Glow Plug Light Question - PeachParts Mercedes-Benz Forum

jkubica · #1 11-07-2009, 01:58 PM

Hi All
On my 1985 300td, my glow plug light stays on for 2 or 3 seconds and then goes out.

If I start it as soon as the light goes out - it starts right away (2 or 3 revolutions) but has a miss for a couple seconds before running smoothly.

But if I count 10 before starting - it starts right away and runs smooth right away.

Is there a problem here?

Thanks,
Joseph

toomany MBZ · #2 11-07-2009, 04:08 PM

The relay is still powering the plugs until you hear a click, around 30 seconds total. The light will go out long before then.
So, no problem really, but check the plug resistance, just for grins.
It's getting to be that time of year for starting issues to become apparent.

Brian Carlton · #3 11-07-2009, 04:28 PM

Quote:

Originally Posted by jkubica

But if I count 10 before starting - it starts right away and runs smooth right away.

Above 50F..........count to 10

Above 30F............count to 15

Above 0F..............count to 30.

YMMV depending on engine and glow plug condition

Skippy · #4 11-07-2009, 05:19 PM

If the light's only staying on a few seconds, you probably have one or more glow plugs out. In Houston weather with five good plugs, your car should start pretty much instantly once the light goes out.

Side note: I have some documents that show my 300D was orignally purchased by a doctor in Houston.

jkubica · #5 11-07-2009, 08:36 PM

Quote:

Originally Posted by Skippy

If the light's only staying on a few seconds, you probably have one or more glow plugs out. In Houston weather with five good plugs, your car should start pretty much instantly once the light goes out.

Side note: I have some documents that show my 300D was orignally purchased by a doctor in Houston.

My thanks to each of you for responding

A few more questions if I may.

Is there a circuit that controls how long the the plug light stays on? If so, how does it make the determination.

Do glow plugs get "weak" ie take longer to get hot as the age or with use?

What role does carbon buildup play in glow plug functioning?

All information appreciated.

Joseph

side note: My doctor drives a Benz - Naw - couldn't be!

#6 11-07-2009, 08:43 PM

The glow plug relay controls how long the light stays lit based on engine temperature.

Normally glow plugs just work until they burn out, but I have had a few that had increased electrical resistance without failing completely. They should be about 0.8 ohms when cold, if one is significantly higher it should be replaced.

I have never had an issue with the carbon build-up affecting the glow plugs.

You will be able to detect when one GP isn't correct, the engine will be a little rougher when first started.

toomany MBZ · #7 11-08-2009, 08:43 AM

If there is enough carbon build up the prechamber won't warm up as much. You want the air to heat up, not a carbon build up.

#8 11-08-2009, 08:50 AM

Quote:

Originally Posted by toomany MBZ

If there is enough carbon build up the prechamber won't warm up as much. You want the air to heat up, not a carbon build up.

Are you suggesting that he needs to periodically remove the carbon from his glow plugs, in Houston?

jkubica · #9 11-08-2009, 10:46 AM

Quote:

Originally Posted by toomany MBZ

If there is enough carbon build up the prechamber won't warm up as much. You want the air to heat up, not a carbon build up.

Hi
The reason I put forth the carbon buildup question, it that I've read on the forum about reaming carbon from the glow plug holes - using a drill bit or special tool.

I'm wondering if this is to make the plugs work better --- or to clear the way for them and to prevent sticking in place?
Thanks
Joseph

Jeremy5848 · #10 11-08-2009, 12:46 PM

The reaming is 90% to prevent sticking, 10% to improve performance. YMMV.

The timer that turns off the pre-glow light on the dash is a different circuit from the glow plug timer. The timer for the light is somewhat temperature sensitive but is more of a "serving suggestion" or "minimum" time before starting. A cold engine will always start more easily and idle more smoothly if you follow Brian's guidelines. Note that the preglow relay has an input from the starter switch: when you release the starter switch after the engine has cranked and caught, the glow plugs will turn off regardless of how long they've been on.

Left to itself, the timer will turn off the glow plugs after about 30 seconds. If you are in a fairly quiet environment, you will hear the relay click off and if you turn on the dome light, it will noticeably brighten when the glow plugs stop drawing down the battery.

Later model M-B diesels also have an "after-glow" circuit that keeps the glow plugs on even after the engine has started. Some people have retrofitted the later relay to their cars. Depending on the year-model of the relay and the engine temperature, the glow plugs will be left on for up to a couple of minutes after the engine starts.

Other folks (including myself -- see this thread for details) have made a modification to the existing circuitry such that if the engine temperature is lower than 50C then the glow plugs stay on for the full 30 seconds. I have found this to be very useful in cold-starting my 1987 300D Turbo (W124, OM603). Combining Brian's advice with the after-glow modification, I pre-glow for ten seconds, then start the engine and let it idle until my custom "after glow" light goes out at 30 seconds. At that point the engine is idling smoothly and the car is ready to drive.

Jeremy

toomany MBZ · #11 11-08-2009, 08:57 PM

Quote:

Originally Posted by Craig

Are you suggesting that he needs to periodically remove the carbon from his glow plugs, in Houston?

Doesn't matter where you live, if the glow plugs activate, you may get build up.

jkubica · #12 11-09-2009, 05:30 AM

Quote:

Originally Posted by Jeremy5848

The reaming is 90% to prevent sticking, 10% to improve performance. YMMV.

The timer that turns off the pre-glow light on the dash is a different circuit from the glow plug timer. The timer for the light is somewhat temperature sensitive but is more of a "serving suggestion" or "minimum" time before starting. A cold engine will always start more easily and idle more smoothly if you follow Brian's guidelines. Note that the preglow relay has an input from the starter switch: when you release the starter switch after the engine has cranked and caught, the glow plugs will turn off regardless of how long they've been on.

Left to itself, the timer will turn off the glow plugs after about 30 seconds. If you are in a fairly quiet environment, you will hear the relay click off and if you turn on the dome light, it will noticeably brighten when the glow plugs stop drawing down the battery.

Later model M-B diesels also have an "after-glow" circuit that keeps the glow plugs on even after the engine has started. Some people have retrofitted the later relay to their cars. Depending on the year-model of the relay and the engine temperature, the glow plugs will be left on for up to a couple of minutes after the engine starts.

Other folks (including myself -- see this thread for details) have made a modification to the existing circuitry such that if the engine temperature is lower than 50C then the glow plugs stay on for the full 30 seconds. I have found this to be very useful in cold-starting my 1987 300D Turbo (W124, OM603). Combining Brian's advice with the after-glow modification, I pre-glow for ten seconds, then start the engine and let it idle until my custom "after glow" light goes out at 30 seconds. At that point the engine is idling smoothly and the car is ready to drive.

Jeremy

Hi
Thanks. Very nice DIY on the Glow mod!
Joseph

kerry · #13 11-09-2009, 08:57 AM

Reaming the glow plug area in my 116 added about 10 degrees to the cold weather starting ability of the engine. There was quite a bit of carbon buildup.