Fuel pressure sensor

[scottwvyc]

I discovered a fuel leak on my strbd side 671n today. It is leaking from some kind of a sensor monted on the secondary fuel filter housing. The sensor has 2 wires running from it, is this a sensor for some kind of low fuel pressure alarm? Thanks Scott

 

[MikeP]

Could be an alarm, could be for a remote gauge, could be to activate engine hobbs meter...hard to tell.

Probably the only way to find out for sure is to trace the wires. If it was installed by a PO, it's quite possible the wires go nowhere or to some very strange destination!

 

[scottwvyc]

The mystery deepens, I dug out my Hatteras manual and found a wiring diagram showing the switch. I seems it is wired in with alternator and battery charger. I am perplexed?

 

[doc g]

Hobbs meter as previously stated.........Pat

 

[scottwvyc]

Please excuse my ignorance, but what is a Hobbs meter?

 

[newinlet]

that should be a meter to show engine hours. My 671N does not have that set up.

 

[MikeP]

Yes, the Hobbs meter shows the eng hours. Normally they are set up so the "clock" doesn't run unless there is fuel pressure. Otherwise, it would run anytime the ignition switch is on.

 

[Genesis]

Yep - that switch goes to your engine hour meter. If you short it you should see the hour meter tick over.

You can buy a suitable replacement at NAPA; a low oil pressure switch works just fine in there.

 

[Canuck Dennis]

If that switch does not work the alternator will not flash, no charging.
D

 

[REBrueckner]

Canuck,
Yes, on my 1972 set up, there are quite a few wires to that fuel pressure switch...one pair IS energyizing either an alternator or generator...that pressure switch acts as a local wiring point to serve as a sort of "ignition switch" as others posted above.....I discovered the set up when the old generators concked out and I added Powerline high output alternators....I never did trace all the other pairs....

When the switch opens (engine off) it prevents the alternator field from draining your battery...like shutting "off" the ignition switch in your car....

 

[captddis]

Yep - that switch goes to your engine hour meter. If you short it you should see the hour meter tick over.

You can buy a suitable replacement at NAPA; a low oil pressure switch works just fine in there.

Napa has the switch, but it is not a low oil press switch as that is NC. You want a NO switch. Sometimes called a 'make' switch.

 

[Brian Degulis]

Here's a link to Stewart Warner / Hobs Lauderdale speedometer is a dealer and will ship. The old switch should be marked for pressure and normaly open or normally closed. They do make 3 terminal replacments that work both ways.


http://www.stewartwarner.com/products/prod1.html
Brian

 

[Fanfare]

When fuel pressure comes up it shuts off your battery charger so the alternator can take over. They fail and leak a lot. DD has a replacement with two sets of switches, one normally open, the other normally closed. I now carry a spare.

 

[Genesis]

Guys, don't overpay for these. I KNOW Napa has 'em because I've bought 'em there before. Just make sure its NO.

 

[captddis]

NAPA # 701 1575 NO 4psi.

 

[scottwvyc]

I found the switch at Napa thanks guys, $42.79 up here in Canada. But just to sure I got a 4 psi switch that closes the circuit when there is fuel pressure,is this the right one? I am a little mixed as to which is NC & NO.

 

[Genesis]

Pressure doesn't matter; 4psi is fine. There either is pressure in that system (~60psi) or there is not (engine off.)

Normally open, closes on pressure.

 

[Nonchalant1]

I found the switch at Napa thanks guys, $42.79 up here in Canada. But just to sure I got a 4 psi switch that closes the circuit when there is fuel pressure,is this the right one? I am a little mixed as to which is NC & NO.

NO = Normally open (will not pass current normally)
NC = Normally closed (will pass current normally)

A NO switch means as you bought it from NAPA will not pass current. When fuel pressure is there, it closes and will pass current. WHen the engine is off and there is no fuel pressure, it won't pass current. Be careful to mark which wires went on one side vs. the other.

Doug

 

[SKYCHENEY]

Be careful to mark which wires went on one side vs. the other.
Doug

It's just a switch. Why would it matter which wire you put on which side?

 

[captddis]

Some switches have multiple leads, you could get a switched lead connected to the hot side.