Incinerated #2 Fuse for A/C

[SCrescue]

Thought I'd post this for anyone who's unaware of the potential for this to occur. I've been reading other threads discussing this issue and figured it will help to make others aware. Apparently this is a result of installing plastic vs ceramic fuses, and is more apt to occur with higher amperage value circuits that generate more heat. Some have expressed additional issues may be related to defective fuse blocks?

Mike...

[gsxrken]

Wow. There is resistance somewhere.
Make sure your front condensor fan is fused and check that your power seats are not blowing or nearly blowing that fuse. You can reach underneath the seat and disconnect the harness plug to diagnose.

[rick-l]

Looks like the point contact of the fuse had some corrosion on it and got real hot. See if you can clean it and get it to work again. Notice the fuse didn't blow the plastic body just melted. They used to be made of ceramic.

[SCrescue]

Originally Posted by gsxrken

Wow. There is resistance somewhere.
Make sure your front condensor fan is fused and check that your power seats are not blowing or nearly blowing that fuse. You can reach underneath the seat and disconnect the harness plug to diagnose.

Most likely the front condenser fan is the point of resistance. The motor is shot and needs to be replaced. I don't have a compressor installed in the system since it doesn't clear the passenger side carburetor, but I do like to run the evaporator blower for ventilation. Btw, my car is not equipped with power seats.

[rick-l]

Originally Posted by SCrescue

Most likely the front condenser fan is the point of resistance. The motor is shot and needs to be replaced. I don't have a compressor installed in the system since it doesn't clear the passenger side carburetor, but I do like to run the evaporator blower for ventilation. Btw, my car is not equipped with power seats.

Just an electrical point, higher than normal resistance will not blow a fuse. I = V/R

Note that the fuse did not blow it got hot from ohmic heating. P=I^2R The corrosion on the fuse contact provided the R.

[donagain1]

Good thing the plastic fusebox cover was in place, what a lousy thing to happen if it was off and the felt carpeting was sitting on the fuse.

[rick-l]

+1 It actually looks like it caught on fire.

[SCrescue]

Originally Posted by donagain1

Good thing the plastic fusebox cover was in place, what a lousy thing to happen if it was off and the felt carpeting was sitting on the fuse.

My thoughts exactly, it could've been a very bad day.

[onewhippedpuppy]

Someone making a plug and play blade style fuse box for the 911 could probably do quite well for themselves. Tempting idea.......

[donagain1]

Isn't the fusable link of a blade style within the plastic. I'd think that there might be enough O2 inside the cavity to support a flame flash.

[T77911S]

Originally Posted by gsxrken

Wow. There is resistance somewhere.
Make sure your front condensor fan is fused and check that your power seats are not blowing or nearly blowing that fuse. You can reach underneath the seat and disconnect the harness plug to diagnose.

the resistance IS the fuse/contact of fuse to fuse block.

resistance=heat

when you have a high current circuit and the fuse/fuse block has resistance, more heat is created, along with a voltage drop.

what i did on my fuse block was to remove each fuse, clean the ends with a fine sandpaper (you could even replace them) and clean the contacts on the block. then i removed each wire to the fuse blocked and cleaned the wire ends. then i cleaned the screw that holds the wire in place. if there was someway to clean the hole the wire slides in, i would.
this should reduce the resitance at the fuse block and help to cut down on the heat.

my fuse blcok was gettting rather warm, mainly due to the fuel pump.
another hi current load would be anything with fan motors or the headlight circuit.


some of those wire ends in that pic dont look too good.

[wwest]

Originally Posted by T77911S

the resistance IS the fuse/contact of fuse to fuse block.

resistance=heat

when you have a high current circuit and the fuse/fuse block has resistance, more heat is created, along with a voltage drop.

what i did on my fuse block was to remove each fuse, clean the ends with a fine sandpaper (you could even replace them) and clean the contacts on the block. then i removed each wire to the fuse blocked and cleaned the wire ends. then i cleaned the screw that holds the wire in place. if there was someway to clean the hole the wire slides in, i would.
this should reduce the resitance at the fuse block and help to cut down on the heat.

my fuse blcok was gettting rather warm, mainly due to the fuel pump.
another hi current load would be anything with fan motors or the headlight circuit.


some of those wire ends in that pic dont look too good.

I have found it not at all unusual for the spring ends holding the fuses in place AND providing the electrical connection to lose their "springiness" over time. My first experience was with the fuel pump fuse.

There are 2 issues that need to be brought up.

1.) the HIGH startup surge currents, often substantually exceeding the actual fuse rating of some devices, fuel pump, etc.

2.) "Switch off" of inductive devices result in a HIGH voltage spike, likely the primary cause of the failure A/C control thermstat contacts.

[SCrescue]

some of those wire ends in that pic dont look too good.

Much of the panel wiring had been hacked into by a previous owner for a high powered stereo system...

[SCrescue]

Originally Posted by onewhippedpuppy

Someone making a plug and play blade style fuse box for the 911 could probably do quite well for themselves. Tempting idea.......

Already being done, search for threads by Fred Cook...

[fred cook]

Your front condenser fan and the evaporator fan are probably served via the same fuse. I possible, move one to a separate fuse even if you have to add a separate fuse holder. The two blower motors together pull between 20 and 25 amps IF everything is in good working order. If the front condenser fan has gone bad, that could explain the zapped fuse!

[fred cook]

ATO/ATC style fuse pane for 1974-1989 911s and 1976-1989 930s.

Originally Posted by onewhippedpuppy

Someone making a plug and play blade style fuse box for the 911 could probably do quite well for themselves. Tempting idea.......

[theweaver]

Hi, This scenario is typical of this type of fuse carrier. Fiat cars also used them, and often headlights would go out as the car was being driven.

You were very lucky this time, as surely this could have turned into a real disaster.
I have recently changed my fuseboard for the same reason, and is very similar to the one posted. He had done an excellent job.

Fuse carriers can be purchased very cheaply, I suggest that you take many photos, and print them out so that if you change your fuseboxes then mistakes are unlikely to happen.

However, one point of caution...the original fuseboxes are linked behind the carrier. You will see this when you turn it over, do not forget to do this should you upgrade yours. The total job can be done for about £25 max.

Good luck!

[onewhippedpuppy]

Originally Posted by fred cook

ATO/ATC style fuse pane for 1974-1989 911s and 1976-1989 930s.

Fred, you just added a new project to my list. I'll contact you after I get the basic stuff on my '88 done.

[T77911S]

if you have hi current devices you can use relays with an inline fuse to the switched side of the relay. its best to locate fuses close to the power source, battery. that way if wires get cut and short to ground, the fuse protects more of the wiring.

the blade fuse panel is nice, but i am not sure if i want to take away from what porsche did, even though it dose have its short comings.

yes, a week spring could also cause resistance. that will usually show up more as an intermittant issue.

[fred cook]

Originally Posted by T77911S

if you have hi current devices you can use relays with an inline fuse to the switched side of the relay. its best to locate fuses close to the power source, battery. that way if wires get cut and short to ground, the fuse protects more of the wiring.

the blade fuse panel is nice, but i am not sure if i want to take away from what porsche did, even though it dose have its short comings.

yes, a week spring could also cause resistance. that will usually show up more as an intermittant issue.

A very good point or two! Relays can help take some of the load off the fuse panel and switches. So, after some requests, this is what I offer to those who want headlight relays. Picture take with a cell phone, please excuse the fuzz! The relays are a high quality unit made in either the USA or in Germany and come from my local NAPA store. By using a socket for their wiring, should a replacement be needed later it is simply a matter of unplugging, remove one screw, replace the relay and plug the socket back in place. No worries about which wire go where! The board comes with the relays (mostly) prewired. Attach one ground, provide a lead to the battery and connect the headlight low and highbeam leads and you're done!