Scottn3n

The fuel pump fuse & connections for it in my '78 get hot. So hot that it melted the plastic where the fuse sits & I had to replace the fuse panel. The fuse does not blow & it is the correct 25 amp fuse with copper (rather than the aluminum). To me the fuse should blow before this much heat builds up. I unplugged all of the connections and cleaned them on the new (used) panel before installation. I also found that the secondary fuel pump was bad & replaced it (why 2 fuel pumps anyhow?). Bad fuel pump - higher load. Now both fuel pumps work fine, car runs fine (ran fine on 1 pump btw). However, the fuse still gets quite warm but takes longer. It's much better, but something isn't right & I don't want to drive for long periods for fear that it may get too hot again. I have not been able to find any bad connections or corrosion near the fuse to cause heat. Any suggestions would be appreciated. I've re cleaned the fuse connections - didn't help. New (used) fuse panel looks fine. Thank you.

GlenL

Clean the connections to the fuse and the fuse. It's getting hot because the corrosion on the tangs and fuse are creating resistance to the ends of the fuse. To avoid this, spin each fuse around a time or two. That'll wear away corrosion and allow good copper-to-copper connection.

MainePorsche

A little Deoxit 100 gingerly applied maybe helpful to your connections too.

Mrmerlin

I would suggest to use the Deoxit 100 spray on all of the fuses,

NOTE this is critical to a good connection,
then spray the rear side of the CE panel where the wires plug into the fuse pins,
these wire connectors on the back can corrode and cause the fuse holder to melt, as well as the fuse itself being corroded and melting

MainePorsche

I like the Deoxit 100 eye dropper, and do (did) front and back of CE panel at all critical points. The spray I think wastes a good volume of the material, and the 100% Deoxit is not cheap.

Mrmerlin

The spray will work better as it shoots into the connectors VS a drop from the needle ,
BTW the Deoxit 100 has a small discharge per push of the spray head,
so only a small amount comes out for every push of the spray head,
so its not wasted, your right it is expensive

Alan

When fuses get hot it is almost never due to higher than normal current. In fact it usually involves lower than normal current due to high contact resistance. Everybody seems to find this very counter intuitive - but its actually very obvious if you think about it...

Fuses melt their encapsulation because their end terminals get hot (not the fuse element - that will quickly blow if it needs to with minimal terminal heating). Terminals get hot because of local heating caused by contact resistance build up between the fuse terminal and the fuse-holder contact. Resistance there causes voltage drop across the resistance that dissipates power as heat.

Example - Say you have a 20A circuit, by V=IR that is about a 0.65 ohm nominal circuit resistance. If you should add just 0.1 ohms of series resistance in the fuse connection through corrosion build up the circuit current will go down to ~17.3A but now you will have ~1.73 volts dropped across the contact resistance that will dissipate 1.73*17.3= ~30W of power in a minute area...

Clean and re-tension all the terminals - and get new high quality fuses.

Alan

jpitman2

I had my pump relay fail on the road, and as I was jumpering the socket I noticed a spark on a fuse out of the corner of my eye - pump fuse! This was second relay to die. There followed much cleaning of fuses and their connectors, and even some making of adaptors to allow use of blade fuses on the high current fuses. If you examine your old fuse panel you will see that the top and bottom fuse receptacles push on to blades coming through from the rear - it might be worth exercising each of these to ensure their contact is good. FWIW, when my fuses and relay were all good I saw ~7A across the pump fuse.
jp 83 Euro S AT 56k

MainePorsche

Absolutely.
It is called the CENTRAL electric panel for a reason. ALL things pass its gates either directly or indirectly.
Take the time. Take it out. Good cleaning/servicing goes along way in this car.

Alan

Actually there are circuits that bypass the CE panel or connect to it in odd ways... e.g. fully bypassing: ABS hydraulics, telephone system, PSD. Direct battery connections through CE (unfused or fused in the wrong place) include: each final stage cooling fan, LH Relay/LH/Fuel Pump/EZK, EZK/ABS Relays.

Individual years have other oddities - central locking fuses, fuse on the radio body, rear hatch fuses on early models etc...

Alan

MainePorsche

Alan,
Just a few things.
Hah ! on me.