Question concerning my 1976 300D; Engine 617.91; Body 115.114. To Quote my engine service manual Job 15.4 505/16 "If the engine is not started when the preglow indicator lamp is extinguished, the preglow system remains switched on up to 90+20 seconds of preglow time. When working in engine compartment, note that in such a case the resistance conductors are red hot."

My Question: I changed the 5 glow plugs this September with new ones (Bosch). I noticed at that time that the conductors in fact did turn red hot. Now 3 months later the conductors get hot but do not turn red. Are my plugs going bad after only 3 months?

Question: Why did you replace all the glow plugs (GP)?

If the car starts easily the glow plugs are OK. The GP on these cars are in series and are 1.2 volts each as if they were made for a 6 volt system. If one GP opens, none will work. You can find which one is open by putting a jumper across each plug until you get a spark. You can also get the car started this way until you replace the bad plug. The resistance wires between the GP drop the voltage so the GP don't over heat. It is a very inefficient system because half of the heat is lost in the resistance wires. I can't understand why MB took so long to replace them or why they didn't get 12volt GP in the first place.

I wouldn't worry about the resistance wires not glowing red as long as the car starts OK. These GP usually last the life of the car and that is why I asked why you replaced them. Glow plugs aren't like spark plugs that have to be replaced often. Gp are only used to start the engine and after they are inert.

P E H

In May of this year I replaced all my plugs with Champion plugs not knowing that Champion is not the quality of Bosch (to be charitable). I was having starting problems and Champion was all my local parts store could get. A word to the wise; use only Bosch.

how hot the resistor wires get (only the zig-zag ones!) depends on how long the glow plugs are operated. In warm weather or with a warm engine, they won't get red hot.

Also, if you have a bad connection somewhere, the current flow will be restricted, leading to failure of the glowplugs to heat properly. You can check this with a voltmeter -- voltage should drop 1.2 volts for each plug and 0.8 volts for the resistor wires, I think -- may be 1.2 V for the resistors, too. Straigh wires are not resistance wire.

The series glowplug system works very nicely, thank, you, and the voltage drop has nothing to do with the old 6 volt systems (which MB didn't use for diesels, anyway!). Pencil plugs have similar resistance per plug, they are just run all at once in parallel rather than series.

The only real drawback to the series plugs is that a bad connection of broken element keeps all of them from working. They are denser and stay hot longer, too, but take much longer to warm up -- 2-3 min in cold weather. MB stayed with them so long because they worked so well and there wasn't a good alternative until about 1978.

With the really old ones (up to W115 pre 1974) there was an extra glow element in the dash -- the temperature of the plug is visible!

And no, Champion glowplugs, series loop type or pencil, are not good. I'm on the second set in the Volvo, and have at least two not working after only a year or so. Bosch is the only way to go.

Peter

__________________
1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!

PSFRED,

The parallel plugs have about 10 times the resistane of the series plugs. I'm talking about hot resistance. Otherwise the parallel plugs would would draw abour 10 times the current of the series plugs because the parallel plugs each get 12 volts where as the series plugs each get 1.2 volts.

I was saying that the series plugs appeared to be from a 6 volt system because adding the voltage across each plug, the total is closer to 6 volts than 12 volts. I always wondered why 2.5 volt or 3 volt series plugs were not used instead of the resistance wires.

P E H

PEH:

The series glow plugs don't ground to the block individually, only through the ground strap at the end of the line. There is a voltage drop to ground at each plug, buy they act as a single large resistor (including the resistance links -- two on a 240 and one on a 300), total voltage drop is about 6 volts. I suspect that a six cylinder version of the engine would have no external resistance links, having instead another glow plug. That way only a single reistance glow plug fits all engines. These were used from the 1930s to 1981 -- can't be all that bad a system!

There are two insulated conductors in each series plug -- the metal shell isn't connected to the elements at all.

The only disadvantates are slow heating and wasted current from the external resistor links. The pencil plugs get hot much faster (less than a minute) compared to the two or three minutes required below zero on the 220D -- quite a wait, holding that tiny knob out against that spring!

Peter

__________________
1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!

PSFRED,

There is not a voltage drop to ground across the series type GP except fot the last one which is connected to ground. There is a 1.2 volt voltage across each GP but only the last one is to ground.

I have plenty experience with series type GP as I have owned a 180D, 3 190Ds, a 220D and a 240D and driven them for over 300,000 miles. I can remember only a few failures of the series plug but I used to carry a jumper wire in case one of the GP opened which whoud prevent the engine from starting. I had one GP in the 220D that shorted to ground and drew so much current that the starter would barely crank the engine. I had to disconnect the shorted GP and jumper around it to start the engine.

I'll admit the series GP are less failure prone but I precer the parallel type because they are so much quicker. I changed one of the 190D to parallel GP. It worked well and started in about 1/3 of the time.

P E H