Did a quick search and didn't get any exact results, so I thought I'd ask:
1985 300dt with new GP's last winter. When I turn the car on, the GP light does not come on. Sometimes if I crank for a few seconds, turn the key off and then turn the key back on, the GP light comes on and then the carstarts immediately. If I try to start it without the GP light coming on, it cranks forever and then stumbles. My hunch is that the relay is not working correctly as it starts easily IF thelight comes and on and starts very difficultly if the light does not come on. The car has always started easily in the 3 years I have owned it, and this is a new problem. Is it possible it is the temp sending unit? What is the best way to diagnose, and can the relay be fixed by a DIY?

thanks guys.

i'd start with the 80A GP fuse, it might have a slight crack in it. and they are cheap.

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-Trevor

OBK #12
1980 300SD 333,XXX miles - Totaled
1986 Mazda RX-7 212,XXX miles - impounded and auctioned off
2005 Jeep Liberty CRD Limited 33,000- SEGR, Provent, Fumoto

Our 300D had weird issues with the relay as well, I just replaced it today with a new modern afterglow one and 5 new glowplugs, the difference is amazing. The engine had ORIGINAL glow plugs in it still! No wonder it didn't start very well! It starts absolutely instantly now. Well worth it.

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-diesel is not just a fuel, its a way of life-
'15 GLK250 Bluetec 118k - mine - (OC-123,800)
'17 Metris(VITO!) - 37k - wifes (OC-41k)
'09 Sprinter 3500 Winnebago View - 62k (OC - 67k)
'13 ML350 Bluetec - 95k - dad's (OC-98k)
'01 SL500 - 103k(km) - dad's (OC-110,000km)
'16 E400 4matic Sedan - 148k - Brothers (OC-155k)

I had the exact same thing happening to me with my 85 300SD. It turns out I had 2 dead glowplugs or one dead one and one intermittent one. And of course they were the two back ones that are a pain to get to if you don't remove a lot of stuff. I was able to replace one before the sleet hit us. My car starts a lot better now and I always get a glow plug light.

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green 85 300SD 200K miles "Das Schlepper Frog" With a OM603 TBO360 turbo ( To be intercooled someday )( Kalifornistani emissons )
white 79 300SD 200K'ish miles "Farfegnugen" (RIP - cracked crank)
desert storm primer 63 T-bird "The Undead" (long term hibernation)

http://ecomodder.com/forum/fe-graphs/sig692a.png

My car does the same thing. You don't need anything new.

Take out your pocket knife, mag-lite, stick, or other similar object, and hit the relay with it sharply. Get back in. It'll work every time.

The relay is hanging sometimes. Cranking for a few seconds makes the engine shake enough to shake the box just like hitting it with your object. It'll work like this until you decide to replace the relay just to get rid of the annoyance factor.

I haven't replaced mine. I just either knife it if I'm not in a hurry, or crank it over and then re-try if I am.

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~Michael S.~
Past cars:

1986 300SDL
1987 300SDL
1982 240D
1982 300SD


Current:

1987 300SDL

I like the mag light idea....If only hitting it with a mag light would fix broken seat springs.....

I'll give it a good wack tomorrow morning and will report back on how it does. Is it possible/has anyone ever rebuilt the relay? Should I scavenge one from a junk yard and see what happens?

Thanks for the suggestions.