1976 300 D 115 Glow plug question - PeachParts Mercedes-Benz Forum

#1 09-07-2003, 07:50 PM

Glow plugs only fire when engine is turning over. No pre-glow sequence. There are 2 wires coming from the 1st plug, one heavy and one light, but I would have to cut the wrap off the harness to trace. I haven't been able to get a wiring diagram at this point and don't know where to start. There is a relay on the right front firewall that clicks on position 2, but I don't know what thats for either. Any help would be greatly appreciated. Thanks.
Zeaman

psfred · #2 09-07-2003, 09:04 PM

Check for voltage on the glow plugs when the relay clicks. The 76 has an oddball system, so a number of things could be wrong.

First, check the fuse (80 strip fuse). If broken replace it or you won't have an idicator lamp.

Second, the relay operates both for pre-glow and during start, I think, so the relay can be bad (if it clicks and the fuse is good and you still don't have voltage), you will then need to replace it. It should stay on for 90 sec unless the engine is cranked. It goes off after cranking.

Third, the indicator lamp is operated off the temp sensor under the #1 glow plug. When the sensor fails, either the lamp stays on all the time the glow plugs are on, or it never illuminates. Can be a true indication that you are only getting voltage during cranking, meaning you have either a wiring problem or a dead relay.

Peter

#3 09-07-2003, 09:18 PM

Peter,
Problem is, the relay that is clicking is located on right front firewall under two fuse blocks and the left one (drivers side of those two) has been disconnected by the PO. There is another relay located on left front fender in front of the main fuse box unerneath two other sets of fuses. I still haven't been able to find the 80 strip fuse yet. I was just wondering if someone in the past has bypassed the time delay and wired it off the starter. I really hate to trace the harness but it's not looking good. I've tried to get some history on the car, but no luck. I do know the car had a major short at some time cause the wires from the starter to the alternator were toast.

psfred · #4 09-08-2003, 01:17 AM

The glow plug relay is under the cowl vent, driver's side. Plug is on the engine side of firewall, relay on the cowl side, you must remove the aluminum cowl vent cover to get to it. I think the fuse is on the relay.

The others are the power window relay and the AC relay.

Check for voltage when the relay is supposed to work before ripping into anything.

If some of the wiring has been "user modified" it will be a pain -- I've seen jerry rigged starter buttons and glow plug switches to replace a bad starter switch -- the vairous bits and the damage caused cost far more than just replacing the switch!

Peter

#5 09-10-2003, 07:03 PM

Well I pulled the cowell apart. The relay under there has a metal cage around outside. It has two springs, two contacts,1 brown wire and 1 red wire. I couldn't find a fuse strip. When I check the 4 wire plug on engine side of the firewall I get 12 volts on all 4 wires with switch in #2 position. I didnt have a helper with so I couldn't observe what the relay itself does with key being turned on or off. The relay doesn't have a plug to remove it either. The contacts were open with the power off, don't know if they close or not at this point. I figure the least I can do is clean the contacts and see what happens. Still trying to locate wiring diagram for this.

psfred · #6 09-10-2003, 08:47 PM

Zeaman:

Plug is on the engine side of the firewall, so are the screws that hold the relay in place.

Check the ground at the temp sensor -- you can jumper around it, I think. I will check the wiring diagram if I can find it to see which end is the ground and which is hot. I think the relay is hot.

It should stay on for 90 sec or so. The dash lamp is operated off the temp sensor (the other wire).

Those are series plugs, so check for voltage all the way down -- should drop some for each plug. If you find no voltage after a plug, it's broken and needs to be replaced, or you have no glow plugs at all.

Peter

Peter

Lady Benz · #7 09-14-2003, 11:57 PM

This may not mean a lot. I have a 77 300D that was having the same problem. The glo plug indicator on the dash only came on when trying turning the engine on. My dad and I worked forever on it. First we replaced the glo plugs, when that did not work we were stumped. May sound silly, but the problem was in the fuse box. There was some corrosion on the fuse that said it was for the carbureator heater. When we cleaned the fuse and the connector it has worked like a charm ever since.

Check it out. You never know.