I drove home from work yesterday and the salt shaker glow plug above the steering column was dead cold despite holding the gorilla knob at position 2 for 30-45 seconds. But when cranking the engine it fired up normally (rough and smoky ).

Is the dash glow plug indicator wired in parallel with the glow plugs on the block? I thought they were all arranged in series?

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1974 240D 3.0L 4(M) - 250K -OM617.910 + 300D FW + Rajay turbo power!
1977 6.9 3(A) - 84k

On my 123 240D I had a bad ground in my instrument cluster and the GP blub wouldn't light up, but the plugs where glowing. The light must be in parallel with the series plugs. So it's likely your 115 is the same.

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The W115 has a manual contact switch via the pull knob so it's not key operated.

The car starts but it starts harder than normal 15-18 cranks instead of the usual 8-12 as I have no idea if the plugs are even working at all.

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1974 240D 3.0L 4(M) - 250K -OM617.910 + 300D FW + Rajay turbo power!
1977 6.9 3(A) - 84k

I am almost sure the GPs and the indicator are in series on the W115 so if the circuit opens or the indicator opens they will not work...check for loose wires under the hood...it has a sort of wireform connector between the plugs as I recall. If you're still in L.A. you hardly would need to glow anyway, but you need it working if you drive anyplace colder than 50-60 degrees. Unless you have really low compression or are starting cold without glowing first it shouldn't take even 8 revs to start it up...it should start on the second or third rev.

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Marty D.

2013 C300 4Matic
1984 BMW 733i
2013 Lincoln MKz

Thanks Marty, I have been looking for loose wires but I haven't found any yet since it went out when I left the office yesterday. There is a wire running to the block (resistor?). It appears to be connected but I haven't touched it since the engine is a greasy mess from all the blowby leakage around the CCV. The engine is starting but it does take a lot of cranks until it just catches suddenly.

The temps are about 50-65F but the engine is worn or acts like it does because of the insane oil consumption of about a quart per 100 miles. It's almost a 2 cycle!

On the other hand the W123 fires in 2-3 revs like you said.

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1974 240D 3.0L 4(M) - 250K -OM617.910 + 300D FW + Rajay turbo power!
1977 6.9 3(A) - 84k

From what I've read about this system: The indicator is in series with the plugs unless someone changed it to the new-style pencil plugs, in which case, the dash resistor would either be bypassed or replaced with a light bulb.

Here's what's happening: During preheat, the four glow plugs and the dash light all heat up together. When you pull the knob out to start, the glow light is bypassed to overcome the starter's current draw but the glow plugs continue to receive current. So, if your dash light filament is burned out, nothing is heating until you pull the knob to start and then the glow plugs start to heat up while you crank. In about 7 seconds things are hot enough for the engine to start....it would be like turning the key right to start on a newer MB diesel without waiting for the yellow light to go off first.

My '76 usually starts in less than one second of cranking even in cold weather; if it takes longer than that it's usually because I have a fuel leak (air getting in). I don't know about the rough idle & smoke right after start up, mine does that too when cold, every time, but my engine has a few hundred thousand miles & moderate blow by as well, but I've seen this behavior on even newer diesels like the Ford PowerStrokes when they're first started.

I have no idea where you can get a replacement dash light filament except Mercedes ($$$) or a wrecking yard but you could check the continuity of yours with an ohmmeter to see if it is "open" (burned out) - another culprit from what I've read is the pull start switch not working right anymore in the pre-glow position (and **************.com sells replacement pull switches).

-AC

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1976 240D "Katja"