I was perusing the Ether thread and a couple other cold start and glow plug threads and I got to wondering: What is the stock MBZ glow plug wiring like?

I'm an auto lighting aficionado. I have relays and upgraded wiring on the headlights of just about all my vehicles. With a halogen bulb, a 10% drop in voltage(very very easy to surpass with undersized corroded wiring) causes a 46% drop in light output.

Are glow plugs the same way? Could undersized or corroded wiring be causing some people to have cold starting issues? Great glow plugs won't work properly if they're not being fed properly. My 617 in my truck didn't come with the car or stock glow plug wiring, I built my own glow plug wiring harness. It's a 4AWG cable running from the battery positive post to a 100 amp fuse, then to a Ford starter solenoid, then out to the engine. From that 4AWG cable, 10 AWG wires run less than a foot to each individual glow plug. I'll have to measure my voltage drop with the glow plugs on when I get home.

This might be something to look into - I'm kind of shooting in the dark here, having no idea what a stock MBZ glow plug harness is like, it could be built massive overkill. On the other hand, it doesn't matter if the battery is at 12.7 volts and your glow plugs are new if your glow plugs are only getting 10 volts or less.

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617 swapped Toyota Pickup, 22-24 MPG, 50k miles on swap

Stock wiring is 4 mm square (10 AWG) from battery to glow relay. 2.5 mm square (12 AWG) from glow relay to glow plugs. Stock wiring could corrode depending on environment. A voltage drop test will tell if wire corrosion is a problem. Since your wiring is higher gauge, you'll likely have less v drop than stock wiring.

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85 300D turbo pristine w 157k when purchased 161K now
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked

The crimped on Eyes on the Glow Plug ends of the Wires likely corrode as they do on other applications.

For the Stock Wires you can clean the exterior and if you can get to the area where the crimp is you can Solder them good and that usually restores a good contact.

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84 300D, 82 Volvo 244Gl Diesel

This looks like a good place to ask this question....

I seem to be having issues with the stock glow plug wiring harness. The wires are cracking and all crusty. I would like to use different wire and have found some nice solid looking solder on ring terminals that fit the glow plug stud....65 cents a piece...they are solid copper lugs. I am going to try to use the stock glow plug relay connector (big one) that plugs into the relay and has the wires going to the glow plugs themselves. The back part of the plug comes apart to reveal the small female tin plugs that are housed in the plug connector itself...I can take them out and un-solder the old crusty wires...however...I think this plug might limit the choice I have regarding what guage of wire to use? I was thinking that I would like to use 8 guage or 10 guage wire to replace whatever guage came from the factory. Will changing the wire size by putting in larger guage size harm anything.....I am concerned mostly with the glow plug relay...Thanks

Just looking at this 8 guage wire. It is pretty big, maybe overkill. I should probably stick to 10 guage, but I still think this is a larger guage size compared to factory stuff. What guage is the factory wire that goes from the g.p relay to the glow plugs? Will 10 guage cause any problems, with the relay?

the stock harness is enough and I believe the feeder is closer to 8gauge, and each runner to the plug around 12ish... maybe smaller, but it's not the wiring that drops the voltage to 10v... it's the draw... 100 amps from the stock battery drops ITS voltage to 10v... you would need two batteries to keep the voltage above 12v when the glows engage. However, after 12or so seconds, the heat from the plugs drops the ampdraw to around 18amps, and the voltage jumps back close to 12v for starting.

would larger feeder wire get a cleaner current to the glows? sure. do I think it would make any difference on any but the corroded battery neglected cars? no.

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John HAUL AWAY, OR CRUSHED CARS!!! HELP ME keep the cars out of the crusher! A/C Thread
"as I ride with my a/c on... I have fond memories of sweaty oily saturdays and spewing R12 into the air. THANKS for all you do!

My drivers:
1987 190D 2.5Turbo
1987 190D 2.5Turbo
1987 190D 2.5-5SPEED!!!

1987 300TD
1987 300TD
1994GMC 2500 6.5Turbo truck... I had to put the ladder somewhere!

I agree, that the factory harness is enough, but I need to build another one cause the one in the car is crusty and stiff and I am afraid I have broken one of the wires off at the ring terminal, there could be more that are as dry as the one I broke and I don't wish to worry about it....

plus... rather than trying to fix this greasy dried out mess and trying to clean these old terminals for solder...I wanted to use these nice new shiny lugs and some new wire. I already purchased new duratherm G.P'S and the lugs and some 8 and 10 guage wire. I have removed an old harness to steal the plug that goes to the relay....all that is left is to warm up the iron and copy the old harness (length) I figure that if I do this and with the new g.p's there should be no reason why these g.ps won't work well for the future and it will be one less thing to second guess, when it is cold. It has only cost me 10 bucks for the supplies

I am only re-doing the portion that goes from the relay to the g.p's. the rest is fine. I have recanted on the idea of using 8 guage for this. The 10 guage looks like it will fit the plug portion. I think the original stuff is 12 guage. The only concern I have is with changing the wire size to be slightly bigger. going factory 12 guage to 10 guage...Will the relay care?

oh, I was responding to the OP, not your message about the crusty wires. sure, if damage is evident, replace!
I'd find the harness from a newer diesel, and snag that harness... get integrated GP nuts and such...

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John HAUL AWAY, OR CRUSHED CARS!!! HELP ME keep the cars out of the crusher! A/C Thread
"as I ride with my a/c on... I have fond memories of sweaty oily saturdays and spewing R12 into the air. THANKS for all you do!

My drivers:
1987 190D 2.5Turbo
1987 190D 2.5Turbo
1987 190D 2.5-5SPEED!!!

1987 300TD
1987 300TD
1994GMC 2500 6.5Turbo truck... I had to put the ladder somewhere!

One thing I think MBZ did that I wish I had emulated is running individual lines to each glow plug instead of one main trunk line to the engine and then separate feeders to each GP. This would make testing for a faulty GP very much easier, just unplug the connector and be able to test each GP right there at the connector, vs. having to disconnect each individual GP.

Either way, the factory system works and my system works. Lots of ways to skin this cat.

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617 swapped Toyota Pickup, 22-24 MPG, 50k miles on swap

So, sorry to be a hijacker of the o.p's thread, but could'nt see starting a new one with him touching on current draw. It was this tthread that spurred me wondering if using larger guage wire compared to the factory harness, would have negative effects on the g.p relay. I have since found another thread where a member has done just as I have been wondering, by going up in guage size from 12 guage to 10 guage....no one seemed to have anything against it in his thread or this one, so we will move forward and use 10 guage wire for the small harness rebuild. With the meager amount invested in re-doing this harness(10bucks), I can't understand why anyone would spend $180 to buy a new harness from MB. The way they have designed the connector at the relay, for just such a repair job is great. thx.

I tested a Glow Plug with an Automotive dash board type Amp Meter and a Glow Plug. Unfortunately the Gauge only went to 60 amps.

What I found is that when you first applied the Electricity the Meter pegged at the 60 amps but within 2 seconds as the tip got hot it settled down to about a steady 16 amps.

I doubt if there is any resistance issues in the Stock Glow Plug Wire Size. If there is an issue it would be due to oxidation of exposed ends and under the Crimped area of the Eye Connectors.

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84 300D, 82 Volvo 244Gl Diesel

We finally finished building a new wiring harness for the relay to glow plug portion. I ended up at a large truck\trailer parts place to get some half inch shrink tubing and noticed that there is some shrink tubing especially for being around diesel. I did'nt use it but it is availible and more expensive....mine worked out nice,anyways. It was very hard and tedious trying to solder 10 gauge wire into the stock terminal ends, but we did it. The stock wire must be 12 gauge, but if you clean all the. Old solder out of the small tin terminals the ten gauge will fit.

Put a very small amount of copper antiseieze on the threads of the new Bosch Duratherms, being very careful not to touch the tips with my fingers or to get antiseieze on the tips, got them installed and re-routed the new 10 gauge harness. There are wires for the coolant sensors on the head and the thermostat housing, that are shrink tubed into the stock wiring harness...I did not put these back into my glow plug harness. You could if you want but...I noticed that one of the wires was cracked, so I will redo that portion later and just zip tie it on . The 85 wagon head has 2 coolant sensor wires inthe cyl.head , the 83 euro harness only had one wire for the cyl.head sensor, otherwise the glow plug portion of the harnesses were identical.The 85 wagon harness part# was 123 540 1510. The 1983 euro 300D part number is #123 540 6108 ks. For reconstruction, we used 4 different sized shrink tube...1\8", 1\4",3\8", and 1\2", I made each wire length 4" longer than the stock harness. The ten gauge wire was from a farm implement manufacturer and the lugs were from the auto electrical store for 65 cents ea...solid copper....(same guys that sold us the glow plugs)

Tested it out that day and it fired up, nice.
Today was perhaps the best test...-37c with windchill -47c. Or -34F and -52.6F. It started with one glow and cranking for only 3 seconds and firing on all 5 with help from a little more throttle, than usual. It was plugged in, overnight. But we are happy. It seems to be running better than it did, before all this. Impressed with these glow plugs too, curious how long they will last.

One thing we noticed was that the glow plug light stayed on today in this cold for much longer than it ever has, hope this is not something to worry about, and could be attributed to the cold weather?? Is there something in the system that accounts for temperature and will leave the light on longer than normal?

When disecting the factory M.B harness and when all the shrink tube is removed, we noticed that each glow plug does have an individual wire from the factory plug at the relay to each individual glow plug. I thought it WAS possible to test each glow plug at the relay connector end?? AND without disconnecting them?? Are you using the factory glow plug timer?

The lamp duration varies inversely with temperature. Same for the glow period.

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When cryptography is outlawed, bayl bhgynjf jvyy unir cevinpl.