x2. However, in the cases of the relay staying on that I recall being posted on this forum, the glowlight did not stay on. Perhaps this is a different kind of failure in the relay. It should be simple to test. Start the car and after 4 minutes or so, check for voltage at the glowplugs.

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1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13

Agreed. However I had little luck finding my GP relay energized with a multimeter when my relay went bad. But my light would only flicker so perhaps he'll have an easier time diagnosing it.

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2009 ML320 Bluetec
1985 300CD
1981 300TD

Past Mercedes
1979 300TD
1982 300TD
2000 E320 4Matic Wagon
1998 E430
1984 300SD
1980 300SD

That wizard of all posters, jbach36, wrote a great piece, explaining all about the glow plug process and changing them out. See the DIY post section in this forum.

jbach36 rocks!

New battery-Started car. Car started immediately. GP light is still on. Checked for voltage at the 5 pins. They all have voltage after 5 minutes. Time for a new relay? I read somewhere in the forum that the relay is repairable, something about changing a capacitor? Opened up the relay and nothing looks burned inside.

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'85 300D Cal 280,000 miles
'14 GLK 350 60000 miles

check your ground gonnection

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1986 300SDL, 211K,Dealership serviced its whole life
1991 190E 2.6(120k)
1983 300D(300k)
1977 300D(211k)

could be the ground. I had the transmission rebuilt. Probably forgot to put the grounding strap. ACC blower doesnt work and the charging light is on also. Will check later.

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'85 300D Cal 280,000 miles
'14 GLK 350 60000 miles

The charging light being on suggests the voltage regulator assembly with the brushes is bad. Especially if it is barely one and goes off now and then, kind of as the engine speed picks up. Brushes wear out and then only intermittently make contact - adds resistance to the circuit and you don't have the juice needed to run the car off the alternator. Thus, the dash "charging" light comes on.

This is likely a separate problem from the one that has your glow plugs energized all the time. I would think the fuse would melt under those conditions though.

Jim

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Own:
1986 Euro 190E 2.3-16 (291,000 miles),
1998 E300D TurboDiesel, 231,000 miles -purchased with 45,000,
1988 300E 5-speed 252,000 miles,
1983 240D 4-speed, purchased w/136,000, now with 222,000 miles.
2009 ML320CDI Bluetec, 89,000 miles

Owned:
1971 220D (250,000 miles plus, sold to father-in-law),
1975 240D (245,000 miles - died of body rot),
1991 350SD (176,560 miles, weakest Benz I have owned),
1999 C230 Sport (45,400 miles),
1982 240D (321,000 miles, put to sleep)

What's your theory on how a bad ground could cause the GP light to remain illuminated at all times?

The temperature sensor is connected directly to ground. If there is no ground, the temp sensor wont work and therefore wont tell the timer to turn off?

Hella GP Relay any good?

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'85 300D Cal 280,000 miles
'14 GLK 350 60000 miles