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#1
10-13-2009, 08:31 PM
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Strange glow plug/ relay problem fixed!
I've been scratching my head why I get the glow plug light sometimes and not other times. This has been happening all summer. Now that it's colder, I been getting hard starts. I checked the glow plugs with an ohmmeter and they all checked ok. I put my non contact ammeter on the battery negative cable and noted sometimes I get full current draw of 5 glow plugs and sometimes I get lower draw of 4 glow plugs. I thought I must have a flakey glow plug that is intermittent, which IMO is very unusual. So I put the non contact ammeter on the individual wires coming off the glow plug relay and found the one that was low current, traced to #1 glow plug. I proceeded to replace that glow plug and guess what? The fricken nut was loose and the ring spade terminal was floating! I tightened the nut, took a non contact ammeter reading to make sure it's drawing full current which it did. Engine starts much better now.
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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 
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#3
10-13-2009, 10:19 PM
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Good work. For people wondering, the reason why the light wasn't working was that the 'test system' which tells the light to go out if there is a glowplug problem, compares the current draw on plug #1 with the draw on the rest. With the loose wire, there was a difference in draw making the glowlight go out.
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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
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#4
10-13-2009, 11:16 PM
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The glow plug relay connector is marked with numbers 1 to 5. I wasn't sure if these numbers correspond to the cyl #'s. The wires are color coded and #1 is blue/ black but I can't see the color of the wires on the glow plug end because they were all covered by shields so I wasn't 100% sure I was working with the right glow plug. When I found the loose nut, I WAS sure.
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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
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#5
10-13-2009, 11:49 PM
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Yup, the numbers do correspond to the cylinder numbers. I replaced #4 this afternoon.
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#6
10-14-2009, 01:04 AM
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THX
I like these elegantly simple fix stories.
It's good stuff for the archives ! THX
__________________
1979 300D 220 K miles 1995 C280 109 K miles 1992 Cadillac Eldorado Touring Coupe 57K miles SOLD ******************** 1979 240D 140Kmiles (bought for parents) *SOLD.SAN FRANCISCO/(*San Diego) 1989 300SE 148 K miles *SOLD
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