Repeat Cummins 5.9 electric lift pump failures

To the OP: Was the problem ever sussed out? I'm just curious what the issue was.

The end of that sentence should have read "to see if the pump failed then". As to the fuse and the relay I forget sometimes that all of us on here don't have the same skills, but yes of course use a fuse and a relay for the test.
Thanks for mentioning it.

bucky wrote:
I'd be tempted to run the next pump straight from the battery to take the coach's electrics out of the equation.

Assume you will use a fuse at the battery and a relay with an "ignition hot" to close the relay.

I'd be tempted to run the next pump straight from the battery to take the coach's electrics out of the equation. The alts on that generation of Cummins are known for AC leakage into the 12V system.
Also, just because a part is new doesn't make it good. Rare, but a whole batch could be bad. Source one from another brand if you have to. Mopar's favorite phrase in trouble shooting is "substitute a known good part".

55watt lamp is a resistive load DC or AC volts it won't care
and it will reduce the current to lamps load

try the amp meter on the new lift pump without the lamp
just for a couple of minutes
less if you see the amps spike, shut it off

Fuel filter. I would be willing to bet you have either contaminated fuel or crud in the tank that is being drawn into the pump.

saraajmele wrote:
We have taken a look at Fass and airdog and realize they are mostly better than the stock Cummins part and are thinking of replacing the VP44 pump once we have the lift pump issue repaired. It will start and run without the lift pump working.

The VP44 will run without the lift pump but it's a considerable strain on the pump. Plus if you lose pressure to the VP44, like changing the fuel filter, you won't be able to start the engine again. The VP44 pump can't overcome the air bubble to restart.

If the VP44 has run a fair amount without the lift pump operating then it's probably not a bad idea to replace it. You can get rebuilt pump for around $1000, but if you have to have it done out on the road you'll be looking at a service/parts bill of around $3000. I got my rebuilt pump for about $800, but that's been a few years ago. It's not terribly hard to install them, just a bit fiddly.

I have an Air Dog Raptor 100 running for over 5 years, but there are lots of complaints about the newer ones. I think if I had to replace mine, I would go with the FASS, even though it's a bit more expensive.

Like mentioned above... I would do a ripple test on alt 1st. Diode failure common IE back feeding current and creating havic and messing up other elec. Not talking about how many volts alt is putting out but what is leaking back if you have any bad diodes. If OK then move on from there. Service manual has flow chart for going thru system. But wiring maps not easy to find.

Since the current lift pump is failed (will install new one Monday for more testing) we’ve installed a 55watt headlight in the circuit finding 12.5 volts dc being supplied 30 seconds after starter bump and at all times with engine running and steady current draw of 3.55 amps if I remember correctly. Will run another test with the metr on A/C and advise. Thanks

We have taken a look at Fass and airdog and realize they are mostly better than the stock Cummins part and are thinking of replacing the VP44 pump once we have the lift pump issue repaired. It will start and run without the lift pump working.