87 300D blowing glow plug fuse
 

My 300D has eaten three 80A glow plug fuses today. I had already replaced the fuse about a year ago when it blew, after having replaced it a year before that. Today it's become much more keen to burn them up so something is clearly wrong.
Per my search of other people's problems with this I have checked the harness for any wires sticking out and contacting the block/head/intake/fuel lines ... I just cannot see anything out of place. Five of the glow plugs measure .8 ohms and one measures .7. I am getting 12.06 volts at the fuse with key on. I cycled it a few times while testing and it did not blow, but the hot end of the fuse gets REALLY hot, burned my finger. It didn't get hot with the harness unplugged. I'm not sure if that's typical or a sign that the problem is in the harness. After a couple test cycles with the harness on, letting it cool between, it blew again and now I'm out of fuses.
Any ideas what other tests I should do? Could it still be the controller causing the issue, maybe I should try replacing it? Under normal conditions, does the fuse get really hot to touch?

The fuse should get warm but not hot. Clean the fuse contact points and see if that helps the fuse to run cooler. Also clean the junction box contacts (top of passenger fender W123), assuming you have one on W124, which is the feed to the glow plug fuse.

On the W124/603 there is a control box mounted on the driver-side fender, with the fuse in it. Everything looks pretty clean but I'll try cleaning nonetheless. Guess it's time to break out the wiring diagrams and see exactly which wires could be causing a short and where.

Remove the cover plate of the larger glow plug connector at the relay so you can isolate each glow plug while glowing. See if you can find the wire/glow plug that makes the fuse get hot. If you're out of fuses, connect each wire to the battery through an ammeter to find the one/s drawing more current than the rest.

Sixto
83 300SD

For the latter method would I just put one lead of the meter on the positive of the battery, and one in the hole on the glow plug harness? I don't think my multimeter will handle high enough amperage though.

If the ohm test is showing around an ohm, each, and the harness isn't shorted anywhere, it's gotta be corrosion heating the fuse.

Clean the feed wire, the screws, the fuse mount etc... You'll find it.

If the ohm test is showing around an ohm, each, and the harness isn't shorted anywhere, it's gotta be corrosion heating the fuse.

Clean the feed wire, the screws, the fuse mount etc... You'll find it.

I'll clean up the bits and pieces tomorrow. Should I just use electrical cleaner and a wire brush? I'll have to run and pick up a bunch of fuses first ... hope I don't have to burn too many more.
Another thought: What is the wire coming out of the back of the harness plug, and running behind the controller box? And can it have any possible effect? It kind of joins a bundle and runs through a firewall towards the fusebox ... maybe a temp gauge?

Use a wire brush, electrical cleaner not needed. Use an IR thermometer if you don't want to burn your hand again.

Jay Bob has pointed out the copper or tin plating removed from wire brushing is bad for fuse contacts. Use metal polish and a soft cloth there.

If one side of the fuse is getting hot, that side has a bad connection. If the fuse is carrying too much current and both ends are making good contact, the thinnest point in the fuse will be hottest, not a contact point.

Metal polish/ soft cloth? We are dealing with surface corrosion of electrical contacts, not expensive chrome plating. A wire brush is fine. A few swipes till you see shiny metal is all that's needed and won't remove any plating.

OK, that makes sense and is telling. The fuses aren't blowing in the middle, but right next to the bottom screw. I'm losing my mind because of course it's a Sunday, so the MB dealer is closed and I can't get anymore fuses. Then I'm out of town two days. I can't stand not figuring it out now. Maybe I can find an extra in my garage somewhere.

Per Ebay pics, W124 glow plug fuse | eBay W124 glow plug fuse is on the relay and looks the same as a W123 glow plug fuse. Maybe you can borrow the fuse from your 240D?

Don't.

You probably will burn the 240D fuse even if there is no problem with the W124 GP circuit. Fuse in 240D is rated for 4 GP, the W124 300D is rated for 6 GP. If the fuse is burning EVERY time then you have a short. GP will NOT normally shortout when it burnt out, it will be open circuit. I will do this test, pull the big connector inside the GP replay and see whether the fuse blow. If it does ( which I doubt ) then the problem is elsewhere and is serious. If it does not then it is one of the GP wires is short to chassis. You can measure the resistance from the big socket pins to chassis. Measure the 6 pins to chassis. You need an accurate meter as the resistance between a short to chassis ( should be 0 ohm ) and GP resistance is about 2-5 ohms.