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.

pdxwaker · #7 10-26-2005, 11:59 AM

Thanks dabenz... very good info. The 220d's are new to me.

I'm used to the plethora of information available on the W123/W126 cars. It sure seems like there are _way_ fewer 115 drivers active around here (from searches and responses to previous queries)...

Maybe I should take the lead and start to document some of my W115 repairs like BoostnBenze does for the W123/W126...

Ralph69220d · #8 10-29-2005, 08:53 PM

The 220d oem glow plugs have inner insulation preventing short circuit's internally, but this substance (I don't recall what is it), can break down over time resulting in partial excess voltage drops across that plug. The manual give's the precise voltage drops that should be seen from plug to plug.

pdxwaker · #9 11-02-2005, 10:48 AM

I'm still having problems with the glow indicator. Here's what I've measured at the glow plugs:

4 in: 7.83V
4 out: 6.58V
w-wire (resistor)
3 in: 4.9V
3 out: 3.74V
u-wire (not meant to be resistive)
2 in: 3.66V
2 out: 2.72V
w-wire (resistor)
1 in: 1.12V
1 out: GND (measured 72mV relative to battery GND)

So... 1.25V at GP4, 1.16V at GP3, 0.94V at GP2, and 1.05V at GP1. Note that these measurements aren't perfect... I had to use the w-wire to connect to the plugs so there is some additional resistance (I tried to measure as close to the plug as possible). The input voltage at 4 seems low to me but I'm not familiar enough with the circuit to say so for sure. Note that my alternator light dims significantly while cycling the plugs indicating somewhat strong current.

I also checked voltage at GP4 (in) when starting... 7.42V... a slight drop due to additional (significant) starter current.

I'm still getting no glow at the indicator regardless of how long I cycle the plugs for. All my shop manual shows for the circuit is a dual position switch on the starter knob which turns on the circuit. There is a 'resistive control unit' in series with the glow plugs and w-wires. No indicator is shown in the circuit... would this be the same as the 'resistive control unit'?

Anyway, before I dig into my dashboard (not easily accessible) I thought I'd seek advice. Is there anyone out there with 220d experience who can better explain the circuit? I'd really be surprised if 50A of glow plug current is supposed to flow through the starter switch. Is there no relay involved? What exactly is the 'resistive control unit' and where is it located?

The good news is that I'm still starting at ~45F... a bit rough but starting. I haven't been able to measure glow plug current yet... I don't have the necessary meter.

Palangi · #10 11-02-2005, 09:06 PM

I never had a 220D, however I had a 110 200D and a 115 240D.

The 200D had a big honkin' resistor coil in a ceramic / metal box under the hood, wired in series ahead of the glow plugs. Shooting from memory, but I don't recall it having resistance wires between the plugs.

The 240D had resistance wires, but I don't think it had a resistor coil. As much heat as the coil put out, it would have to be in the engine compartment. It would be a hazard if it was under the dash somewhere.

If your car has both resistance wires AND a dropping resistor, that would seem very suspicious to me. Unfortunately, I no longer have my 115 manuals, so I cannot verify if my suspicions are correct or not.

If I'm not mistaken, the voltage is stamped on the glow plugs. Maybe pull ope and see what voltage they are supposed to have across them.

pdxwaker · #11 11-03-2005, 10:49 AM

I haven't found the 'resitive control unit'... only the dash indicator. I did measure ~45A of current during glow... pretty descent but not quite spec of 50A.

I'm working on getting the Chilton manual. The Mercedes documentation for the 220d is horrible. The owner's manual doesn't even provide fuse locations and the service manual isn't much better. I'm hoping for more with the Chilton's book. Unfortunately I've been spoiled by the unbelievable amount of information available for the W123.

Palangi · #12 11-03-2005, 11:36 PM

I found the Chiltons manual to be worthless as a milking stool under a bull. I don't remember it having any information on the glow plug circuit, but I will look when I get back home (which may be saturday or sunday).

Ralph69220d · #13 11-05-2005, 02:38 PM

My MB 220d says:
the GP's have an input of approx 50-60 amps (though some w115's use a 50amp fuse, so input current must vary with year ) with a voltage drop of 0.9 volts per plug.
the GP's remain energized when cranking the engine.
too high of preglow current is a result of a gp failure or poor ground. or short circuit in system.
oil and/or carbon bridges are the leading cause of gp failure's according to mb manual.
(my favorite): there is an insulation material within each gp to prevent internal short circuit's. you can see from the filament end of the gp the "end" of one of the two chamber's containing the insulating material (non-specified as to what this substance is). if you see erosion at the end, imo, that is an indication to not trust the gp. any carbon buildup is a big danger sign. bad injector(s) with poor spray patterns are listed as major cause of gp failure.
too low pressure from the ip is a culprit in gp failure. ip timing set too far advanced is a culprit in early gp failure.
note, that all of the above refers to the loop filament type of gp. testing for short circuits in a cold gp can be problematic if the insulation material is cold, that is, the short doesn't show up until the gp is hot.
other piece of info: in the 220d combustion temps are around 800 degrees centigrade. in preglow, the filament is around 900-1000 degrees centigrade. the threaded bore of the head into which the gp threads must be cleaned meticulously. using the tool to clean the prechamber opening into which gp slides is very necessary.
sorry for the disordered post. i'm just quoting or paraphrasing specific points contained in the manual.

dabenz · #14 11-06-2005, 10:49 AM

pdxwaker: let's back up a bit (for us old guys). You can figure out the month and year of manufacture from the chassis and motor numbers. I have the CD - a "well worth it" purchase from here - and there are 3 or four different configurations from 1970-1972 or 1973.

Here's an analogy: if you wire a 240V baseboard heater for 120V then you get 1/3 the heat output. The heater (and your glow plugs) need proper input voltage as well as enough current. The salt shaker isn't going to glow if the input voltage is too low. Are the hangers heating up? They should get hot - not just warm.

I think you're losing voltage before the salt shaker. Don't remove it from the circuit as it's the resistive control unit. Measure voltage before and across the salt shaker - easy to reach once you remove the kick panel under the dash. Depending on the version, the salt shaker gets power from the gorilla knob via two parallel wires from either the key or the headlight switch. And please measure battery voltage - across the battery - before and after glowing.