220d glow plug problem - PeachParts Mercedes-Benz Forum

pdxwaker · #1 10-25-2005, 04:17 PM

My 220d salt shaker indicator on the dash does not light when I cycle the glow plugs. However, the brake light dims and the car does start indicating that I'm at least getting some 'glow'. I'm having a hard time finding a troubleshooting procedure for series type glow plugs that includes the dash indicator. It also looks like the online factory manual does not include the 115 section (at least clicking on this section doesn't seem to work).

Does anyone have a good test procedure for testing all components in the series style glow plug system (i.e. measured voltage at each plug should be within xx range)?

Jim H · #2 10-25-2005, 07:30 PM

Quote:

Originally Posted by pdxwaker

...Does anyone have a good test procedure for testing all components in the series style glow plug system (i.e. measured voltage at each plug should be within xx range)?

Well, in a series circuit, if any one of them is open, none of them will work.

Take voltage readings to ground along the 'string' and see if any look like they don't belong in the series. Readings should start at battery voltage, and drop off in proportion, such as 12V / 4 plugs = 3 volts per plug, so 12V, 9V, 6V and 3V would be expected.

Good luck. Others will add to this.

pdxwaker · #3 10-25-2005, 08:11 PM

Quote:

Originally Posted by Jim H

Well, in a series circuit, if any one of them is open, none of them will work.

Take voltage readings to ground along the 'string' and see if any look like they don't belong in the series. Readings should start at battery voltage, and drop off in proportion, such as 12V / 4 plugs = 3 volts per plug, so 12V, 9V, 6V and 3V would be expected.

Good luck. Others will add to this.

It doesn't seem quite that simple... there are voltage dropping wires between the plugs and not all of those seem to drop the same voltage. I'm also not getting a full 12V at the first plug in series (#4)... the salt shaker indicator and/or relay/wiring must be dropping voltage before this. A diagram sure would help...

BluEyes · #4 10-25-2005, 11:19 PM

How long are you holding the knob in the pre-glow position? My 5th glow plug (that's all it is) doesn't get red hot untill a count of 20-30 depending on the temp outside.
If it's not that cold, I'll only preglow for a count of 10-15 and the engine starts right up, but the indicator never gets red.

As far as testing the plugs themselves, if you have voltage anywhere at any of the plugs, or on any of the resistance wires then they all have to be good because they are in series.
If you don't have an assistant to hold the glowplugs on while you test, you could also test for contunity across each plug.

pdxwaker · #5 10-26-2005, 10:50 AM

Quote:

Originally Posted by BluEyes

As far as testing the plugs themselves, if you have voltage anywhere at any of the plugs, or on any of the resistance wires then they all have to be good because they are in series.

I've read some posts that say the plugs can fail shorted (not just open). In this case all plugs will have voltage. I pulled a plug and it is marked '0.9V'. For now I'll use this spec +/-20% for good/bad threshold.

Thanks for the tip on the indicator... I'll try proglowing a bit longer to see if I get glow at the indicator.

Is there any sort of fuse in the system that affects only the indicator?

I'm still confused as to why I only see ~9V at the first plug in series. The only thing I can think of is that ~3V is dropped at the indicator. I also can't figure out how to pull the indicator for testing... any tips?

dabenz · #6 10-26-2005, 11:38 AM

First quick and dirty is to feel the "hangers". They should get hot as they're really resistors just like the glow plugs. Then I work backwards from the ground at front cylinder, checking the dead system for shorts to ground with an ohmmeter. Make sure the ground is good and clean. Three places to check at a glow plug: plug tip (the end you see now) and both sides of the ceramic. The system can short from carbon or a cracked ceramic, then blow (open) the plug upstream of the short (from too much current). Carbon means you need to ream out the hole. A blown plug without a short means you probably have a drippy injector (water or dirt in fuel). Treat those glow plugs as a red flag that points you to the real problem. The salt shaker is a "glow plug": you'd better get less than battery voltage downstream of the salt shaker. 9v is a bit low: check and clean the battery connectors and the starter ground strap.