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!

Re: High Resistance..........
 

And, IIRC, your installation of the ATO fuse blocks was fantastic! http://www.pelicanparts.com/support/smileys/clap.gif You might post some pictures to give everyone an idea of what you did.

I just noticed there is a ferrule on the end of the wire. What does that look like?

Changing to ATO.........
 

Basically I built an aluminum mounting bracket that holds 24 ATO 20 amp rated fuse holders. The mounting bracket bolts directly to the old base. There are several jumpers on the backside of the old style fuse holders that you will have to replicate. I spent a lot more time studying the wiring harness than actually doing the conversion! This is no place or time to mess up. Here are some pictures:

This is the new fuse block before mounting:
http://forums.pelicanparts.com/uploa...1179799690.jpg

This is the fuse block mounted in the car and wired:
http://forums.pelicanparts.com/uploa...1179799783.jpg

Notice that there are 3 extra fuse positions available.

This is the new fuse block with the original cover installed:

http://forums.pelicanparts.com/uploa...1179799902.jpg

Since doing the fuse conversion, I have had only one wiring issue and that was due to a bad connector crimp (my fault).

Nice work Fred!

No luck on the fuse block clean up & repair. Melted another fuse on the way home from work. Stopped by dealership on the way - new fuse block should be in tomorrow. If that does not solve problem then on to further troubleshooting with the multimeter.

Is this supposed to be fun?

Hmmmm............
 

Have you checked the load that is being pulled across that fuse? A quick check with an ammeter will tell you if the problem is the holder or the load. I could not tell how many distribution wires are coming off that fuse, but if more than one, you might have to separate the load and add a fuse.

Good luck!

Fred, I have not checked the load yet.

I have inquired what the total amp draw should be for the compressor, evap fan and front condenser fan should be (< 25 amps obviously) but no replies.

I have thought about adding an addl circuit and taking some of the load off the A/C circuit, but want to make sure I don't have an overload problem with one of the components in circuit first.

BTW, there is only one distribution wire coming out of the fuse.

Thanks for your help!

Long Shot....................You haven't overcharged the sytem by any chance putting more load on the compressor???
Dave.

Specifically asked the tech who charged the system right B4 the problem started this question and he emphatically stated that charging the system would not cause this.

He recommended replacing the fuse block as a starting point citing that the fuse block is a common failure in this age car.

Fuse load..........
 

I think you will find that only the condenser fan and the evaporator fan work off the fuse in question. The evaporator fan works thru a relay but the front condenser fan I believe does not.
On my SC the combined load for the two fans was slightly over 20 amps. When I rebuilt my fusebox I split the load between two fuses.

25 amps is greater than the range of most multimeter's measuring current. So you would have to go to Radio Shack and buy a 0.1 ohm resistor.

A multimeter does not seem to be an item in his toolbox. :(
The electrical load the clutch presents has no relation to the load on the compressor.

Re: Fuse load..........
 

I believe the power seats, if so equipped, run off this fuse as well.

Busted!!!!!!

He's a carpenter for Godsake, not an electrician...

SmileWavy

I've been exposed! ;)

OK, fuse block replaced with new one and overheating problem remains. Disconnected front condenser blower and circuit still overheats, so evap blower or A/C clutch is drawing too much amperage. Could there be a prob with the smuggler's box relay?

I think this is beyond my capability to troubleshoot this any further so next step may be the dealer.:(

Next step...........
 

unplug the wire going to the a/c clutch compressor. If you still melt fuses, the problem (almost) has to be in the evap. fan. In the process of being replace, the wrench could have done something to bind the fan or armature.

By the way, my multimeter will measure up to (I think) 25 amps. I got it at Lowe's or Home Depot from the electrical department. Or, you could go to your local auto parts store and buy a cheap ammeter gauge. They usually read up to about 30 amps. It would not be as accurate as a multimeter, but would be "close". For a little more $$$ you could buy a digital ammeter gauge and be very close!

I seem to have the overheating/melting problem only when running the evap fan at the highest speed.

Any chance something is amiss with the fan switch/resistor pack?

the evap motor looks to be very much like the footwell and the front condenser motors. I know that the footwell type motor pulls about 8 amps running in the fast speed. I would guess that you motor is on the way out. The bearings might be beginning to bind and causing the motor to labor. You might get of cheap by taking it apart and cleaning and oiling the bearings.

The problem almost has to be in the fuse block.. They don't catch fire to interrupt the circuit.

What gets hot first? Does the fuse bridge wire itself get hot? Which side of the terminals gets hot? Is that crimp on the wire corroded?

I know you don't have a multimeter but maybe you could put your left index finger on the upper screw and your right one on the bottom screw and see which one leaves a bigger burn mark.

I would bet a cheapo Radio Shack multimeter and some playing around learning what it does would be cheaper than driving by the dealer mechanics.

OK Rick, you're making me admit that I have a meter but don't know how to use it other than to do simple continuity tests. I don't find the insructions very useful.

Found some tutorial info on the web that I need to study.

The jumper on the top of the fuse block gets hot first. The fuses melt on both ends so its hot on both sides.

Confirmed that it heats up regardless of fan speed. When the compressor kicks in it starts to heat up.

Mark

Lets asssume the new fuse block is good. It seems that if the fuse is still getting hot and melting the fuse body at BOTH ends there is one of two things causing it.

1) Too much resistance between the fuse and fuse block posts.
or
2) Too much current thru the fuse.

If it is "1" (excessive resistance) there is a problem between the contact points at both ends resulting in excessive heat. Since we assume the new fuse block is good that leaves the metal of the fuse strip as suspect.

If it is "2" (excessive current) the fuse should "blow", but it doesn't. So if there is too much current (which must be checked and corrected if it is the case), the fuse should be blowing. So again the fuse is suspect.

So my question is - have you been using the same "brand" of fuses? And if so, my potentially over-simplified suggestion is to try another brand of fuse and see if it melts too.

Good luck
Andrew