I'm sure this has been posted someplace BUT.....
On a 1993 300D, 2.5 Turbo Diesel, Anyone know what the voltage is to the Glow Plugs and the time duration that they have voltage to them. Do they stay on at a lower voltage and what are these numbers & times, also how do you make this check and what controls all this action?

Thanks.

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Den
1993 MB 300D 2.5
2001 VW Jetta TDI 5 spd.
2001 new bug TDI 5 spd.
Motorcycles

The glow plugs are designed for 11.5 volts. I don't think voltage checks are useful since your car has the glow plugs hooked up in parallel. Each glow plug gets 30 amps to begin. As the coil heats up resistance limits the current. The current spec is thus something like 8-16 amps per glow plug. The indicator lamp will go off after a time determined by the ambient temp inside the relay. Glow will continue for another 20-25 seconds while the key is in position 2 (run), or until the key goes to position 3 (start) and returns to position 2. Glow will continue under the same 20-25 second limit while the key is in position 3.

If your car has the afterglow system, the indicator lamp will go off as determined by engine coolant temp. Afterglow time is also determined by engine coolant temp. Whew.

Sixto
95 S420
87 300SDL

It has been posted may times, and using the Search feature will reveal a wealth of knowledge.

In general:

Full battery voltage is applied through a relay to each GP. (Some older models had series-wired GPs, where the full battery voltage applied across a series string.)

Glow time duration is determined by the relay sensing engine coolant temperature. The GPs are on longer when the coolant temperature is lower.

The relay may continue to apply power to the GPs after the starter is released, again dependent on engine model. I don't know which models apply.

As you have gathered by now, it's all controlled by a Glow Plug Relay, which is an electromagetic device with an operating coil and one contact per GP. When the coil is energized by the electronics, the contacts close and apply battery voltage to the GP.

There are electronics that sense when the key has been turned to Pos II, closing the contacts and lighting the GP light. Electronics then determine when enough time has passed to turn off the GP light, and how much longer the relay is to be energized.

If the GPs stay on after the starter has been engaged, they stay at full battery voltage, since the GP relay is just a switch.

A crude, but cheap way to check is to observe the dome light. When you turn the key to Pos II, the current drawn by the GPs should cause the light to dim. The GP light goes out after the minimum time required, indicating OK to Start, but the GPs are still energized. If you don't start the engine, you may see the dome light brighten again when the GP relay releases.

A voltmeter can also tell you a lot.

Hope this helps, and I'm sure others will add information I have omitted.

Best Regards,
Jim

Thanks for the great quick response. Now all i have to do is figure out if everything is working correctly. I may have a faulty relay, get a quick start up but 10 - 20 sec. later develop a rough idle and some times develop a slight missing or stalling until we reach 40 to 60 C. temp. Then she runs like a kitten.
Today I waited for the rough idle to start, waited a few sec. shut the engine off and went thru the start up procedure again. Results, a quick start up as usual, 10 -20 sec. later a little roughness but not as bad as the first try. Something smells here, yes. I'm being told something but I just didn't get it. Duhh...
Thanks again for the quick response and the info.

__________________
Den
1993 MB 300D 2.5
2001 VW Jetta TDI 5 spd.
2001 new bug TDI 5 spd.
Motorcycles