2nd Gen Non-Engine/Transmission Why Fuel # increase when lights on???

[Joe Mc]

I always run with my driving lamps on and the fuel pressure's alway been 20 +/- a pound - regardless of throttle and comp settings.



Anyway, the other day I noticed FP was down to about 18#, wondered what the heck? A while later, forgetting about the FP, I noticed I forgot to turn on the driving lights, then I noticed the FP go up to 20#



After getting home I tried lights off 18#, parking lamps (or any combo of lights) on 20#???



Westach gages, FP gage is a 0 - 30 and matching Westach sending unit.



Any thoughts?

 

[davemac]

Joe,

I`ve got the same situation. Westach gauge (but only 16#) & sending unit. I turn on the headlights & the FP increases by 1#.

Hit the brakes, brake lights go on, FP goes up 1/2 #.

Turn on off road lights & FP goes up 1#. I have asked about this before but never gotten a satisfactory answer. I`ve learned to live with it. As long as the FP stays above 10# I`m happy.

 

[Gary - K7GLD]

I would suspect the alternator control operates on demand - at lower demand, the charging rate and system voltage is maybe a bit lower than when more accessories are operated - and the regulator increases alternator output to compensate - and then the fuel pump gets a bit more voltage and increases system PSI...

 

[Joe Mc]

Gary - That makes sense but, shouldn't FP be "constant"? I guess as long as it's not lower I'm fine - thanks.

 

[WestTN]

Ground the sensor and the meter to one point only. This will avoid small stray current flow caused by currents from other grounded component elsewhere in the truck.

 

[JKaeseberg]

Voltage Drop

Two thing would cause the change, the pressure changing or the voltage changing. I believe your getting a voltage drop to your guage. The power for the guages in the dash are fed by a voltage regulator that keeps the voltage constant. When you turn on different electrical accessories the voltage at your guage changes, giving a different reading. Take a voltage reading at your guage with a digital volt meter to verify. Hope this helps.

 

[Moose Truck]

Ground the sensor and the meter to one point only. This will avoid small stray current flow caused by currents from other grounded component elsewhere in the truck.

That's It!! The Gage's ground wire comes out to one side of the sender and from there it goes to ground. Mine is grounded to a stud under the hood. The other side of the sender goes to the pin (# 5??) on the back of the gage. Moose

 

[DieselDoghouse]

Ditto on the ground issue. No Mr. Spock jokes about photons repelling fuel, etc.

Somehow there's an offset voltage being generated by light activity. Its a shame that a digital-based gauge would be fooled by input voltage variation but—maybe add a voltage regulator to the input voltage (like a LM317 IC regulator) so 12V is REALLY, ALWAYS 12V?

 

[WestTN]

Well, these are actually analog gauges. They are reading a bridge circuit in the sensor. The actual signal out is a small percentage of the bridge current. The error offset from the combination of the bridge balance current and the stray ground current becomes fairly substantial.

 

[DieselDoghouse]

WOW! even more reason to stabilize the input voltage and secure all grounds!!

 

[Joe Mc]

cool have to give it a try once I get the rear diff back together - doing new hub bearings and races. . . hope this weekend the wifey mobile is driving me nuts - 01 Volvo XC-70, it's like driving a couch . . .