madhat

We are doing an install on a 99, and the AD is not getting 12v from the ECM plug that origionally supplied power to the in tank lift pump. I want to run the 12v to the AD from a location that is hot when the key is on. I got it to crank and run fine doing this last night, but I was told that the VP44 does not like full pressure when cranking.


What will I hurt if I run full pressure to the VP44 when it is cranking?

TIA.

mad

nickg

I believe the issue is hard starts on some trucks if over 20psi

Carolinablue

As I've been told, it shouldn't matter as the overflow valve opens at 14 psi returning to tank.

madhat

Ok, so I'm good. The AD is set at 15 PSI according to the manual.

pind

hey Mark, why not install a 4 psi oil pressure switch for controlling the AD.

That way, the pump will not come on until the switch sees engine oil pressure, and in the event of oil pressure loss, the AD will shut off. Kind of a safety feature as well.

madhat

I'm not sure why I would want to do that for start up... the starter motor would just crank longer, and the VP would be with out fuel. Also, if I lost oil pressure, I would not want to shut my truck down by starving the VP.

Not trying to kick you in the knee, this is just what I'm thinking. I know next to nothing when it comes to VP's...

pind

What it comes down to is safety.

Now, normally when you first start a VP truck, the LP is not running. It starts to run just as the engine cranks. It takes less that two revolutions of most engines to see 4 psi or higher, so you won't be starving the VP.

The safety aspect kicks in, when you lose oil pressure, the pump shuts off. Which means, in the event of a crash, when the engine stops, so does the pump. Most of us don't remember to turn the key off when an accident happens, we're too worried about other things. This means, if your pump is wired to IGN hot, it is still running, trying to pump fuel, if there is a ruptured line, guess where the fuel is going. When responding to an emergency call, the last thing those guys need, is to be waiting for something extra to catch fire, or be working in a lake of fuel.

Just food for thought.

madhat

I see where you're going. Good points.

dkenny

this might be dumb question..
you do have a fuel pressure gauge right?

don't go by the manual from AD..mine was below 15psi from the start.
even after changing gauges. the only other gauge I had was a manual vacuum/boost gauge..every see 10in vacuum on the fuel lines..I did..and I wasn't happy about it either..but after many emails with AD..I just needed to add some washers to the pressure spring..now I run 18 idle and 15 or so WOT..I don't think I see less than 10psi since adding the washers.

so I have you have gauge.

I do like the other suggestions if you remove the pump-ecm wiring..

something else I would suggest if you an AD..is a switch under the hood to short out the relay contacts..for a manual pump operation..I going to be adding this to my truck..I needed it today..but that's another story.
-dkenny

Clunk

Except for a lift pump "grunt" when the key switch is first turned on, you won't see any more lift pump run unless you "bump" the starter to get The Free 25 Second Lift Pump Run or start the engine which will turn the lift pump on for constant run until you turn the keyswitch off...or at least it's supposed to.

How 'bout some service manual info,

torquefan

If you wanted to get fancy, you could add a secondary power source to the lift pump and install another relay wired to trigger the pump during starter cranking.

madhat

Yeah, I put 12v to the pump through a switch. He kicks the switch on, then cranks the truck.

We're gonna call AD on Monday and see what the deal is.

rattlerbob5.9

Madhat,

You guy's are trying to hard all you need to do is take the original hot lead (+) that ran the OEM carter lift pump and use it to trigger (turn on) a relay that feeds the new AD.

It will then give you the little 1-2 second prime to the vp-44 when you key up before starting the truck and still retain the ability to bump the starter to get the 30 second run cycle as well which works good when changing the fuel filter.

The reason for the relay is to carry the larger than OEM electrical draw of the new pump without damage to the PCM .

You want a little fuel to the vp before cranking but not a steady high pressure which can case hard starts.

torquefan

Ya know, my thought was that that wouldn't work because the ECM doesn't always provide a full 12 volts to the factory LP, and the reduced voltage may not be enough to turn on the relay, but thinking it over, that's exactly how Dodge's in-tank retrofit pump works. So it should work fine.