Hi everyone new to the forum, read though here a bit for info when working on my merc.

I had a problem with my 300d today when i was vacuuming the carpet. I turned the key to on and moved the seat forward then turned the key off and began to finish cleaning the car.

When i finished vacuuming i looked up and noticed smoke coming from under the hood. I popped the hood and opened it up to a fire. I was able to get the fire out with no issues so the car is fine. looking a the area the fire was (under the intake manifold) it appears to be the glow plug wires. the circuit still works as i tried and the glow plug light still comes on and the car still starts normally not hard like glow plugs are bad just a little smoke.

I know i will need to replace the harness because that is a bad situation. Does anyone know what would cause an issue like that? Would it be a sticking relay or something else seems like a fuse in the system would have blown but nothing did.

the wiring harness goes to a box with part # 0075459932 which is the glow plug main relay i believe could this relay be going bad and sticking open?

all help would be appreciated.

droze

If it was an electrical fire, your current got way to high and your fuse definitely should have blown. High current = short to ground. Check the resistance of each glow plug to ground.

Did you recently replace the glow plugs or do anything under the intake? I've heard of folks getting the harness trapped under the intake with out noticing it or having a wire end touch a metal surface.

nope haven't done any work to the glow plugs just happened. I will check the resistance to ground on the glow plugs. cannot explain why the fuse would not have blown just a really weird turn of events.

My guess, the fuse is 60 amps right? Well that current would typically be spread out through all you glow plugs, unless you have a short. Then most of the current will go through the wire with short. You don't need 60 amps to melt a wire and something was limiting the resistance to keep the fuse from blowing, so you had a bad ground to short in a since

A 91 won't have a glow plug fuse. My guess is with the key on, the plugs ran the full 60 seconds before timing out, which made the harness get hot. If there was corrosion or some other issue it could have caused a fire. The relay could have gotten stuck on as well.

-J

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1991 350SDL. 230,000 miles (new motor @ 150,000). Blown head gasket

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I think you are right and it was the relay getting stuck on. I just pulled the intake manifold off to make it easier to remove the harness and test the glow plugs. all the glow plugs ohm'd out at .8-1 ohm so looks like they are good. Im gonna fix the harness and test the glow plug controller. I may just replace the relay there is a chance it may never do it again and it could do it in a week.

Sounds like a stuck relay, happened on my brothers former '83 300D, but it ended up melting the power supply post/blowing the fuse in the relay rather than a fire...

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'13 ML350 Bluetec - 95k - dad's (OC-98k)
'01 SL500 - 103k(km) - dad's (OC-110,000km)
'16 E400 4matic Sedan - 148k - Brothers (OC-155k)

Wish I was so lucky. I pulled it all apart and am going to rebuild the harness. I have to get a couple new nuts because the retainers did not hold them and they fell down the side of the block and are stuck.

How would I check the relay? Would I just go from a post on the relay to ground with my meter and see how long it is active with voltage? That's how I would check it for auto door repair anyway, this one just looks a little more complicated than a standard dumb relay. I guess it would work like any other time delay relay system active for a determined amount of time then releases. I don't have any schematics for this car or any documents for the relays operation.

I really won't k ow what it's doing till I get most of it back together but that will have to wait till Monday since I'm leaving town for work over the weekend.

I have two questions...
first, why did you turn the key on to move the seat? the seats should always have power, with the door open.
2nd, are you sure you turned the key off while you were vacuuming?

as to the fire, what was burning? what got hot? I'm sure the GP harness got damaged in the fire, but I'm confused as to what was burning? is there a lot of oil and gunk all over the motor? is there a problem with leaves and such?

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John HAUL AWAY, OR CRUSHED CARS!!! HELP ME keep the cars out of the crusher! A/C Thread
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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!

Wierd my seats won't move without the key on, or maybye i have never tried with the door open. The key was off as it was the first thing I checked when I realized what was happening.

The rubber wire loom that travels from the glow plug relay to the glow plugs was actually burning. It was burning where it comes around the water neck on the top of the engine to the glow plugs.