W115 240D Glow Indicator Filament Woes - PeachParts Mercedes-Benz Forum

Cairo1966_W110 · #1 12-26-2022, 12:56 PM

I'm having some trouble with my W115 glow plug indicator - the "salt shaker" in the dash. The filament gets red/orange very quickly within just a few seconds (3) of applying the glow with the pull starter (gorilla knob). If I am not careful, it will blow the filament within 6 or 7 seconds.

The glow plugs themselves do not heat enough to start the engine and the bendy resistor wire between plugs 1 - 2 and 3 - 4 is only just a tiny bit warm to the touch.

To me this indicates a short somewhere. (Or a bad or wrong filament - ?)

However, if I remove the ground strap (which goes from the last glow plug to the engine block, made of braided copper) - then the filament doesn't light at all. So, it seems no short, because the only way the circuit is complete is if the ground strap is in place. Is my thinking correct there? If there was a short, with the ground strap disconnected, and current applied, the filament would glow, because the short would "complete the circuit.". But it remains dark.

I have tested each glow plug, removed them all many times, used the multimeter, I can detect no fault or short. None of the glow plugs show connectivity to the engine block, except when the ground strap is in place. I've swapped them with a new set (and a working set from my W110). I've cleaned the carbon out of each glow plug hole, they are pretty clean.

If I disconnect the regular wire to the first glow plug, and I use a separate wire directly from this glow plug to the positive terminal of the battery, all the glow plugs work, the heat quickly (and as far as I can tell, evenly). The two bendy resistor wires get very hot to the touch. Motor can start from cold (14 C day) after just 4 or 5 seconds of glow this way.

I have the wiring diagram, and the system seems quite simple - power is supplied from the battery when the key is in the run position to the gorilla knob, the gorilla knob in the glow position provides power, through the indicator filament, directly to the first glow plug, through all the plugs to the ground strap and ground to the engine block. As far as I can tell that system is in place, wires are in good shape, and no shorts.

Anyone have any ideas or hints on what the problem could be? Advice would be greatly appreciated and I'm stumped here for more than a week (and blown 2 filaments).

PS. this is the older "loop" style glow plugs, in series with each other.

cleeves · #2 12-29-2022, 06:58 PM

Quote:

Originally Posted by Cairo1966_W110

I'm having some trouble with my W115 glow plug indicator - the "salt shaker" in the dash. The filament gets red/orange very quickly within just a few seconds (3) of applying the glow with the pull starter (gorilla knob). If I am not careful, it will blow the filament within 6 or 7 seconds.

The glow plugs themselves do not heat enough to start the engine and the bendy resistor wire between plugs 1 - 2 and 3 - 4 is only just a tiny bit warm to the touch.

To me this indicates a short somewhere. (Or a bad or wrong filament - ?)

However, if I remove the ground strap (which goes from the last glow plug to the engine block, made of braided copper) - then the filament doesn't light at all. So, it seems no short, because the only way the circuit is complete is if the ground strap is in place. Is my thinking correct there? If there was a short, with the ground strap disconnected, and current applied, the filament would glow, because the short would "complete the circuit.". But it remains dark.

I have tested each glow plug, removed them all many times, used the multimeter, I can detect no fault or short. None of the glow plugs show connectivity to the engine block, except when the ground strap is in place. I've swapped them with a new set (and a working set from my W110). I've cleaned the carbon out of each glow plug hole, they are pretty clean.

If I disconnect the regular wire to the first glow plug, and I use a separate wire directly from this glow plug to the positive terminal of the battery, all the glow plugs work, the heat quickly (and as far as I can tell, evenly). The two bendy resistor wires get very hot to the touch. Motor can start from cold (14 C day) after just 4 or 5 seconds of glow this way.

I have the wiring diagram, and the system seems quite simple - power is supplied from the battery when the key is in the run position to the gorilla knob, the gorilla knob in the glow position provides power, through the indicator filament, directly to the first glow plug, through all the plugs to the ground strap and ground to the engine block. As far as I can tell that system is in place, wires are in good shape, and no shorts.

Anyone have any ideas or hints on what the problem could be? Advice would be greatly appreciated and I'm stumped here for more than a week (and blown 2 filaments).

PS. this is the older "loop" style glow plugs, in series with each other.

Others might have a more direct answer but if it were me I'd continue on the trouble shooting route and measure the voltage drop to ground on each of the glowplugs. Each of them should be something like 12/4 = 3Volts, so something a bit under 12V on the other side of the dash filament, then a bit under 9 on the next plug, to 6 and 3. Since it blows quickly hook up your multimeter and put it facing the windsheidl so you can pull the glow starter only 1-2 seconds without blowing the filament and momentarily see each of them.

If each of these voltages is normal, I would suspect it is the wrong filament, and then connect the (+) terminal of the battery to the first glow plug and heat them up without the filament and see if you can get it going, if so there is some issue with the filament.

-Henry

vox_incognita · #3 12-29-2022, 10:20 PM

These might help a bit

http://eva2.homeip.net/W115/program/Engine/615/15-510.pdf

http://eva2.homeip.net/W115/program/Engine/615/15-505.pdf

I would run a min.6mm2 wire through the ballast resistor on the firewall from + term. to GP on cyl.4 and check function. This will say whether the short is somewhere upstream (pre-glow/starter switch). Then investigate. Measure the resistance and voltage as shown in the docs ,also A between the ballast resistor and GP #4.
Do not run battery current w/o the ballast resistor, it'll fry the GP's.

IIRC at starting (when you pull the knob to operate the starter) the salt shaker is bypassed to give the GP more current.

t walgamuth · #4 12-30-2022, 06:59 AM

Maybe you have the wrong glow plugs. Do you have the single loop style or the "pencil style?

HGV · #5 01-01-2023, 10:18 PM

You have burned out plugs. Replace them with the upgraded pencil plugs that are not in series but in parallel and get rid of the bendy wire things. The glow plugs will then each recieve battery voltage of 12v instean of 3v and all will be good. Ibsuspect you have two bad plugs.

Cairo1966_W110 · #6 01-26-2023, 07:44 AM

Thanks everyone for posting your thoughts and advice, especially the PDF pages of the manual were great.

So, in the end, it was the filament. This is the filament inside the "salt shaker" on the dashboard. I was using a too thin filament, too much resistance, and it was blowing before any current could get to the actual glow plugs. On a recent trip to Germany, I picked up a few original bosch filaments, as you can see much thicker, and now everything is normal.

Anyone ever seen these thinner filaments or know for what application they are for?

Incidentally, you mentioned a "ballast resistor" on the firewall. My W110 has this in place (no coil inside), but it is bypassed and the W115 doesn't have it at all. Everything seems to be working ok.... I have tried to find replacements, but it seems this part is really unobtainable. If I can find one, I will restore the system to use the resistor.

Both cars with the loop style in-series glow plugs.

Anyway, thanks again everyone for your input! This Sunday a new rebuilt automatic transmission for the W115 .... !