Lossing Glow Plugs right and left! - PeachParts Mercedes-Benz Forum

Jim Sutton · #1 02-07-2009, 09:46 AM

I have been losing glowplugs on my 84 300sd. Here is the story:

Engine is impossible to start. Ohm out all the plugs and find one plug at 35 ohms. Swap it out with one measuring 1 ohm. Car runs great for two days then impossible to start. Check plugs and two more fail. replace the plugs and verify that relay turns on and off. 10 volts at the plugs, car starts. couple days later and repeat. Change the plugs and exchange the relay with my spare. Confirm relay operation. Good for two days and last night, nothing (no relay light).

My guess is the relay is leaving the glow plugs on too long and burning out the plugs. There must be another component external to the relay that sets the relay on time.

This seemed to start when I replaced my battery. All of the plugs I have been replacing are used. When a plug goes bad the Glow light stops coming on the dash.

JimmyL · #2 02-07-2009, 09:49 AM

Did you ream out the glow plug holes during any of these installations?

oldsinner111 · #3 02-07-2009, 10:26 AM

Ream holes and buy bosch duratherms and updated relay.

Jim Sutton · #4 02-07-2009, 10:32 AM

No did not ream out the holes.

What are the chances the both relays were bad?

How does the relay determine how long to leave the plugs on?

kerry · #5 02-07-2009, 11:00 AM

Confirm the length of time the plugs stay on with a voltmeter on the plugs. There's a timer in the relay itself I believe.

Palangi · #6 02-07-2009, 12:01 PM

Relay contacts have been known to stick in the ON position ocasionally. Usually that will run the battery down.

Brian Carlton · #7 02-07-2009, 12:13 PM

Quote:

Originally Posted by Jim Sutton

Check plugs and two more fail. replace the plugs

What brand plug are you installing? Are these new plugs?

funola · #8 02-07-2009, 07:50 PM

Quote:

Originally Posted by Brian Carlton

What brand plug are you installing? Are these new plugs?

"All of the plugs I have been replacing are used." per his first post.

Brian Carlton · #9 02-07-2009, 08:23 PM

Quote:

Originally Posted by funola

"All of the plugs I have been replacing are used." per his first post.

Yes, I saw it originally. I wanted some additional confirmation with some details.

leathermang · #10 02-07-2009, 05:07 PM

Since the engine won't start... my guess is that the suggestion about reaming is more logical... the carbon has pretty much closed up the hole and is causing both the non start and then a shortout/burnout of the GP...
If it would start but kept burning out then the potential for the relay sticking is higher.

Diesel911 · #11 02-07-2009, 08:35 PM

Summary of the good questions.
Did you ream or otherwise clean the Glow Plug holes.
What brand of plugs were you using.
More questions.
In the couple of days the car started OK did it idle well and was there any smoke.
How many miles are on the Engine.
Do you think swapping the GP Relay and a (+) or (-) effect or caused no change in the failure rate.

Question for anyone. Can a poor ground cause the GPs to fail?

Some Members on this and other forums believe that if the GP holes are not reamed or otherwise cleaned out the heat gets transfered through the carbon to the block instead of heating where it is supposed to. Others think it might insulate the GP and cause it to hold the heat in in the plug itself.

funola · #12 02-07-2009, 09:09 PM

"Can a poor ground cause the GPs to fail?"

The glow plugs are grounded pretty good to the head/ block. You mean from a bad ground strap? I would say no. That would result in less power to the glow plugs and they just won't get as hot as with a good ground.

Diesel911 · #13 02-08-2009, 01:01 AM

Quote:

Originally Posted by funola

"Can a poor ground cause the GPs to fail?"

The glow plugs are grounded pretty good to the head/ block. You mean from a bad ground strap? I would say no. That would result in less power to the glow plugs and they just won't get as hot as with a good ground.

Yes.
I do not know one way or the other myself but he said the problem started after he changed his battery.
He swapped the Relay so it is also unlikey that 2 Relays would have the same problem
So I was fishing for what other type of electrical problem might cause the plugs to overheat.
I ask the bad ground question as I have read that a bad ground can burn out other electrical stuff (not only on Mercedes), mostly gauges and ect.

Also the other thing we do not think of is that in the Automotive Theory of Electricity Electrons flow from (+) to (-); we accept that for automotive use. However, the opposite is true.

With the Electro Theory:
"Electrons in the conductor will flow from negative to positive because of the displacement of negatively charged electrons in the outer rings of the electron shell of the material. These electrons flow away from the negatively charged source and towards the positively charged source.Electrons in the conductor will flow from negative to positive because of the displacement of negatively charged electrons in the outer rings of the electron shell of the material. These electrons flow away from the negatively charged source and towards the positively charged source."

The above Quote from, Electron theory in applied electronics : http://www.helium.com/items/1009209-electron-theory-in-applied-electronics?page=2

So in reality the current is flowing through the Ground Strap and block to the Glow Plugs.
So I was asking my question to see if anyone else has had the same GP problem and had found that renewing/cleaning the Engine Ground helped.

Ryan Z · #14 02-08-2009, 05:01 PM

Where do you get a GP reamer? I looked around, no luck.

---------------------
1984 MB 300SD Turbo

JimmyL · #15 02-08-2009, 05:20 PM

Quote:

Originally Posted by Ryan Z

Where do you get a GP reamer? I looked around, no luck.

---------------------
1984 MB 300SD Turbo

PM Diesel911. I have purchased 2 reamers from him. One for an OM616/617 and the other for an OM606. Very happy with the price, extremely happy with the product!