ok i got this 79 240 d auto... its been a week the car is acting up when i start it.. usually youll turn the key on wait for the glow plug light to go off and then start it right? well when i turn the key on, theres no glow plug light, i try starting it then the glow plug light goes on while im starting it. and then sometimes if im lucky it will start, but today it was cold and it just drained the battery and didnt start.. it has the old loop style plugs, the fuse is good, i doubt theres a relay .. is there? so the glow plugs do work, but only when i attempt to start , while its turning over its heating the glow plugs. strange... any help... any ideas????? thank you.

check the fuse. Its either under the drivers side under dash panel in a black box with a removable lid, or somewhere in the engine compartment(follow the glow plugs lines)

the loop style suck if one fails, they all wont work.

or the light can be intermittent. Get an ohm meter and check them for continuity and resistance.

The relay is up under the panel above your knees when driving. It sounds like yours is tired. Fairly common failure item on that year.
Borrow one for a test or perhaps get one at a wrecking yard. Very few cars are junked because of a faulty glow plug relay. You kind of have the classic signs. Intermittent operation at best.

If the glow light is coming on in the start position but not in the glow position is sounds to me as if the ignition switch is faulty.
You can get by until you fix it by making a jumper wire of pretty heavy wire that you can use to go from the positive post of the battery to the first glowplug at the firewall end, bypassing the glowplug relay/ignition switch. Hook up the wire, heat up the glowplugs, disconnect and jump in the car quickly and start it. It's a useful technique to know if your fuse or relay fails and you really need to get it going.

__________________
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

Kerry is posibly right if it acts consistantly the same. When you move the key to the start position the glow light always lights?