1984 300D, Dangerous fuse failure (a brass one, not aluminum)

[Nuker]

So I have been reading here for about the last 6 months since I bought my first MB. I was lucky to find a 2 owner, 1984 300D here with 98,000 miles on it for about $3,300. The car was in good shape with the exception of little to no maintenance other than oil changes for the last 40,000 miles.

So I set out to do everything right and make it a clean, reliable daily driver. I started out by changing all the oil, filters transmission fluid, filter, motor mounts, air cleaner, etc.

All of this complete about 2 months ago, I also changed out all the fuses as they were a mix of copper, brass and aluminum types, all looked old. I read here to use the brass type, not the aluminum. So I ordered a number of the Bussmann BP/GBC types in 8, 16 & 25 amp.
I removed all the old fuses and kept the copper/brass ones as spares, threw away the aluminum ones. I then cleaned every fuse holder and installed all new brass fuses of the correct rating in every holder, checking that they were seated correctly and the holders were tight. All is good!

Fast-forward to this week, my A/C has been working on & off (terrible in 105F weather). So I am troubleshooting per thread recommendations here. I evacuate all the refrigerant, replace all o-rings and flush the condenser & evaporator. Still working intermittently. I find no leaks and my charge is still good - evacuated, weighed & re-charged yesterday.

So tonight, I read about the Klima relay, accumulator/drier pressure switch and temperature switch. Test both and they are fine. Look for Klima relay and can't find one... Look in the fuse box and find this scary scene:


Yep, that new fuse overheated without melting the element and got hot enough to melt/shrink the body material. It was collapsed enough that once I broke it free, it could bounce around in the holder. No wonder my A/C was going in and out. Anyway, I am glad this thing didn't somehow start a fire. Left to right is the burnt Bussmann, a new Bussmann and the older one I will use instead:


So to fix this issue, I took green abrasive and cleaned all the soot off each side of the fuse holder in question. I then installed a clean, good fuse and measured across the holder...7 Ohms - not good! I scrubbed more and more and then applied some No-Ox A special grease to the fuse and holder (this is what is used on the contacts of power circuit breakers, to keep corrosion at bay). I seated and re-checked the fuse, 0.4 Ohms - Good!
Here is how to check the fuse across the holder to ensure good contact:


So now I will try it out tomorrow and check the fuse once I am back home. I will update this thread if I find out some other issue was causing this overcurrent condition or if it was just due to the old fuse having poor contact.

Thanks all. Hope this helps out someone, I have gotten lots of help from the forum already.

[JHZR2]

Ummmm, great pics, but what does the inside of your fuse box indicate wrt rating?

Believe fuse 8 is supposed to be a red 16A fuse. Going to 25A will get local contact points too hot with the continuous use of the blower when old and higher-draw. And melt and burn...

There’s a fuse bypass kit that puts a bypass strip fuse on the fender... otherwise please check the correct rating.

[Maxbumpo]

The fuse that melted / shrank was not the problem, the corrosion that prevented clean contact was the issue.

[BWhitmore]

I believe the KLIMA relay was used on 1985 models forward. I do not think your 1984 model used a KLIMA relay. My 1985 300D has the KLIMA relay mounted on the left fender well under the hood.

[Diseasel300]

Originally Posted by Maxbumpo

The fuse that melted / shrank was not the problem, the corrosion that prevented clean contact was the issue.

The root cause is the craptastic design of those "torpedo" fuses. The only contact area is basically a circle outline where the end of the fuse sits in the holder. Fine for lower amperages, questionable for 16A loads, and absolutely ridiculous for 25A loads. It's exactly why later cars moved the blower fuse to an auxiliary holder external to the fusebox. Over time the tiny connection on the fuse becomes resistive and the continuous high current draw nature of the blower motor cooks it.


To the OP....make sure you use ceramic body fuses. Even if you have a poor/hot connection, they won't melt and deform.

[JHZR2]

Originally Posted by Diseasel300

The root cause is the craptastic design of those "torpedo" fuses. The only contact area is basically a circle outline where the end of the fuse sits in the holder. Fine for lower amperages, questionable for 16A loads, and absolutely ridiculous for 25A loads. It's exactly why later cars moved the blower fuse to an auxiliary holder external to the fusebox. Over time the tiny connection on the fuse becomes resistive and the continuous high current draw nature of the blower motor cooks it.


To the OP....make sure you use ceramic body fuses. Even if you have a poor/hot connection, they won't melt and deform.

Agree. And if the corrosion/oxide is removed mechanically, the abrasive nature of such removal can reduce the contact area even more. No good solution. Believe the strip fuse retrofit uses a ring terminal and screws into the backside of the fuse holder.

Haven’t looked at the fuse box chart for ratings in a while, but I would doubt many of the spots that take 25A (blue fuses) at least in a w123, are continuous draw loads the way the blower on fuse 8 is...

[ROLLGUY]

Originally Posted by Diseasel300

........................

To the OP....make sure you use ceramic body fuses. Even if you have a poor/hot connection, they won't melt and deform.

Totally agree. Also, with my Sanden retrofit, I suggest using the relay kit as well. It has a separate fuse for the cooling fan and compressor, and leaves fuse #8 for only the blower. I have heard that a possible reason for compressor and/or clutch failure is voltage loss to the clutch coil. When this happens, it overheats and causes a compressor shaft seal leak. By removing the clutch from fuse #8, it takes the load off of an already overloaded fuse.

[JHZR2]

Originally Posted by ROLLGUY

Totally agree. Also, with my Sanden retrofit, I suggest using the relay kit as well. It has a separate fuse for the cooling fan and compressor, and leaves fuse #8 for only the blower. I have heard that a possible reason for compressor and/or clutch failure is voltage loss to the clutch coil. When this happens, it overheats and causes a compressor shaft seal leak. By removing the clutch from fuse #8, it takes the load off of an already overloaded fuse.

+1 to the rollguy relay kit.

[ROLLGUY]

Originally Posted by JHZR2

+1 to the rollguy relay kit.

I now have a new dual relay model available (actually, it is the ONLY one available). It has two relays, and two fuse holders. That way, both the compressor and fan have their own separate fuse and relay.

[Diesel911]

Originally Posted by JHZR2

Ummmm, great pics, but what does the inside of your fuse box indicate wrt rating?

Believe fuse 8 is supposed to be a red 16A fuse. Going to 25A will get local contact points too hot with the continuous use of the blower when old and higher-draw. And melt and burn...

There’s a fuse bypass kit that puts a bypass strip fuse on the fender... otherwise please check the correct rating.

What I remember from past threads about it is that the Fuse Kit is for early version of the SD's (W124?) not for W123's

[Diesel911]

Can't tell from the pictures if the melted fuse body is plastic or not. Use fuses with ceramic bodies and I don't think they can melt.

[JHZR2]

Originally Posted by Diesel911

What I remember from past threads about it is that the Fuse Kit is for early version of the SD's (W124?) not for W123's

I know for a fact that they work, as I had one on my ‘83 after the fuse box nearly started on fire.

Im an old-timer on here:

#8 fuse blowing, AC condenser drain

Inline Fuse on a HVAC Blower Motor

Edit:

Knowing now more than I knew then, I’d want to do a much better job of diagnosing the blower, and add a derated fuse (key is to fix a bad spot in the fuse box to something with no contact resistance issues), and try to splice more larger gauge wire in the circuit where sensible, to further reduce voltage drop.

[Diesel911]

Originally Posted by JHZR2

I know for a fact that they work, as I had one on my ‘83 after the fuse box nearly started on fire.

Im an old-timer on here:

#8 fuse blowing, AC condenser drain

Inline Fuse on a HVAC Blower Motor

Edit:

Knowing now more than I knew then, I’d want to do a much better job of diagnosing the blower, and add a derated fuse (key is to fix a bad spot in the fuse box to something with no contact resistance issues), and try to splice more larger gauge wire in the circuit where sensible, to further reduce voltage drop.

I am only saying what I read in the past.

In my mind if you are blowing the Blower Fuse you are better off fixing the issue that is causing that.

I went into my Blow Motor cleaned all of the gunk out, lube the rear bushing. I can't recall if I replaced the Ball Bearing or not.

Then there was the main cause of my blower issue which you will see in the attached pics.

The left pic shows a good connection with no evidence of overheating.

The right pic shows evidence of overheating I believe due to corrosion in the crimped connection. That was fixed by bridging the connection with Solder.

In my case if I had added the fuse kit with a higher amperage fuse the heat caused by the resistance from the corroded connection would have eventually damaged the Blower Motor.

Instead I fixed it and have had zero issues since then.

[JHZR2]

Totally agree that’s prudent. In my case, somehow there was a 25A fuse there, worked great for a couple years then got hot. I noticed it because I got a bit of smoke out the vent which was charring of the paper card in the fuse box. The strip fuse repair was a better idea then putting another fuse in the box, IMO.

[Diesel911]

Originally Posted by JHZR2

Totally agree that’s prudent. In my case, somehow there was a 25A fuse there, worked great for a couple years then got hot. I noticed it because I got a bit of smoke out the vent which was charring of the paper card in the fuse box. The strip fuse repair was a better idea then putting another fuse in the box, IMO.

If the wiring is intact (no shorts or corroded connections) it kind of has to be an issue with the items in the fuse circuit.

An inductive amp meter can help you sort out things. You just need enough room for one strand of the wire to hook inside of the channel (you don't have to tape into the circuit) the back of the gauge.

There is also a wider channel to slide a battery cable in so that you can check the amperage when you Starter is cranking.

The one pictured is from sears but NAPA used to ones similar in function for around $25 but there is no metal shield on them.

So if I burned a Blower Motor Fuse and I inserted another Fuse and got the inductive Amp Meter on one of the wires right next to the Blower and turned the Blower on I would get the amperage that the Blower Motor.
If you check both wires and they are both pulling the same amperage that is too much then it is likely the Motor or something keeping the motor from turning freely.