My 1979 W123 300D is presenting a major problem related to the glow plugs.

I was having starting problems in the cold weather, and so replaced the glow plugs with OEM Bosch components. The engine will still not start, and the glow plug light remains on much longer than normal--about 60-90 seconds.

While troubleshooting the light issue, I happened to look under the hood and noticed that, approximately after the time when the glow plug light would turn off (20-30 seconds) the connector rails between the glow plugs are glowing red hot! I must assume this is not normal.

Are the glowing connectors due to the glow plug light remaining on, possibly due to a stuck glow plug relay?

I always believed if the relay was bad, the light would not come on at all.

Any help will be very much appreciated.

Couple of items: Series plug relays stay on longer than parallel plug relays (in my experience) because the series plugs are far less efficient (as indicated by the loss of heat thru the connector wires). The glowing wires are normal. They will glow when the plugs are warm and continue to glow as long as they are energized. Time the glowing wires and you will know how long your plugs are staying on. 20 or 30 seconds for series plugs is inadequate in cold weather.
I would remove the series plugs and install the retrofit parallel plugs. The modification is simple. Just remove the old plugs. Get rid of the front grounding wire and the heavy squiggly wires, screw in new plugs, attach the hot wire to the plug in the back and make short 8 or 10 gauge jumper wires between the plugs. The result never fails to please.
There are two other modifications which I have done which substantially improved cold weather starting: switch to synthetic oil and replace the starter with one from a turbo model.

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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

Thanks very much for this information.

I am still seeing a problem with the glowing wires, however. They continue glowing after the glow plug light goes out. In fact they continue glowing until the ignition switch is shut off. The reason I thought the glowing wires was abnormal is that it didn't seem right to have live glowing red wires in the engine compartment, due to a potential fire hazard.

Does the fact that the wires continue glowing point to a faulty relay? Or is the relay ruled out by the fact that the glow plug panel light goes on and off more or less as normal?

Thanks again!

If they are on all the time, then the relay is sticking on. Have you timed them? The light and the length of time the plugs stay energized are independent of each other so don't expect them to go off when the light goes 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

Just do what Kerry suggested, hes 100% on the mark, you wont regret it. Dont even waste your time with those "loop' style GPs.

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1985 Euro 240D 5 spd 140K
1979 240D 5 spd, 40K on engine rebuild
1994 Dodge/Cummins, 5 spd, 121K
1964 Allice Chalmers D15 tractor
2014 Kubota L3800 tractor
1964 VW bug

"Lifes too short to drive a boring car"

I tend to agree with the theory that the relay is sticking.

The light stays on about 90 seconds, more than twice as long as normal. The wire connectors get red hot and stay red hot.

Is there a repair for a sticking relay or just replace?

I have had 2 early glow plug type engines all but fail to start with the early series type glow plugs. I did what Kerry says to do and they started better than new. I mean RIGHT NOW! Just use your old relay and self time them about 10 seconds and hit the starter.

Are you sure you hooked the series type up right??

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My Truck.. 2007 DODGE, 5.9 Cummins, 6spd stick, 4X4. My car..1977 240D, OTHER WHEELS...1955 VW Oval window bug, European Delivery (Holland) with a 1700cc, 2 barrel, Porsche drum brakes. 1939 WILLYS Pick-up. 1967 Triumph 200cc Tiger Cub. 1976 Honda 550F 4cyl Motor Cycle.

I will not comment on the type of plugs or starting difficulties. If the wires are still glowing a couple of minutes after turning the glow circuit on I think the glow plug relay is sticking on. Funny thing is it cannot be sticking on all the time or it would take your battery down. Best test for that might be to get another relay from a salvage yard if cheap enough.

Thanks. I think you're right. The wires don't glow with the ignition switch off, so that's what has saved the battery so far, but it has taken the battery down considerably in the process of troubleshooting all this.

In regard to the series, yes, I believe it is properly installed. There are few ways to go wrong with this, as long as the wires are separated on the plugs and the insulators are on correctly.

That's a good point. I think the glow plugs are energized for approximately 10-30 seconds beyond the time that the light is on. I will check that and see of the wires cool down after that much additional time beyond the light.

Thanks again.

Then I don't think there's a problem with the relay. If the plugs turn off at 90 seconds or less, I wouldn't worry about the relay. In fact, a little afterglow when the engine starts improves running. Some people have modified their relays to obtain an afterglow.

__________________
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

I was more or less able to confirm that the relay is not the problem:

I turned on the switch, the glow plug light came on, stayed on too long, while the glow plug connector wires got red hot again. The glow plug light turned off, and this time, I left the switch on and waited about 30 seconds. I heard the relay make its noise and the glow plug wires immediately stopped glowing red. However, when I went ahead and turned the ignition switch to try and start the engine the glow plug light came BACK ON, which it normally doesn't do, and the engine would not start or even fire a little, like it did before.