Hi all,
My 25 amp fuse for the under car pump keeps blowing. Does this indicate the relay is bad or is something else wrong?
Thanks for the advice!

 

Since you didn't say specifically, I will assume you have a 740/940 since it's a 25 amp fuse.
The most common reason for a fuse that big blowing is a fuel pump about to die. It binds and pulls more amps. A bad connection can occasionally produce the same result. Does it blow instantly or over time? Generally fuel pump relays don't cause the fuse to blow although in theory it can. There's a set of points inside the fpr that open and shut. They could carbon up and cause a poor connection, increase resistance and cause an increase in amp draw. Before I started buying new parts, I'd 1. Check every inch of wiring that I could see, looking for insulation breaks and corroded connections 2. Drag out the trusty digital multi-meter and start investigating.

 

Thanks for your fast reply. Yes, it's a 1985 740 turbo wagon. The fuse blows instantly. Initially the fuse went while I was going down the interstate at 60mph. When I got it home I replaced both fuel pumps, still the fuse blows as soon as I turn the car on.

I appreciate the assistance, thank you!

 

OK...a couple of things come to mind. First, your 85 is withing the period of biodegradable wire harnesses. The heat from turbo cars makes it just that much worse. All wiring outside of the cabin is subject to disintegrating insulation inside of the black vinyl sheath that several wires reside within. So...could easily be a short. Second, it's rather simple to see if the relay is the issue. Pull it and make a short jumper wire out of 12 or 14 gauge wire. There are two parallel rows of three contacts where it plugs into the fuse panel. Jumper from the lower left to the middle on the right. This will make your fuel pumps run non-stop, whether the key is on or not. If the fuse doesn't blow, then it could certainly be the relay.