Engine/Transmission (1998.5 - 2002) LP Voltage?

[nuv22]

I was wondering if the following voltages to the LP sound normal.



8. 53 when the ignition is on (truck off)

12. 33 when i "bump" the starter and the pump runs for a bit.

12. 37 at idle.



i am just trying to diagnose a problem with the new relocated LP in my truck. it will be fine then psi drops to 6 then i get on the gas and shoots back up to 12-13.

 

[nuv22]

wow no post? come on i know someone out there has to know this

 

[Fishin Guide]

Voltages sound correct. There is a reduced voltage to the pump under start conditions vs run.



My last lift pump did exactly what yours is doing. pressure would run to 5 psi at cruise and if I romped on it, the pressure would shoot back up to 13 after I let up. Then it woud be fine for miles or only a few seconds. The pedal romp fixed it.



It was another bad lift pump. I would think yours is the gone as well. I would recommed a relay on that pump so you get full voltage all the time as well.



Dave

 

[nuv22]

thanks for the info. does the relay in the fuse box even run anything ? or does the pump go directly in to the computer? also are they using pulse modulation on these pumps ? i would think that the motors wouldent like that.

 

[BK]

I agree with Dave, All the readings look fine.



Except that I would not add the relay. Very early on when some folks tried that during relocation, or when just adding a pushers, the higher voltage = high pressure duing start, produced some hard starting condition for some folks.



If I remember right they actually chop the voltage ~ 25% to produce a lower pressure during the starting cycle (7psi). . so if you use a relay, you may not get the relay to engage or you may have to high of a pressure experience the hard starting issue.

 

[Fishin Guide]

I would have to go back and trace the wiring path for the pump to answer where the power comes from or if it's even fused.



I don't think you would have an issue with a relay with a single stock pump. Many guys are unning Fass's and the like and making 20+ psi with the key on and no issues. But, I could be wrong.



Dave

 

[BK]

thanks for the info. does the relay in the fuse box even run anything ? or does the pump go directly in to the computer? also are they using pulse modulation on these pumps ? i would think that the motors wouldent like that.

They are chopping the dc... so yes it's a form of pulse modulation.

Just don't know if they are doing pulse width modulation.

I doubt it, it's most likely a fixed duty cycle for mod 2, not actively changing it.





The FSM wiring diagram shows no fusing, right from the ECM to the LP.

Pins 15 and 35 are in parallel on the ECM , they come together at the connector Pin 18 at the ECM connector, YL/WHT wire, into the harness and then feeding the + voltage to the pump.

 

[lsenus]

You may also want to take a look at your intank pickup. Some here have reported a thick film on the intank screen which seemed to be limiting fuel flow to the pickup.



One other member here had a problem with the alternator, sorry don't remember the post. But it seemed that the alternator was putting out dirty power to the system and affecting many electronic systems.

 

[BK]

Isenus...



That reminds me, also thought some reported a problem with a vent line/fitting on the tank... . ??



And your right Isenus. .

The ECM does do strange things in the dodges, both gas and diesel when the ripple is high, either from a bad alternator or weak / tired batteries.



Forgot about those two things.

 

[lsenus]

BK, Good memory about the tank vent. It seems that some trucks, probably made on Fridays after paychecks and lunch at a local tavern, had a nipple cover left on the tank vent on the top of the pickup module. As yo burn fuel, a vacuum gets created in the tank, causing the LP to have to draw not only the wieght of the fuel through the line, but also to attempt to collapse the sides of the tank.

 

[BK]

Thats it--- even better memory than me !