing does not have a clamp ammeter, then that would not be the easiest way to test the glow plugs. Most everyone has a VOM. I have tested GP's with an ohm meter, and have had mixed results. Some GP's that ohm'd out turned out to be bad anyway. The easiest and cheapest way by far to test GP's in the car is as follows:
(1) Remove the GP harness from the GP relay
(2) take a piece of wire (or clip lead) and connect one end to +12v (terminal on GP relay is a good place)
(3) touch the glow plug connection (at the GP) with the other end of the wire or clip lead. If it sparks a little, the GP is good. If no spark, the GP is bad. It does not get more simple than that (or cheaper, as everyone has a piece of wire laying around). I have found bad GP's this way that ohm'd good, bench tested them afterward, and they were in fact bad.