97 D-90 SW blowing fuel pump fuses intermittantly

[BarryO]

Hello all,

First Post and I hope you can help.

I had a problem this past summer that's recently re-surfaced. The truck would die while I was driving it. I'd check the fuel pump fuse and find it blown. If I replaced the fuse, it would blow immediately when the ignition is turned on. I could keep going through fuses; each would blow immediately. However, if I waited a while, I could replace the fuse, and everything would work fine, until the next occurance (typically in a few days).

I replaced the fuel filter and the problem went away, although it was never clear to me why this was the source of the problem.

Anyway, the problem has re-appeared. I plan to pick up a new fuel filter today, though I don't understand how this would cause a problem wherein the fuses would blow immediately once the condition arises, but the condition goes away once the truck sits for a few hours. Also, six months seems very quick for a fuel filter to go bad.

If this wasn't an intermittant problem it would be easy to diagnose, but by the time I tow the truck home, the problem goes away.

[rover4x4]

Could it be related the the fuel pump relay?
get a bigger fuse:eek:

[Mike Hippert]

Sounds like an intermittent short. Pull the fuse, measure the resistance from the output side of the fuse holder to ground. Keep your meter with you, when the fuse blows again measure the resistance again. If it is considerably lower I would guess that you have a short in the wiring. BTW the output side of the fuse holder is the side that does not have 12VDC on it when the ignition is on and the fuse is out. This all assumes you have a meter. If you don't you can pick one up at Sears for under $30, you will use it allot!

[BarryO]

Does the fuel pump fuse feed power to anything else?

OK it happened again. 'left it sitting by the side of the road for a few hours; no good (still blowing fuses). When I went back the next day, it started just fine and I drove it home.

I'm afraid to dirve it now until I look at it, in case I get stuck in a really bad location (e.g., a road with no shoulders). 'must be an electrical thing to blow the fuses immediately.

I'll try to trace the wiring next.

[milli]

Barry,
If you play any golf at Quail Valley, say hi to Doug Hixson (HP) for me. Thanks!

[Mike Hippert]

Fuel pump fuse should only power the fuel pump. I agree with tracing the wires, it is the easiest thing to fix.

[dante18]

Barry check the wire connections on top of your fuel pump. Sometimes the epoxy that holds them to the pump gets cruddy and the lose a good connection. Just a thought.

Dante

[BarryO]

Originally posted by dante18
Barry check the wire connections on top of your fuel pump. Sometimes the epoxy that holds them to the pump gets cruddy and the lose a good connection. Just a thought.

Dante

Do I have to drop the tank to check this? 'never worked on this area of the truck before.

[dante18]

I am not sure on the 97's. Can anyone else help us out here. I seem to remember somebody saying something about a check plate under the rubber mat in the rear of the truck?? Does this sound correct to anyone out there? I also agree with Mike, if you do not have a meter buy one they are indispensable when you own a British auto.

Dante

[BarryO]

FINALLY!!! Solved this one.

A mechanic over on the RN board told me that the fuel pump fuse also powers the O2 sensor heaters. Wrong! I got the wiring diagrams from Alldata: The O2 sensor heater fuse also powers the coil of the fuel pump relay, but the pump itself is powered by the fuel pump fuse, which powers nothing else: battery -> fuse -> fuel pump relay contacts -> internia switch -> pump.

The relay module and inertia switch looked OK, so I resigned myself to dropping the tank. When I started doing this, I noticed the wirings to the pump had bee pinched between the gas tank and the chassis. The dealer had replaced the pump a few years ago. So rather than totally disconnecting the tank, I just dropped it enough to unplug the wiring from the pump: thanks alot to member dmarchand for the suggestion to remove the left tire; this made this possible to do.

So when the dealer put in a new pump, they attached a new connector at the same time. 'don't know why. The covered each splice with heat-shrink tubing, but the splice going to the pump power lead is the one that got pinched. There was a nice little hole worn on the top of the tubing on that splice: and a little weld mark on the chassis where it would short out every once and a while.

'can't tell you how happy I am to conclusively trace this down. This is supposed to be my daily driver, but I was becoming afraid to drive it as it would die unexpectly.