Have a great ole 240d always been a runner, several weeks ago started to get harder to start in the mornings. Then one morning it would not start at all, hear some slight firing... Replaced the fuel filters, no start, replaced the glow plugs, no start, replaced with hi cca battery, no start.

I did notice that when i pull the glow plug handle out to warm them up then dash out under the hood and feel for warmth the rear most one is hot, the metal connector between that one and the 3rd back is glowing red..but no other plugs are warm.. is this normal? I have double checked my reinstall of the new glow plugs and they seem to be correct? Also when i pump the fuel primer i see surges in the clear secondary fuel filter and when i pulled an injector, i have some drips when turning the engine over, not squirts but drips, again unsure what is normal.

Also checked the voltage and was over 12 volts at the tip of number 4 plug and less then 2 volts at each of the remaining plugs...

yet the metal connectors all appear to be right?

Assuming you still have glowplugs in series, it sounds like the third glow plug is damaged or the wires at it are grounding against something. Perhaps you overtightened it and damaged it (I did this) or perhaps there is something touching that isn't supposed to. I would start investigating there and see if anything looks wrong. If not, I would swap out that plug with a new one.

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1968 220D, w115, /8, OM615, Automatic transmission.
My 1987 300TD wagon was sold and my 2003 W210 E320 wagon was totaled (sheds tear).

What Shortsguy1 said. I cracked an insulator once too. I've also seen an IP accelerator rod hit an improperly installed metal bar between number 3 and 4.

Either number 3 plug or the metal bar between 3 and 4 is shorted. Is your dash indicator glowing? In the scenario you describe the dash indicator should be glowing brightly and way too quickly.

When the start knob on a 115 is pulled to the glow position the dash indicator is connected in series with the 4 glow plugs. The result is a series string of 5 (4 glow plugs plus dash indicator) with about 2 volts across each plug. If number 3 is shorted then the dash indicator and number 4 will have 6 volts across them causing the dash indicator to glow brightly.

Also, when the start knob is pulled to the start position the dash indicator is bypassed. By design this reduces the series string to the 4 engine glow plugs resulting in nearly 3 volts across each glow plug when cranking to help start the engine. Unfortunately this means that in your scenario when you crank number 4 is getting nearly 12 volts. I say nearly because the metal connecting bars between the glow plugs have some resistance by design.

Because of this, at the very least, I would replace number 4 even if the short in number 3 is external. The life of number 4 has very likely been reduced and its electrical characteristics may have changed enough cause a problem. For the voltage drop across each plug to be equal they must draw the same amount of current.

It would be unfortunate to fix the short but end up with poor starting anyway. Especially this winter!

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1969 220D 5 Speed (OM616)
1983 240D 4 Speed
1985 300D Auto 376K
1985 300D Auto 275K

I think you guys have hit on something, the glow indicator is turning bright red really fast...

I will replace number 4 tomorrow and attempt to find the short.

thx

I would find and repair the short before you replace number 4. You don't want to power up the new glow plug with a shorted number 3. That may damage the new number 4. Since the metal bar between 3 and 4 is hot the short is definitely at number 3 and the existing number 4 still works well enough for testing.

If it's in the budget and reliability is important you might just want to replace them all. Either way find the problem first. Good luck!

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1969 220D 5 Speed (OM616)
1983 240D 4 Speed
1985 300D Auto 376K
1985 300D Auto 275K

After glow plugs, adjust the valves if not done recently. Ideally valves should be adjusted every fall before the onset of cold weather.

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Respectfully,
/s/
M. Dillon
'87 124.193 (300TD) "White Whale", ~392k miles, 3.5l IP fitted
'95 124.131 (E300) "Sapphire", 380k miles
'73 Balboa 20 "Sanctification"
Charleston SC

Either number three plug is shorted internally or the heavy wires are shorted to ground nearby. I even wonder if the loop end on that series plug might be contacting carbon in the number three cylinder of the engine. If carbon might be the issue decarbon all the glow plug holes. It makes the glow plugs work more efficient anyways.

Remove the plug and check it for any short between the exposed element and the plugs base threaded area with a meter. If none I might check for carbon buildup. Really though I expect the plug to be internally shorted to ground. .

Make up a small jumper cable with a wire size comparable to the hard wires in your system. Throw it in the glove compartment or trunk. That way if a plug burns out sometime you can just jump pass the burnt out glow plug and the car will usually start on three plugs.

This of course after you repair your current issue.

Very appreciated all the words of wisdom. The problem is solved. Steps taken were:

Have a spare 240d engine in the barn, removed two of the ceramic caps, the stiff wire connector between 3 and 4 swapped these into for my "runner".
Also put a new glow plug into #4 and moved #4 over to #3 spot (it tested fine with ohm).

This did the trick, so not sure what was the issue although I think with the stiff wire connector turning bright red that runs to #3 that the #3 plug grounding out in some way must of been the culprit.

Again thanks all...

Glad it is working again for you. That is what is so great about these old cars. They are simple enough that it is possible to diagnose and fix most problems relatively easily. Thanks for taking the time to close the loop on this thread, so that others can be helped as well.

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1968 220D, w115, /8, OM615, Automatic transmission.
My 1987 300TD wagon was sold and my 2003 W210 E320 wagon was totaled (sheds tear).