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1978 300D
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7 Posts
Discussion Starter #1
I replaced the glow plugs with Bosch, mine are the loop style, they are connected in series by bare metal wavy interconnects, and grounded at the end of the line by a ground strap to the block. I cleaned all connections to a shiny metal. When I glow the plugs, the wavy interconnects glow a dull red. The ground strap does not glow. I took a meter to the new plugs before i connected them up out of curiosity and I couldn't get resistance out of them. One showed some resistance, that is the one on the end, connected to the ground.
Is this normal or do I have defective plugs? Before I replaced them the first connector was glowing a little. That is what prompted me to change them.
Any ideas?
 

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2001 Volvo V40
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2,954 Posts
The wavy interconnects are resistance wires, so they tend to glow as well as the glow plugs themselves, certainly if you glow long enough. If they are not becoming hot and red, than something is wrong.

Checking the loop-style glow plugs is not done by a resistance measurement. You can measure the drop in voltage, or measure the current going through the system with an amp clamp, it should measure 50-55 A. The checking procedure is decribed very thoroughly in the FSM.

Alternatively: if it starts all right, the glowers are probably all right.
 

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1978 300D
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7 Posts
Discussion Starter #3
I checked the voltage on each side of the plug with power on, as the Haynes manual describes. They state .75-1.25V for each one, Cylinders #5 and 4 read 1V, but the series is dead after that. It is very hard to start.
 

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2001 Volvo V40
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2,954 Posts
If the input of cylinder 4 has voltage, but no voltage is coming through the glow plug, that glow plug is faulty and should be examined/replaced. The glow plugs are wired in series, current is flowing through each glow plug to the next one. If one is broken, the whole system stops working. Sometimes faulty glow plugs indicate faulty injectors, especially if part of the glow plug is melted away.

FSM = Factory Service Manual. It can be bought from MB, to get an idea of what is in there, look here: 15 Electrical System Engine (Preglow) - OM617 and select 15-505 and 15-510 for the pre-glow system.
 

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'73 450SL, '83 300CD, '01 E320 4matic
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2,446 Posts
It sounds like you have a plug or wire which is shorted to ground. Inspect your connections before you condemn a plug. If one of the resistance wires is stretched or bent in such a way that it's contacting a ground, you'll lose everything else down the line. More often than not you can also blow the fuse and/or damage the plugs that are getting voltage, because they're all pulling much harder than they should be.

Glowing resistance wires is normal- but consider that all of that glowing is wasted heat.

The old system works, and it's ok in more temperate climates, but if you find that you'll need to start replacing things, you should very seriously consider updating to the newer style plugs and relay. You can get the relay and wiring harness at your local friendly junkyard, for very little money in most cases. Just find an 82ish or newer 617 turbodiesel. Everything you'll need is right under the hood. The wiring is not difficult.

The GPs will run you a few dollars, but you'll thank yourself for the investment the first time you're sitting outside in the bitter cold, and it actually starts. I think the newer style plugs are a bit more effective at starting older engines as they wear, since they heat more quickly (and hotter). If your compression is good, with the new plugs you can start down to 0 fairly easily with a couple of cycles.
 

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1982 Mercedes 300D Turbodiesel 225000 miles & 1977 Mercedes 240D 534000 miles
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4 Posts
Closely inspect your ground wire at the end of the circuitry for fraying, rust, or disintegration. If so, then replace with new one. For a series glow plug system you need a vey good ground to be able to glow the plugs effeciently.
Today I changed my frayed ground wire after I noticed that the wavy metal connectors were glowing like you mentioned. Now it takes only 10 to 15 seconds to get the glow light out. Hope this will help you.
 
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