Well my 240D glow plugs failed this morning - PeachParts Mercedes-Benz Forum

funola · #1 02-22-2015, 09:43 AM

Quote:

Originally Posted by Zacharias

I.........
No. You want to measure the resistance by putting the red lead into each female receptacle in the large plug at the end of the harness that runs from the relay to the glowplugs. The black lead goes to a good ground, preferably the negative battery terminal if the lead is long enough. The engine block is NOT a good ground.

Before anything else, check that you can hear the relay engage when you turn the key to the preglow position. If you don't hear it click, try tapping it on the side or on top. Also make sure that the strip fuse inside the glow relay is intact.

Why is the engine block not a good ground? It is where the glow plugs are grounded so it better be a good ground. The engine block is grounded via the ground strap on the driver's side bell housing and if it is not making a good connection, it could affect glow plug operation.

BTW, measuring resistance is not polarity sensitive (unless you are measuring the resistance of a diode), where you put the red and black leads does not matter. You want to make sure power is off while measuring resistance otherwise the reading will be wrong.

toomany MBZ · #2 02-22-2015, 10:49 AM

The glow plug relay is highlighted.

VT220D · #3 02-22-2015, 01:17 PM

Quote:

Originally Posted by funola

Why is the engine block not a good ground? It is where the glow plugs are grounded so it better be a good ground. The engine block is grounded via the ground strap on the driver's side bell housing and if it is not making a good connection, it could affect glow plug operation.

BTW, measuring resistance is not polarity sensitive (unless you are measuring the resistance of a diode), where you put the red and black leads does not matter. You want to make sure power is off while measuring resistance otherwise the reading will be wrong.

I think Zacharias meant that connecting the multimeter probe to the block can give a poor connection due to rust, dirt and oil. The engine ground does need to be good for the glow plugs to work properly. Since the starter draws about 250 amps I would expect to see problems cranking with a poor engine ground.

OP Because your glow light isn't coming on I suspect a strip fuse or glow control relay failure. If your strip fuse is blown your relay may be sticking. Even if every glow plug fails the relay will still operate and turn on the glow light for the appropriate amount of time.

If a single glow plug is bad the glow control relay will function normally and light the remaining plugs. However, after the car starts the glow light will flash. This is to warn you that a glow plug has failed. The control unit determines that a plug has failed by comparing the resistance of pairs of plugs. It can be fooled if both plugs in a pair are dead but that's uncommon.

Zacharias · #4 02-22-2015, 02:41 PM

Quote:

Originally Posted by funola

Why is the engine block not a good ground? It is where the glow plugs are grounded so it better be a good ground.

Good question. I tried using the block when I measured mine and got strange readings.

When I asked for advice here, I was told that only reliable ground was the negative cable. By several respondents.

When using a multimeter or test light on other cars I have always used the engine, or something attached to it, as a ground.

funola · #5 02-22-2015, 05:10 PM

Quote:

Originally Posted by Zacharias

Good question. I tried using the block when I measured mine and got strange readings.

When I asked for advice here, I was told that only reliable ground was the negative cable. By several respondents.

When using a multimeter or test light on other cars I have always used the engine, or something attached to it, as a ground.

You have to clean the corrosion off the engine ground point first by scraping or sharpen the probe to puncture through the corrosion to make a good ground connection.