Frying fuel pump fuse traced to O2 sensor

[Russ Maki]

Hi folks; I thought I'd pass a recent troubleshooting experience along, as I
didn't find much on this in the archives.

A few weeks ago my '87 4000q started mysteriously blowing fuel pump fuses.
Of course the problem cropped up at midnight while I was 40 miles from
home...I managed to limp back by cannibalizing my fuse box, swapping out
fuses for the radio, rear window defogger and seat heaters. The circuit was
frying 25-amp fuses in an eyeblink. so I knew something was very wrong.

I know I was engaging in risky behavior by driving, but I stopped and
checked a couple times to see if the pump was getting warm, and it wasn't.

The weird thing was: the circuit was fine as long as the car was moving --
75 mph on the highway was no problem.. As soon as I stopped, the car would
die, the result of a blown fuel pump fuse.

Digging into the Bentley, I learned that the fuel pump shares the circuit
with the oxygen sensor heater -- in fact, I believe it's the only other
thing on the circuit. I disconnected the heater and the problem went away.
Then I checked resistance in the O2 heater circuit and got a reading of
infinity. So I popped the sensor out and examined it.

I found that the installer had zip-tied the sensor wire bundle to a CV
shield that had worked itself loose. The vibrating shield  eventually wore
through the wires and produced a dead short in the 02 heater circuit.

A rewired 02 sensor heater circuit and a rerouting of the wires, and all is
as it should be once again. But I wish I'd known I could have reduced the
tension on that white-knucke trip home by simply unplugging the O2 sensor!

Cheers,
Russ Maki
Ixonia, Wis.