I know the Cummins computer supplies direct voltage to the lift pump. I notice a psi drop after a hour or so of steady running. If I shut it off and restart it the pressure is ok for a while. I am going to back probe it and run a voltage test when it does it again. Seems to me I read where others have reported a voltage drop. The supply system is stock and the pump only has 14,000 miles on it. I know its not the pump so please dont jump on this as a junk pump issue. I am pretty sure its a voltage drop. I wiggled the connector and no difference. A shut off and restart and its back to normal. I know low voltage is not good for any pump. Any one with ideas?

 

Bump.

 

Do you have a way to check how many amps the pump is drawing? Like a clamp on amp meter? I was working on a 99 ford powerstroke the other day and the fuel pump would work for a while and the longer it ran the truck would lose power and then just quit. I checked the amperage draw and it was about 10-11 amps which I didnt think was too much at first but I didn't have anything to compare it to. Then I did and signal check with an amperage probe and an oscilloscope. The signal looked bad so we replaced the pump based on that. After putting the new pump in, we found that it only drew 6-7 amps. That might not seem like much but it was enough to get it hot to the point that it didn't work. I realize the difference here is that your pump isn't old but you can't beat hard facts. Sorry for the long post. Just thought it would help.

 

Thanks for the reply. Reason I believe it is a voltage drop from the ecm is that if I do a restart the pump pressure immediately changes. When I did check the voltage it was close to battery voltage but the pressures were up in the normal 12 to 16 range. It does this at random and it always seems its after a longer run time. There is no rhyme or reason.

 

The ECM monitors battery temp and controls alternator output voltaged based on that. The battery could be getting hot and the ECM is droping the voltage. I see a .5V difference after about an hour or so of driving, once the battery warms up.

 

I understand the ecm and temperature. I still feel I get a 40 percent drop. One of these days I will catch it acting up and without shutting it off I will back probe the connector and verify voltage and ground.
I dont drive long distances so its hard to say when it will act up again. I did verify the pressure readings last summer when it did it the first time. It has done this maybe 10 times since mid summer.

 

I put a solenoid in line with mine so the ECM turns that on and off...the full current draw from the LP goes through the solenoid and the ECM is the trigger. I see no appreciable drop now.

 

I was thinking of going with a relay and wiring it like you have. I was just curious if any others have seen a voltage drop.

 

Relay!!! I was trying to come up with that word. Now I feel like an idiot! I have not measured a voltage drop because I never hooked a vtvm to it...I just always thought it was weird that my pressure would drop so I tried the relay. The only issue is that you don't get the partial pressure when starting. It has created harder starts, but that could be because my VP has 135,XXX miles on it.

 

I relocated my fuel pump to the frame on my truck and installed a relay from vulcan to fire it off. I was thinking you could do that too but if the voltage drops too much it might open the relay and not work at all. When I have experienced intermittent voltage drops when working on anything, it is usually a ground. Thats where I would look first. Then maybe a vulcan relay/harness for good measure.Just my $0.02.