Under 10 volts at glowplugs - PeachParts Mercedes-Benz Forum

Zacharias · #1 02-10-2013, 01:58 PM

I have had cold start issues for a while, in this thread: Persistent cold start problem - now below -11 C / 12 F

Today I did some checking and rechecking:

1. Redid OHM test with block heater inactive. Four plugs now measure .5-.6, the one that previously gave huge OHM reading now gives infinity.

2. Have power to top right pin in small connector at relay with ignition on.

3. When relay active I have 11.98 volts at all five male pins in relay.

4. HOWEVER, only have 9.75 volts at glowplugs when relay is activated. (I had previously used a test light on the GPs, today I used the multimeter.)

What am I seeing here?

cooljjay · #2 02-10-2013, 02:05 PM

Save yourself the headache and thinking.....just replace all your glow plugs and forget about the readings. I get 10v at each plug and my car starts fine...my glows all ohmed fine but my car wouldn't start....after i replaced all the glows...it fires right up....

Zacharias · #3 02-10-2013, 02:14 PM

The plugs were new in October. At this point I have no problem replacing them -- need to change one anyway -- but I would like a solid reason for doing all five rather than thinking that I am throwing parts at the issue (I will have to have a shop do it, I am working outside).

I am leaning more toward the battery as a starting point as the relay shouldn't be showing under 12v, as far as I know.

Fattyman · #4 02-10-2013, 02:29 PM

Quote:

Originally Posted by Zacharias

The plugs were new in October. At this point I have no problem replacing them -- need to change one anyway -- but I would like a solid reason for doing all five rather than thinking that I am throwing parts at the issue (I will have to have a shop do it, I am working outside).

I am leaning more toward the battery as a starting point as the relay shouldn't be showing under 12v, as far as I know.

Did the recess for the glow plugs get reamed when they were replaced?

cooljjay · #5 02-10-2013, 02:33 PM

When was the starter last replaced on the car? Are the battery cables original? If the plugs were just replaced I wouldn't suspect them....

Zacharias · #6 02-10-2013, 03:00 PM

Quote:

Originally Posted by Fattyman

Did the recess for the glow plugs get reamed when they were replaced?

Ah. No. I borrowed the tool and when I went to use it, it wasn't the right one. Larger diameter.

Zacharias · #7 02-10-2013, 03:04 PM

Quote:

Originally Posted by cooljjay

When was the starter last replaced on the car? Are the battery cables original? If the plugs were just replaced I wouldn't suspect them....

No idea on the starter. It turns plenty fast for the compression on this car.

I understand where you are going but we're talking about a car that fires on the first crank at around -5 C then basically acts like it has no glow at -10 C. It is getting fuel, you can smell it out the exhaust as it cranks.

I have swapped between two relays along the way as well.

The valves were adjusted in September.

cooljjay · #8 02-10-2013, 03:12 PM

Well guess its time too smash the car....

I would replace the starter, the brushes do ware down and the old grease will get hard and a bad starter will kill the battery....and cause a strain on voltage....but you say no sooooo.....

kerry · #9 02-10-2013, 04:19 PM

I'd begin by just swapping out the one known bad plug and go from there. It's only like 5 minutes to do that.

Diesel911 · #10 02-10-2013, 05:31 PM

Quote:

Originally Posted by Zacharias

The plugs were new in October. At this point I have no problem replacing them -- need to change one anyway -- but I would like a solid reason for doing all five rather than thinking that I am throwing parts at the issue (I will have to have a shop do it, I am working outside).

I am leaning more toward the battery as a starting point as the relay shouldn't be showing under 12v, as far as I know.

About 10.5 volts at the Glow Plugs is normal. However, cold temps sap your Battery so if it was really cold you would expect the Voltage at the Plugs to be lower.

Try the same test after the Car has be running and it is nice and hot under the Hood and the Battery fully charged and see if the Voltage goes up.
If not it could be likely it is your Battery.

There is at least 2 Members who have said removing the Glow Plugs and reaming or otherwise cleaning the Carbon out of the Holes improved their starting.

As far as replacing all of the Plugs instead of only replacing the bad ones that seems to be a personal choice.

But, you can see the though going on with that. If one is bad speculation is the others could also be close to the end of their life.

New Plugs with the exception of occasional duds are though to be at the beginning of their life and the hope is it will be several years before any of them have issues.

Here in S CA where it would be a reall cold day if it was 48 degrees F I would have no issues only replacing the bad ones.

If I lived someplace where the temps are minus something (617.952 Engine) I would if one Glow Plug was bad and they all had been in there along time I would seriously consider replacing them all because I would hope to make it through the Winter without any further Glow Plug Issues.

On those Engines with Aluminu Heads that is a different story. In the Winter if a Plug is bad replace only the one Plug and hope it does not Break when you try to get it out.
If someone plans to remove the Glow Plugs on Engine with the Aluminum Heads that is better done when it is warm enough to remove the Cylinder head if you had to.

Diesel911 · #11 02-10-2013, 05:33 PM

Quote:

Originally Posted by Zacharias

Ah. No. I borrowed the tool and when I went to use it, it wasn't the right one. Larger diameter.

People have come up with some alternative methods to clean the Holes. If you have the pencil type Glow Plugs The actual Reamer end is 7mm.

barry12345 · #12 02-10-2013, 06:16 PM

New glow plugs a few months ago. Just change the bad one and go. You may want to do a free test of the battery if it is older depending on how the car starts with all the glow plugs working.

ah-kay · #13 02-10-2013, 06:26 PM

Quote:

Originally Posted by Zacharias

3. When relay active I have 11.98 volts at all five male pins in relay.

4. HOWEVER, only have 9.75 volts at glowplugs when relay is activated. (I had previously used a test light on the GPs, today I used the multimeter.)

What am I seeing here?

You are missing nothing. 9.75V, could be less, at the plug is normal The GP is telling you it is drawing a lot of current and the terminal voltage drops. Measuring the resistance of the GP is good enough. You may have a weak battery if the car does not crank as fast as it should. Otherwise there is nothing wrong with the GP.

vstech · #14 02-10-2013, 07:56 PM

what brand plugs did you use?

Fattyman · #15 02-10-2013, 08:24 PM

Ream the glow plug holes before putting them back in.
You won't be sorry.