I read a few threads but got some conflicting information. On my 84 300d, do the plugs glow during starting? Is there an afterglow?

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1984 300D w/ 370k mi. daily driver
1994 7.3 idi non-turbo 180k mi.

Yes, the Glow Plugs are on during Starter Cranking.
If you look at the Terminal/Junction Block that is in front of the Battery the most forward position has a White and an Violet Wire (Purple?) See the pic.

The White Wire goes down to the Same Terminal on the Starter that the Ignition Switch goes to. When you start Cranking that Terminal gets Voltage and that Voltage goes up the White Wire to the Violet Wire. The Violet Wire connects to the Glow Plug Relay and turns your Glow Plugs on.

If you want to check that loosen the Violet Wire from the Terminal Block and pull it away from the White Wire.
If the Violet Wire is long enough simply touch it to one of the 2 rear ward Screws (clean the Rust off of the Screw Head if needed; if the Violet Wire does not reach us a jumper Wire) and you hear the Glow Plug Relay click on.
I don't know if when the Relay is activated it is entirely Manual or if the Timer is also activated.

If you think that is an odd way to do it. Even when the Engine is new no matter how fast the Engine spins during cranking the heat of compression is not going to be as hot as the Glow Plugs. That means that by not having the Glow Plugs on the extra Electricity available to crank the Engine is not going to accomplish as much heat wise as the Glow Plugs can. (This assumes your Starter, Cables, Grounds and Ignition Switch are within specs.)

In a worn Engine during cold Weather a lot of the heat of compression is absorbed by the cold Engine parts.
(See the Chart for details)
The Glow Plug heat zooms up to over 2,000 degrees F and settles down to 1,800 something degrees F and that higher heat is localized in the Pre-chamber where the Ignition of Fuel Is going to take place.
That is a higher heat than heat of compression can create; especially in a cold Engine.

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84 300D, 82 Volvo 244Gl Diesel

GP's usually time out in ~10 seconds

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1985 300D 198K sold
1982 300D 202K
1989 300E 125K
1992 940T

"If you dont have time to do it safely, you dont have time to do it"

"The democracy will cease to exist when you take away from those who are willing to work and give to those who would not."

The Glow Plug Light is a prompt for the Driver and does not control the Glow Plug Relay Timer (this is on the Pencil Type Glow Plugs and ones that do not have the afterglow feature).

If you keep the Key in the Pre-glow position after the Glow Plug light goes off here in Southern California Weather you get an other 20 seconds of Pre-glow time for a total of about 30 seconds.

If it is extremely cold you can cycle your key again and start over with another 30 seconds or so as many times as your Battery can take that.

The translation in the Mercedes Manual is not Clear.

But, you can test it Yourself.

Open your Hood and roll down the Drivers Window so you can hear the Glow Plug Relay click on and off.

Turn the Key to the Pre-glow Position and you hear a click when the glow Plug Relay turns on; start timing or counting the seconds.

Watch as the Glow Plug Light goes out and keep the Key in the Pre-glow position and keep counting/timing until you hear the Click when the Timer inside of the Glow Plug Relay turns it off.

Keep in mind that the Glow Plug Relay and Light do not know the condition of your Engine.

There is a Temperature Sensor built into the Relay an on some of the Older Pencil Glow Plug Relays it is connected to a Coolant Temp Sensor.

I don't live in an area that has cold Weather and have not seen it demonstrated if the Temp Sensors just make the Glow Plug Light stay on longer or it actually makes the Relay Timer stay on longer.
My best guess is that it only has an effect on how long that Light stays on.

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84 300D, 82 Volvo 244Gl Diesel

This is My own guess.

The Glow Plugs job is to heat the Air in the Pre-chamber.

When the Heating Element/Tip of the Glow Plug becomes surrounded with Carbon that Carbon cut off exposure to the Air and if it is solid enough allows heat to be conducted into the Cylinder Head/Coolant; not heating the Air as well.

When I tested a single Glow Plug connected to a 60amp Dash Board Automotive type Amp Peter when I applied the Voltage the Amp Meter pegged at 60 amps. As the Tip of the GP heated up the Amperage settled down to about 16 amps.

So when the GP is cold it pulls more amperage.
I think if the GP is surrounded by Carbon and heat is conducted into the Cylinder Head that keeps the GPs colder and pulling higher amperage for a longer period. I believe pulling that higher amperage for a longer time helps to shorten the life of the GPs

Besides that more then a Few Members have claimed reaming the Carbon out from the Glow Plug holes made their Winter Starting easier.

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84 300D, 82 Volvo 244Gl Diesel

Thanks diesel911 for the info. I have verified my relay clicks off after about 30 seconds. The light is worthless IMO. It never stays on more than 1-3 seconds no matter the temp. I suppose the sensor in the relay is malfunctioning. But the light doesn't matter bc it doesn't affect glow time in any way?

I thought maybe the plugs glowed AFTER starting based perhaps on the temp sensor in the relay. I'm familiar with older ford diesels (7.3 idi) which cycle the plugs after starting. The battery meter in those trucks makes this apparent.

If one were to install a manual control switch (momentary) what would be the correct way to operate it- considering cold temps and a worn engine? Would there be any problems with preglowing, glowing while starting, and then after glowing for another 5-10 seconds or so?

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1984 300D w/ 370k mi. daily driver
1994 7.3 idi non-turbo 180k mi.

[QUOTE=Danman;3229204]Thanks diesel911 for the info. I have verified my relay clicks off after about 30 seconds. The light is worthless IMO. It never stays on more than 1-3 seconds no matter the temp. I suppose the sensor in the relay is malfunctioning. But the light doesn't matter bc it doesn't affect glow time in any way?

I thought maybe the plugs glowed AFTER starting based perhaps on the temp sensor in the relay. I'm familiar with older ford diesels (7.3 idi) which cycle the plugs after starting. The battery meter in those trucks makes this apparent.

If one were to install a manual control switch (momentary) what would be the correct way to operate it- considering cold temps and a worn engine? Would there be any problems with preglowing, glowing while starting, and then after glowing for another 5-10 seconds or so?[/QUOTE]

In theory the Glow Plugs are only supposed to operate at Battery -11.5 Volts. It is not supposed to be good for them to have more Voltage than that. So it depends on how fast your Alternator catches up to Charge after the Engine is going for that.
I practice I don't know if it would be a problem or not.

I have a Manual Button on My Volvo Diesel and it uses similar but shorter Glow Plugs (6 of them). And, I frequently push the button after the engine is running. And, I have not had to replace a whole bunch of Glow Plugs doing that. So for Me in practice it has not been a problem.

I don't see any problem at all for the initial Start and keeping them on during cranking. That is what they do already.

Some of the later model Mercedes Diesels have a Glow Plug Relay with an afterglow function similar to the Ford one you spoke Of.
I believe the Plugs for them are slightly Shorter.
It was used on some of the 6 cylinder Diesels that have Iron Heads.

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84 300D, 82 Volvo 244Gl Diesel