1969 220D Failing Glow Plugs - Mercedes-Benz Forum

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post #1 of 11 (permalink) Old 02-15-2011, 04:04 PM Thread Starter
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1969 220D Failing Glow Plugs

I just finished replacing the fuel injection pump with a good used one and it was running fine, but I had a glow plug failure. As most of you probably know these old diesels glow plugs are series wired so if one goes none of them work. I ordered a new set and replaced them. Two failed almost immediately. I replaced the failed ones with 2 of the old ones that seemed to still be fine. I suspected that I got a bad set of GP (Bosch but made in India); however, after several days one of the old ones failed. Seems like too much of a coincidence. I am getting 12V at supply when I pull out the starting knob (verified with volt meter). It's not shorting anywhere that I can see.

Any suggestions?
THanks,
Greg
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post #2 of 11 (permalink) Old 02-15-2011, 04:51 PM
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I have had experience with the glow plug systems on the earlier W110 bodied cars and I believe this system is virtually identical. On those cars you have the glow plug resistor located on the dash and another resistor located on the engine side of the firewall. There should be 8 VDC coming out of this second resistor, which connects directly to the glow plugs. If you have 12VDC there (or at the first glow plug), then the firewall resistor is shorted and sending too much power to the glow plugs, which will burn one out quickly.

Check the metal guard on the firewall resistor. Sometimes they can come loose and touch the resistor, shorting it. Another place to check is to make sure that none of the connecting wires between the glow plugs is shorted to the engine block or head. If one is, this will put too much current through the others and burn one out.

Last edited by KCM; 02-15-2011 at 09:17 PM.
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post #3 of 11 (permalink) Old 02-15-2011, 09:16 PM
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After doing some more research, turns out your car does not use the firewall resistor. But if you have 12 VDC at the lead to the first glow plug (#4), you still have a problem, as it should be 8 VDC. Either you have an open circuit between the power wire from the dash resistor and the ground wire on the #1 plug, or your dash resistor is shorted out. Each glow plug is 0.9 VDC and the squiggly connecting wires take up the rest of the voltage as they are also resistors. So you should have a voltage drop of around 0.9 VDC before and after each plug, and the squiggly resistor wires should have a drop of roughly 2.2 VDC each. You can measure this with your voltage meter.
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post #4 of 11 (permalink) Old 02-16-2011, 07:07 AM
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How do they fail? Can you post a picture of a failed glow plug?

Failing glow plugs can be electrical (to much voltage as described above) but could also be related to the injection system: injection timing too early, injectors faulty.
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post #5 of 11 (permalink) Old 02-16-2011, 09:47 AM Thread Starter
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Originally Posted by KCM View Post
After doing some more research, turns out your car does not use the firewall resistor. But if you have 12 VDC at the lead to the first glow plug (#4), you still have a problem, as it should be 8 VDC. Either you have an open circuit between the power wire from the dash resistor and the ground wire on the #1 plug, or your dash resistor is shorted out. Each glow plug is 0.9 VDC and the squiggly connecting wires take up the rest of the voltage as they are also resistors. So you should have a voltage drop of around 0.9 VDC before and after each plug, and the squiggly resistor wires should have a drop of roughly 2.2 VDC each. You can measure this with your voltage meter.
Thanks KCM. Yes, I looked for a resistor and couldn't find one. The wire leads directly from the #4 GP to a bundle of wires without anything intervening. I will check the voltage again (when it stops raining)
Greg
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post #6 of 11 (permalink) Old 02-16-2011, 11:10 AM Thread Starter
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How do they fail? Can you post a picture of a failed glow plug?

Failing glow plugs can be electrical (to much voltage as described above) but could also be related to the injection system: injection timing too early, injectors faulty.
Hi Govert, Yes I wondered about timing and if it was making a "rough ride" for the glow plugs (and other things). As you probably remember, immediately after installing the "new" pump I had trouble getting it to deliver fluid when turned over by hand. Who knows how long the donor car sat unused. Eventually it started to pump on all 4, but #1 was least of all and there seemed to be a big discrepancy in onset between #4 and #1 (#1 was much later) so I timed with #4 reasoning that #1 was not giving me an accurate onset of injection. Onset of injection was after 30 BTDC on #4 but not by a lot. It sounds like I should check again this weekend.

Regarding the failure mode of the glow plugs and their appearance; when it first happened I thought that maybe the current supply was faulty (I was trying to leave work and didn't have many resources at hand), so I tried to "jump" them with battery cables. A rather awkward thing to do and I can't say that I might not have made things worse. When I changed the G.P.s #1 filament was parted; it looked like a weld cut with a small amount of bead at each broken end, but again this might have been my doing with the jumper cables rather than the original problem. The other G.P.s looked normal but one was open, or nearly so to an ohm meter. When the new ones failed (t out of 4) they looked fine but two had high resistance.
Greg
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post #7 of 11 (permalink) Old 02-16-2011, 01:10 PM
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Jumping the glowers on a W115 isn't advisable (as described above), that would have made things worse.

The glower can short-circuit, for instance by the gas linkage touching the resistance wires between the glowers. It is also possible that carbon build up inside the glow plugs hole causes short circuits. Do some measurements with a multimeter.

The dash glower does work (usually takes about 20-30 seconds to glow)?

If there is no electrical problem and the glowers keep burning, than it is timing.

If you go to this page: 15 Electrical System Engine (Preglow) - OM616 and there select 15-512 Damage on filament-type glow plugs and its cause you can see some typical damage to glow plugs and its causes.

There should be no more than 2 degrees difference between the timing of cylinder 1 and 4 and 30 BTDC is too early anyway.
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post #8 of 11 (permalink) Old 02-16-2011, 05:00 PM Thread Starter
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Thanks Govert, They've failed twice. The first time #1 GP looked like it had literally melted, but that could have been my fault due to trying to jump it, the other failed plug looked normal. The second time the two failed plugs also looked normal, nothing like the photos in the link you sent me. Both times they were were very easily removed with no excessive carbon or evidence of mechanical failure or impingment. During both failures the dash glow stopped working and the ignition light would no longer dim when I pulled on the starting knob to engage the GPs (but short of activating the starter). This suggests to me that the circuit is open and one of the GP has failed, but at other times everything has been normal. I'd post the picture but it's asking for a URL (web site right?).
Greg
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post #9 of 11 (permalink) Old 02-16-2011, 06:31 PM
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You can attach a picture. Just go to the advanced posting options and there is a paper clip icon.
Click on that and attach the picture you want to post.
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post #10 of 11 (permalink) Old 02-17-2011, 12:11 PM Thread Starter
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Trying again with uploading a photo. Just to reiterate, the failed glow plugs pretty much looked like new, except one of the two that failed the first time when I tried to jump them. This one looked like #3 in the link that Govert sent. Unfortunately I threw this one away, so no picture of it.
Greg
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