New A/C Problem - melting fuses???
 

HELP! My A/C has developed a new problem. For the last three days in a row, my A/C quits after melting the plastic part of the fuse. The metal part of the fuse has remained in tact and has not blown. This has happened 5 or 6 times now.

While this is happening, I get a funky, musky smell in the cabin similar to bradford pear tree blossoms (aka "cum" trees as I have heard them referred to - LOL). I have noticed this odor each case prior to the A/C crapping out.

I've never run across anything like this on the boards RE A/C, but I have seen rotton egg smells associated with the alternator and battery going bad. FWIW, I've got 152k on the clock. Alternator was replaced at 100k and battery was last replaced at 140,000.

A/C was recharged last Wed with 1 12oz can of R-12 and problem first surfaced on Friday. Sometimes the A/C will not quit while running. After shutting down and cooling off the melted fuse appears to shrink enough to break the electrical contact so when I start the car - no A/C

What's goin on?

Bad or dirty "riveted" connections in the fuse block maybe?

This is a shot in the dark, but have you cleaned the fuse block contacts where the fuse fits?

As a preventative measure (and I don't do many of these for my car) every year or two I disconnect the battery, pull out all of the fuses and clean the contacts with a brass wire "toothbrush" like you can buy at Home Depot etc.

I am thinking that maybe the dirty contacts are heating up the fuse enough to melt the plastic, instead of frying the metal strip on the fuse like you would normally expect with a high current draw. Maybe with the new refrigerant load your compressor is working harder, and still functioning normally, but drawing more current than before.

But, another possibility is that your compressor is drawing too much current, just enough to heat the fuse strip up and melt the plastic, but not enough to blow the fuse. Kind of an in-between level of failure. Maybe the compressor is bad, or working harder since you refilled the system. Did the shop put too much refrigerant in?

Just stuff to think about. Maybe someone who has been there will respond with better info.

BTW nice analogies about the smells. Have not smelled a "cum" tree myself, still a nice thought. Or not. :)

Good luck
Scott

I would first check the fuse block terminals (the metal pieces the fuse snaps into) for corrosion and lack of spring tension. If the fuse element is not blowing then I would suspect too much electrical resistance at these contact points which leads to the heat causing the melting.

This just happened to me.

http://forums.pelicanparts.com/porsche-911-technical-forum/346186-how-can-happen.html

Once I cleaned the melted plastic goop of the contacts they didn't look bad, but cleaning the goop would likely have removed some corrosion if it was there. I went ahead & thoroughly cleaned the contacts the ends of the screws and the jumper at the top. I've only driven a very little bit since then, but the AC did seem to run a tad stronger (unless I'm imagining it) and the problem hasn't returned - yet. Will have to wait a bit to be sure because I'm under the knife right now for voltage regulator replacement.

Well, fuse block and contacts all cleaned up with a wire brush and I'm still melting the A/C fuse.

Any other ideas?

Here's a pic of the fuse. This one has not failed yet but is close....getting the funky melting plastic smell.

http://forums.pelicanparts.com/uploa...1179424871.jpg

You still have dirty contacts.

The corrosion forms an electrical resistance and and when you pass alot of current through it you get ohmic heating (I^2*R).

Put a voltmeter across the wiring to the fuse and see what kind of a voltage drop you have there. It shold be ~0.

Remember those 500 watt halogen lights they quit selling because they started fires? Same thing.

Mark...I guess we are talking about the center fuse. The power is coming in on the right. The terminal on the left still looks dirty. It should really shine. A high resistance at the terminal or the wire connection will cause the fuse to heat up and blow. I would clean it again. Next I would loosen the screw on the left and remove the red wire. The wire has a brass bushing around the conductor. I would clean the brass until it too shines. I would try to spray some cleaner and then some dielectric like CRC 2-26 to improve the contact between copper conductor and the brass bushing. From your picture I can see the copper is a bit funky. I would try to re crimp the brass bushing. If you are handy with a soldering iron I would flow some solder into the bushing to make a more last connection.
Once you have that area cleaned up you can try to run the system again. If the fuse blows then you need to disconnect whatever is attached downstream and systematically re-attach each component while measuring the current draw.

Melting fuses........
 

The fuse that you are having problems with powers (I think) both the evaporator fan and the front condensor fan. You need to check both fans to see if one of them is or has frozen up and is thus pulling too much load. Since this is the only fuse point for both circuits, you could have a wiring fire if the problem is not fixed! If you can't tell which fan is the problem, disconnect the condensor fan first (easiest to get to) and run the a/c. If it is the condensor fan, the fuse will be ok. If the problem is in the evaporator fan, the fuse will continue to melt. You also need to loosen the screws that holds the wiring to the fuse block and clean the corrosion off the wires and out of the hole in the block. A .22 caliber rifle wire cleaning brush is just the right size for the job. Use the brush and some spray electrical cleaner to get both the hole and the wires completely clean. If the two fans are ok, cleaning the connections may fix your "issue". Hope this helps.

Rick, Ron & Fred, thanks for the quick reply and suggestions.

You're right the contacts still look bad....you should have seen them before!

I stopped by the shop on the way back from my last "melt-down" and the tech said that they sometimes replace the posts or the entire fuse box....specifically after looking at mine. I've got the same kind of tarnish/crud built up on the wires that go into the block......Ron what type of cleaner should I buy and where can I find it?

Fred, the Evap motor and front condenser fan motor were replaced last year and are functioning properly and I've added a fuse to the front condenser fan.

Given the problem and the poor condition of the fuse block I've ordered a new 3 - post fuse block that will be here on Monday. I will clean up the wires as suggested by Ron and continue with his recomendations from there. Hopefully the problem will be solved without having to resort to his last step of disconnecting and reconnecting.

BTW, my compressor appears to be the orig Nippondenso (sp?)
could it be giving up the ghost and drawing too much amperage......what's the lifespan on these?

Thanks!!!!

If it were drawing too much current the narrow center metallic part of the fuse would be getting hot, not the point contact on the end.

Thanks, thats what I wanted to hear!!!

Mark, you may need to dis-assemble the area marked "A" and sand the contact points between the post and the flat blade.

You also should scrape/sand the surface "B" so that you have "shiny" metal exposed. I had to do something similar on my 911 and on my daughter's 924. I solved my problem by doing the above.

http://forums.pelicanparts.com/uploa...1179487902.jpg

I've used emory cloth with good results when cleaning fuse and other electrical contacts.

Update: New 3-post fuse block is supposed to be here tomorrow. Nevertheless, could not resist the temptation to tinker further with it by removing the fuse block for some more intensive cleaning with a dremel wire wheel.

On removal. I noticed a couple of the rivet post connections were a little loose with the problematic A/C top post being the loosest. There was also a small crack in the plastic at that post connection contributing to the problem.

After cleaning everything up, I tightened all the rivets using a large punch with the post resting on the vise. Will have opportunity to test before new fuse block arrives. If it works, I'll just let the dealer keep their $65.00 part!

Shiny bits
http://forums.pelicanparts.com/uploa...1179689793.jpg
Crack at center post bottom row
http://forums.pelicanparts.com/uploa...1179689888.jpg

All put back together. Ran A/C for about five minutes. Fuse block was hot to the touch.........should the fuse block remain cool? Does heat = problem?

http://forums.pelicanparts.com/uploa...1179695068.jpg

I installed an external relay and wiring for the a/c, no problems after that since 5 years.

Yes it does.

As pretty as you cleaned up the fuse block I can't believe corrosion/oxidation causing resistance is a problem.

If I remember fuses right a 25 amp fuse should have about 0.005 ohms of resistance. I^2 * R = 625 * 0.005 = 3.125 Watts. If you touch the fuse itself it should be warm if it is drawing that much current.

look at a 25 amp AGC FUSE AGC-25 for a reference. Granted this is not an antique open ceramic fuse but it says at rated current it should have 0.11 volts drop across it. (2.75 watts)


Maybe it is time to break out the voltmeter. You could measure the voltage across the fuse wire bridge (all within the fuse itself) (with the AC running) an then measure the voltage across the contacts at each end of the fuse (2 of them) and then measure the voltage drop from the wire to the terminals (2 of them also).

High Resistance..........
 

Looking at the picture of the back of the fuse holder, the brass jumper appears to have suffered high heat. You may have fixed the problem by restaking the rivit. If the problem persists, you might try touching it up with some silver solder.

These are the kinds of problems that finally pushed me into replacing the fuse panel with one that uses ATO fuses.

Good luck!