In the ongoing saga of a simple glow plug replacement, I am now in need of advice regarding some very strange developments, to wit:

1. Glow plug relay seems to be working; I was told it might not be, but the glow plug light comes on and cycles off approximately as it should;

2. Connector wires BETWEEN glow plugs glow red hot approximately 25 seconds into the glow cycle (about the time the glow plug light normally extinguishes). After another 30 seconds, the wires cool after the relay makes its "thunk" sound;

3. Engine will not even fire, despite having done so with the old worn-out glow plugs;

4. Glow plug light comes back on when the switch is turned from "glow" to "ignition". It never did this before.

Is there any way to hook the glow plug series up incorrectly?

If there is electric current to the connector wires, is it reasonable to assume that the glow plugs themselves are receiving current?

I am making this last plea for help prior to calling the M-B shop and having the car towed there, something I am quite reluctant to do.

Thanks for any input.

year make model, please.

did you upgrade the plugs? if so, they need to be wired differently.

__________________
1984 123.193 372,xxx miles, room for Seven.

1999 Dodge Durango Cummins 4BTAA 47RE 5k lb 4x4 getting 25+mpgs, room for Seven.

1979 W123 300D Non-turbo. Standard "loop-type" plugs.

Check somewhere about the part numbers you have purchased. There were some twelve volt plugs that look exactly like your original large type of glow plugs. . I suspect you might have them installed. Yet your ability to wire them the old way seems strange.

Yet I have never seen one of those twelve volt large based plugs. There is a chance they might look identical with the same tip insulator.

Again a part number check should verify. They will draw current but not get hot enough if at all. In a way it would be good if they were the twelve volt version. Once proven just change your wiring but not before being absolutly sure. Twelve volts will very quickly burn them out. Almost if not instantainiously if they are the same low voltage plugs like you had.

Hope this helps. Remember some parts suppliers are not very knowledgeable on an obscure item like this. So check it out.

The 12v plugs that fit into the loops style holes are the pencil style plugs that are meant for the parallel system. The ends very different. Instead of a loop of wire it will be a pencil tip.

Make sure you have the bus bar wires on correctly. They should alternate on either side of the insulator. Also if the first GP is burned out the rest won't work. I would check the voltage at each plug you should have 12v at the first one and I believe you should loose 1.5 volts as you go down the line. If you get zero at one then you have a bad plug. The Haynes manual has a great diagram on how to do this.

Also make sure the last glow plug has a wire that goes to the block. If you don't have that things won't work properly.

__________________

This is a good suggestion and plausible, and I have had parts houses sell me the wrong part in the past. I'll check it out immediately.

Thanks!

Thanks very much. Will do. I do have the Haynes manual but not my Mercedes CD here, and will check the voltage/Ohms as per the Haynes diagram. Other than that, everything is wired correctly, including the ground wire.

I'm confused by Barry's post. I am under the impression that the OE loop plugs require 12 volts which is reduced down the line as each plug draws its power.
If the heavy squiggly wires are glowing red all the way to the plug at the front of the block, you are getting power to all the plugs I believe. If you have a bad plug, it interrupts the circuit and you won't get any glowing wires at all.
Did you do anything else when you changed the plugs like replace the fuel filters for instance?

The only thing I can think of is that you reinstalled the wires and insulators incorrectly and current is passing along the heavy squiggle wires to the ground wire on the front of the block but not thru each glow plug. Is each squiggly wire separated from its partner by an insulator on each glow plug, forcing the current thru the glow plug?

__________________
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

Yes, the insulators are all in their correct place. I agree with the assessment that if the connectors are getting enough power to turn red, then the circuit is getting power. This all leads me to conjecture that perhaps somehow, I got the wrong glow plugs. And yet I am told by my long-serving parts house manager that there are only two (2) Bosch types for that year, the pin type and the "loop" type. Mine are the "loop" type. I am going to take one of them into the Mercedes-Benz dealership tomorrow and check on the remote possibility that I have the wrong ones installed.

OK. Here's a simple test. Feel each glow plug when cold. Then energize them. Feel each glow plug once the relay has gone thru it's sequence to see if each glow plug is warm. Be careful, they get hot and those wires burn human flesh (don't ask me how I know). If you have a non-contact thermometer you can substitute it for your fingers. This will confirm whether the glow plugs are functioning or not. If they are, it's likely your problem is elsewhere What is the temperature at your location?
I don't think you have the wrong plugs.

__________________
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

From what the present more knowledgeable posters are saying. Basically the loop plugs are always low voltage units. The twelve volt large base replacements being always pencil tip type. It does look that plugs of the wrong value are not in your car. As I said I had never seen the upgraded plugs for the low voltage loop type. Others apparently have.

Since you had some action or at least the car was trying to start with the old glow plugs. It still looks like the new plugs at this point. They apparently are doing nothing much.

I have heard of people removing loop plugs and assembling the christmas tree string with the ends of the glow plugs visable and applying voltage at the top of the string to observe if the loops go white hot. Might be an ideal as one last gasp before taking the car in.

You could have got a defective batch of plugs. Unlikely but possible as you really suspect the new plugs yourself at this point I think.

The christmas string must include the squiggly resistance wires as well of course. The normal voltage drop across the two connection points on a loop plug is .75-1.25 volts with two-two point six volts lost across each resistance wire.

I just thought I would throw this in if you did not have the information. Also because sometimes these old diesels do not like to start with one or two glow plugs not functioning I would check for the small voltage drop across any plug past the squiggly wire towards the front of the engine. This eliminates any possibility of one of those forward plugs having an internal to ground short.

This before removing the plugs if you are going to set up the test. Also scan the fuel shutoff on the injection pump to make sure it is not in the closed position before starting to do anything with the glow plugs.

I can easily visualise your frustration by the way. It is the simple things sometimes.

This relates more to a Ferguson tractor than Mercedes, but don't laugh too loud, the Ferguson has series wired loop type glow plugs. The system is power to coil type wire resister to each GP in sequence to ground. If the heavy gauge power leads are shorted and the power does not have to pass through each GP the wire WILL go cherry red. The Ferguson does not have a relay,and the bus wire (about 10 gauge) will melt if the GP switch is held on. I don't know if the Mercedes loop GP system has a resister, my 300 appears to have small parallel wired GPs. (It was dark when I just looked, and the flashlight needs new batteries.)

__________________
1984 300 D runs, interior
1984 300 D light hit in front, RUST, mint/perfect dash, decent interior (see above), parts car
(hate to kill one)
1998 Dodge diesel

I'll attempt this test. It seems very odd that so much energy is evident on those connector rails, and no start, as if there was no energy to the plugs themselves.

The plugs I have installed are said by the parts house to be the correct ones, and they certainly look identical to the ones that came out.

This is what made me think, when I first saw these red-glowing wires, that the plugs were shorted. I don't see how that is possible, as it appears to me that the whole glow plug system is wired up correctly. And yet these connector wires, rails, conductor wires, whatever you want to call them, are grounded directly to the engine block through the hex collars of the glow plugs, and so it makes one wonder how they wouldn't get hot. It's worth noting, however, that the glow plug relay is activating for about 2-3 times as long as normal, or about 90 seconds. This is when the wires get red--not before the normal time limit of the glow plug relay.

The wires on my old loop system glowed red towards the end of the normal cycle. The glowing wires are normal unless they are glowing more than usual but that is very hard to evaluate via the internet. It may be time for a YouTube video. The glowing is easier to judge at night.
Given your problems, if I were you, I would remove the loop plugs and replace the with retrofit pencil plug. They are a FAR superior plug.

__________________
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