Fuel Pump Fuse Issue

Westy · 08-28-2015, 11:28 AM

The PO put a 10 amp spade-type fuse attached to the 10 amp Porsche fuse already there. Many years ago this added-on fuse blew. Then today, I was driving slowly on a backroad, doing a weave (20mph) and the fuse blew again. I learned and carry spares, but why? I can only guess the extra fuse was put in because they are easier and cheaper to replace. But if he knew they blow, then there's a reason. I looked at the wires, nothing appears to be out of place, but this fuse disintegrated and was hot. Something's going on, it's only when turning sharply, and it's only happened twice. I've replaced the fuel pump, and there are no wiring problems down there. Hmmmmmmmmm.......................................

donporfi · 08-28-2015, 11:50 AM

Turning the same way ? , someting might be loose and shorts with a live conductor.
Dirty contacts ?

Westy · 08-28-2015, 11:53 AM

Loose, bad, dirty contacts where? Behind the fuse panel? The PO would have had to remove the panel to wire in this fuse. I'd a thunk he could have seen a loose connection. I suppose I can pull the panel so I can look behind it.

jason2guy · 08-28-2015, 12:15 PM

what i did. moved the fuel pump fuse to a new location, installed a new fuel pump.
then to get rid of that problematic relay, i made a small harness that plugs in to the stock relay position, that harness terminates at a standard relay that uses the fuel pump 12v to trigger the new relay and takes power directly from the battery

Westy · 08-28-2015, 12:20 PM

And my response is,,,,,,,HUH? I'll bet that's what has been done, maybe sans relay. Only one way to find out!

scarceller · scarceller's Garage

Not sure but maybe the pump is old? As they age they tend to consume a bit more power. Maybe 10amps is borderline? If you have a DC current clamp you could test to see how much current it's drawing while at idle. You may discover your very near 10amps possibly more.

scarceller · scarceller's Garage

I just looked at the 84-89 schematic, those pumps are fused with 25Amp fuse. Is this 10A fuse that keeps blowing actually powering the pump or the relay coil? 10A fuse directly to the pump seems small to me?

Westy · 08-28-2015, 12:30 PM

Pump's brand new.

Westy · 08-28-2015, 12:32 PM

Quote:

Originally Posted by scarceller

I just looked at the 84-89 schematic, those pumps are fused with 25Amp fuse. Is this 10A fuse that keeps blowing actually powering the pump or the relay coil?

Can't answer that. I've never been in there. I would also guess Motronic is waaaay different than CIS. No??

scarceller · scarceller's Garage

Quote:

Originally Posted by Westy

Can't answer that. I've never been in there. I would also guess Motronic is waaaay different than CIS. No??

Motronic is different but the pumps are not, actually the CIS pumps work harder and at much higher pressures than Motronic pump. I'm just asking: are you sure the fuse is supposed to be 10A?

Westy · 08-28-2015, 12:51 PM

No, not sure at all. Pretty sure the 2nd owner, who owns Poboys in Oakland CA, is the one who did this. It was his car, and his business is European cars, specifically Porsche. I will admit fully, when it comes to wiring electronics, I'm a neophyte at best!! I'll get a picture of the panel and post it up.

scarceller · scarceller's Garage

Can anyone with a CIS car confirm the correct fuse size for the fuel pump?

pampadori · pampadori's Garage

as far as the fuse and wiring being hot to touch, mine is often hot too. It seems to be borderline for the gauge wire used even when new and then toss in corrosion on the connections and you only get more heat.
Mine isn't melt-the-fuse-holder hot yet, but it will let you know when you touch your forearm against it that it ain't ambient temp.

Westy · 08-28-2015, 01:53 PM

Hopefully these pics help

SpyderMike · 08-28-2015, 02:47 PM

My Haynes Manual shows a 25amp fuse in position 16 for the fuel pump on a 1981/1982.

allaircooled · allaircooled's Garage

On my car, the fuel pump wasn't protected by a fuse. That is an upgrade I plan on doing myself. Not sure when they started fusing the pump, but maybe that is why you have the inline fuse holder.

scarceller · scarceller's Garage

I think a 10A fuse may simply be to small. 10A X 16V = 160watts I think that pump needs more than that. Only way to know is to measure the current draw but the fact that the 84-89 use 25A is a good indication of what you may need. If indeed it never was even fused I'd go with 25A.

Westy · 08-28-2015, 05:08 PM

Wow! Fire scares me!!!! Would you run both the factory and jumper at 25, or just the jumper? Man that seems high!!!

Paulporsche · 08-28-2015, 05:49 PM

25amp for CIS.

MrScott · 05:54 PM

Maybe not the solution to your problem (or not the whole solution) but I had an intermittent no start problem on hot start. The wire going to the fuse was hot and the screw terminal and wire end had some corrosion. Corrosion -> Resistance -> Heat, no good. Undid the screw, cleaned the wire and terminal and it's been fine ever since.

Is the wire to the factory fuse holder darker near the holder and/or brittle? If so and it's not bad enough to need replacement or cutting I'd suggest cleaning off the corrosion with a wire brush or sand paper, followed up with contact cleaner. The wire 5th from the left looks iffy too.

I don't like that the wire's loose in there - no good can come from it moving around. If both fuses are in-line what's the point of having two? Seems like a bandaid over the real problem. Now he'll have to upgrade both fuses to 25A? That's a little silly.