Engine/Transmission (1998.5 - 2002) lift pump voltage?

[PSKelly]

installed my 3rd stock lift pump and i am still getting low fuel pressure. I decided to check voltage at the pump with the key turned on and i am only getting 8. 84 volts. I should mention that i am in colorado and it is very cold out so i don't know if that voltage is common with all the heaters and stuff that cycles when it is cold. coild this low voltage be the reason i'm not getting my good fuel pressure? I have installed a low fuel pressure light that comes on @ 5psi and when i step on it it will turn the light on. right know i am seriosly considering a FASS system or the GDP pump. any suggestions?

 

[PSKelly]

just got back into my pickup and my check ingine light was on. I thought I would tell you guys that while I was diagnosing the low voltage of the lift pump I removed a couple relays under the hood trying to find the one that provided power for the lift pump. After removing several relays that I thought would have cut the power to the lift pump, I never did find one that would. I am pretty sure this is why I am now getting a few codes on my odometer. Does anyone know how to clear the codes? My check engine light finally did go out but the codes are still there.

 

[DCreed]

most any parts store can read/clear codes these days.

 

[OPoole]

The FASS is a great system to look into with a 6yr warr.

 

[PSKelly]

I totally agree and am going to install one after christmas,but I believe the FASS plugs into the original wire harness and if i'm not getting the proper voltage for the stock pump. I hate to send only 8. 5 volts to a FASS system and expect it to work 100 percent. Does the FASS plug into the original lift pump harness? Has anyone ever checked their voltage going to their lift pump before determining that the pump was actually bad? It just seems that after installing my 3rd pump in a year and the 2nd and 3rd pumps within the last 2 weeks that their might be an underlying problem. Do any of you guys have any suggestions as to what else might be the problem?

 

[jhoughton]

The FASS is relay controlled using the original LP wires. However the actual voltage comes strait off the batteries. How old are your batteries? When mine were getting old and it was winter here with temps down to 15 deg at lowest the relays for the grid heater will hold your voltage down around 8. 5-9 volts until things warm up. I would get the check gauges light each morning. With good strong batteries the effect is less. I too have a FASS and it holds a steady 16psi.

 

[gtwitch]

lift pump voltage

4 months ago I put a Holley Blue on the frame with a screw on cart filter between the pump and the on engine (stock) filter, I can hear the pump running and when the engine is still cool and/or under no load the pump will change speed, I believe the ecu is controling the voltage to the pump as the bypass from the VP44 does its thing. I have been meaning to put a scope on the power wires to the pump whhich are comming from the origional on engine pump location and I did run #12 wires so there would be no voltage drop. I want to look at both the voltage and/or wavefrom as I suspect the ECU may be controlling by some sort of duty-cycle or PCM mode. By the way the fuel pressure at the engine fuel filter output (98. 5-99) is never below 12 at WOT and most of the time is 14 psi.



gtwitch in Wyoming

 

[MAV1]

testing lift pump voltage

PSKelly,

Unable to tell how you tested your lift pump for voltage so will offer this:
In stock form, i. e. no relay harness added, the ecm does control and determine how much voltage goes to the lift pump. Assuming there is proper ground for the lift pump, often there isn't, and that the driver in the ecm for the lift pump isn't damaged, there should be battery voltage to the lift pump, except for when cranking the engine. When cranking the engine, the ecm clips the voltage back to around 6-8 volts until engine start is obtained. To properly test the lift pump for volts available to it, warm the engine to normal operating temperature and with engine running, measure voltage at the red wire of the lift pump harness connector. It should match battery voltage.

The FASS comes with a relay harness, which plugs into the oe lift pump harness and in turn plugs into the FASS. The rest of the FASS relay harness consists of a relay and fuse, and leads that connect to the respective posts of one of the batteries.

You are on the right track in attempting to diagnose low lift pump fuel pressure. If after determining there is proper voltage to the lift pump, there is adequate ground and there is no inlet restriction from the fuel tank, the lift pump is bad. After going thru 7-8 Carter lift pumps, I did this;
https://www.turbodieselregister.com.../191376-finally-some-one-puts-end-carter.html

Hope this helps and good luck,
MAV1

 

[RJWEST]

If the grid heaters are cycling the holly blue will also change speed with voltage, At least mine did... ( but I had mine hooked to a relay controled by the OEM LP input wire...

Also There is a bypass in the pump, Mine was making lot's
of noise when it was bypassing, Installed the Bypass regulator
and cranked up the Pump Bypass to were it stays closed.

Note, My Holly Blue is running, but I know it is not Healthy,
as it experienced a pressure drop over time... .

 

[PSKelly]

Thanks for the advice MAV1. I checked the votage with the key turned on and the pump unplugged. I probed the two sockets and got something like 8. 8 volts and then I checked the hot wire on the OE harness to the ground on the battery and got the same thing. I then checked the resistance between that ground on the harness and the battery and got about 2. 5 - 3 ohms( I thought that would be a little better than that). Thats a good idea about checking it with the engine running after it has warmed up. I'll have to rig something up to check that tomorrow. The FASS system sounds like it uses direct fused current from the batteries and does not rely on the OE harness for the voltage supply. Is that right?



thanks

pskelly

 

[MAV1]

That's correct. The voltage to the lift pump is whatever battery voltage is, using the FASS harness, or any other relay harness such as the one available from Vulcan Performance Home Page, do a search for harness. That is where I obtained the fuel pump relay currently on my truck. Best 50 bucks I ever spent. No more ground, ecm driver, voltage problems! FWIW, to date I have seen 3 Carter lift pumps bad right out of the box, one would not pump at all and the other two delivered low pressure. One as low as 8lbs. max pressure.

2. 5-3 ohms is very good. That is very little current resistance, especially over that long of a wire run. 8. 8 volts engine off, key on is very low, even if the grid heater is on. My Juice with A2, shows around 11. 5 volts engine off, key on, grid heater on. As jhoughton pointed out, I wonder if your batteries are playing out. Would be worth while to have them tested.

MAV1

 

[PSKelly]

yes my truck is a 2001 and i'm still running on the stock batteries. thought that might be sneaking up on me soon also. the more i think about it i think i should just replace the the pump with the FASS as soon as possible. going to check the voltage while the motor is running tomorrow and while at it i'll have the batteries checked also.



thanks



pskelly