"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.

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85 300D turbo pristine w 157k when purchased 167,870 July 2025
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked

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.

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84 300D, 82 Volvo 244Gl Diesel

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


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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!

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Jimmy L.
'05 Acura TL 6MT
2001 ML430 My Spare

Gone:
'95 E300 188K "Batmobile" Texas Unfriendly Black
'85 300TD 235K "The Wagon" Texas Friendly White
'80 240D 154K "China" Scar engine installed
'81 300TD 240K "Smash"
'80 240D 230K "The Squash"
'81 240D 293K"Scar" Rear ended harder than Elton John

duplicate

Forget the ground strap or ground system. The only thing that would happen if it was somewhat compromised is the glow plugs will not heat up as much.

What caught my attention was the random substitution of used glow plugs to replace failed plugs. Your random failing plugs may be because of actual differences in resistance in various plugs. Since all the plugs are in parallel. Total current flow through one plug is determined by its actual resistance.

Inefficiencies usually eliminate any one given plug really heating up to it's maximum temperature. Yet with plugs of different resistances you may be increasing the efficiency and allowing a couple of plugs to reach their actual rated current flow. This is higher than they have been pre conditioned to so they fail. Either get five good plugs from the same engine and test them for about the same static resistance and install them. Or buy a new set in a reliable brand name.
Another possibility is the relay is coming back on sporadically. I know you changed relays yet this could be an external to the relay defect turning it on. I would consider driving with a digital meter or 12 v bulb wired to a glow plug but extending back into the car to eliminate that as a possibility for a period of time.

The problem starting when you changed out the battery is logical. More voltage than the old plugs were conditioned to was made available. Probably had a poor battery for quite awhile.

Also check the battery charging voltage with engine running. You do not want to see more than 14.8 volts or the regulator in the alternator is bad.

At least you really cannot lose glow plugs left and right. They are all on the same side of the engine.