Fuse 4 rating

Ron-one · 12-06-2009, 02:22 PM

Hi
Whilst looking over the fuse boxes this weekend I found that I have a 40 amp fuse in Fuse 4 (Rear Window heater) of the auxilliary box.

All the info that I have been able to find indicates a 25 amp rating for this fuse, however a 25 amp fuse overheated & failed within minutes of the heater being switched on.

The car ('88 924S) has run without problems for the last 5 years on the 40 amp fuse.

So, is the 40 amp rating correct or do I have an accident waiting to happen?

Your thoughts would be much appreciated.

mikepellegrini · mikepellegrini's Garage

You have an accident waiting to happen. The circuit is rated at 25 amps. If it blows 25 amp fuses, then you have problems. Going to a bigger fuse will quite possibly produce melted wiring...

13.0 Electrical

jpk · 04:48 PM

I'm looking at the wiring diagram for the '84 (I don't have the 924S diagrams but I'm 99.9% sure they're the same) The correct rating for fuse #4 should be 25 amps.The only thing that fuse #4 feeds is the rear defog through the relay.
The wire in the circuit up to the connection through the hatch struts is indicated as 4mm area, which translates to about 2.25mm diameter and slots in just slightly smaller than 11 gauge. 11 gauge wire can sustain 47 amps, so you're OK there.
After the hatch struts, (between the tab on the strut and the connection on the glass grid) the wire goes down to 2.5mm area, which is a bit bigger than 14 gauge. 14 gauge is rated for 32 amps. That's just a short jumper and it's out in the open, so even if it went inferno, I don't think it would cause any problems. Just to be safe, you should switch the defog off as soon as the window has cleared.
The question remains - why is the circuit drawing more than the 25 amps it was designed for? Did a PO tap that wire to run a high-power stereo? Do all of the lines in the defog work or only some of them?

Ron-one · 12-07-2009, 02:57 PM

Thanks for your responses.

Talking to my son, who runs the car, he says that he uses the heater/defogger pretty frequently at this time of the year & apart from being a little slow (compared to a modern) it appears to be ok with all wires working.

Regarding the stereo although the head unit is not standard fitting (presumably Blaupunkt?) it seems pretty bog standard stuff & was fitted by the dealer when the car was purchased. Indeed we are in the process of changing the speakers which appear to OE.

The fuses appear to me to be a pretty grey area. In response to my last query (Dashlights /Fuse 9) Mike attached a pic of his auxilliary fuse box, the fuses left in place didn't seem to correspond to the list in the 13.0 Electrical link. This list didn't tie in with the list in the Porsche handbook that came with our car & the fuses that we had in place were different again.

Makes it difficult to determine Porsche upgrades from PO work.

Many thanks, any further thoughts would be great. I won't see the car for another 2 weeks so will look at it again then.

Thanks again

Ron