1999 Dodge lift pump

[WesHawkins]

  I have a puzzle to try and figure out.  A friend brought his truck to me because it was bucking and surging.  I drove it, it was.  I checked fuel pressure and found zero fuel pressure from the lift pump.  Truck was running without lift pump fuel pressure.  He said to me he had just replaced that intank pump about a year ago.  This truck had the original lift pump on the side of the engine replace ten or twelve years ago so this is the second intank pump.  What I found was when I bypassed the relay the pump would run, put out about 14.5 PSI.  I found that the in line fuse that is in the power feed line to the relay was disintegrated.  The only thing left were the prongs of the fuse.  When I put a new fuse it the pump ran but the fuse holder got very hot to the touch.  I checked amperage draw of the pump, it was 1.48 amps.  Is this to much and causing the fuse holder to get hot and the fuse to disintegrate?  Or did the previous intank pump cause this when it went out?  I check the amperage draw on my truck with the FASS pump and it’s only .5 amps.  Now that I have pump pressure restored the truck runs great.  

[hdpwipmonkey]

I would see if I could find someone that has the in tank pump and measure their amp draw so you would have an apples to apples comparison but if a Fass (with a much bigger pump) is only pulling .5 amps than I would be suspicious of the little in tank pulling three times as many.  

[Tractorman]

On 9/8/2023 at 6:00 PM, WesHawkins said:

When I put a new fuse it the pump ran but the fuse holder got very hot to the touch.  I checked amperage draw of the pump, it was 1.48 amps.  Is this to much and causing the fuse holder to get hot and the fuse to disintegrate?  Or did the previous intank pump cause this when it went out?  I check the amperage draw on my truck with the FASS pump and it’s only .5 amps.

 

Not sure how you are testing for current draw, but those numbers are way low.  I don't think  they are real.  The current draw should be more like 8-12 amps.

 

I would suspect that you are getting a severe voltage drop across the fuse connections while the pump is running, which will generate lots of heat, even from a perfectly good lift pump.  Since the fuse holder was already damaged from a previous situation, the fuse holder needs to be replaced.

 

I would jumper the existing fuse holder with an in-line 20 amp fuse and re-test the lift pump.  If the fuse holder still gets hot, then you have a lift pump that is beginning to seize.  If the fuse holder stays cool, then you have found the problem.

 

- John

 

[WesHawkins]

  I was using an amp clamp at both the power lead going into the relay at the firewall and at the power wire along the frame rail back by the tank.  I did replace the fuse holder with a larger wire one and it doesn’t get hot anymore but current draw is the same.    I cut the old fuse holder and removed the contacts to inspect them, they were corroded and burnt.  Fuel pressure is 14.5 psi between the filter and the injector pump.  I moved the fuse holder away from the battery post and connected to the positive post at the under hood power distribution box.  I’m going to replace the relay also and recommend to the owner that he/I install a fuel pressure gauge.  I was surprised the truck ran at all without lift any lift pump pressure.

[Mopar1973Man]

Anything that gets hot is a resistive connection. This mean the load stays the same but voltage gets reduced but current flow will increase typically. I would replace that fuse holder and check again.

[Tractorman]

1 hour ago, WesHawkins said:

I was using an amp clamp at both the power lead going into the relay at the firewall and at the power wire along the frame rail back by the tank.

 

Personally, I have never found a multi-meter with a DC amp clamp connector to be reliable for direct current measurements.  I think you are having the same issue with your meter.  From a couple of other sources, 12 amps was the response for current draw for stock lift pumps.  Years ago my truck had a factory in-tank lift pump with relay installed under warranty by a Dealer.  The power to the relay was supplied by an added 20 amp fuse.  If the lift pump truly only drew .5 to 1.5 amps, then a 5 amp fuse would be more than adequate. 

 

1 hour ago, WesHawkins said:

 I was surprised the truck ran at all without lift any lift pump pressure.

 

There is a positive displacement vane pump (called the feed pump) inside the VP44 injection pump.  It is this pump that drew the fuel into the VP44 without the lift pump operating.  As long as there is not a blockage from the fuel tank, the engine will run.  The purpose of the lift pump is only to supply a positive pressure at the inlet of the VP44.

 

- John

 

Edited September 11, 2023 by Tractorman

[WesHawkins]

37 minutes ago, Tractorman said:

 

Personally, I have never found a multi-meter with a DC amp clamp connector to be reliable for direct current measurements.  I think you are having the same issue with your meter.  From a couple of other sources, 12 amps was the response for current draw for stock lift pumps.  Years ago my truck had a factory in-tank lift pump with relay installed under warranty by a Dealer.  The power to the relay was supplied by an added 20 amp fuse.  If the lift pump truly only drew .5 to 1.5 amps, then a 5 amp fuse would be more than adequate. 

 

 

There is a positive displacement vane pump (called the feed pump) inside the VP44 injection pump.  It is this pump that drew the fuel into the VP44 without the lift pump operating.  As long as there is not a blockage from the fuel tank, the engine will run.  The purpose of the lift pump is only to supply a positive pressure at the inlet of the VP44.

 

- John

 

Thanks John

  Thanks for the response.  I’m going to recheck the amp draw using my voltmeter here in a little bit.  I don’t think that pump draws to much amperage give the small size of the wire running from the relay to the pump.  I’ve  changed out a few VP44’s but I’ve not had to diagnose many of these lift pumps, they usually have already died.  I changed out the factory lift pump on my 2000 Dodge before it went out with a FASS pump about ten years ago.  I’ve never had any issues with mine except the APPS while up at McCall, Idaho years ago.

[Mopar1973Man]

I typically use my Fluke DVM for measuring the Amp draw of devices but it is normal probe type DVM. 

 

 

[WesHawkins]

6 hours ago, Mopar1973Man said:

I typically use my Fluke DVM for measuring the Amp draw of devices but it is normal probe type DVM. 

 

 

  I rechecked the draw with my DVM and it came to almost 10 amps.  I think the old pump that was replaced a year ago after going out damage this fuse and holder and no one bothered checking it.  I upgraded the fuse holder and put in a 20 amp fuse and all seems good now.  

[Tractorman]

@WesHawkins, the 10 amp draw sounds like a healthy lift pump.  You are probably right regarding an overlooked fuse holder condition. 

 

- John

[WesHawkins]

1 hour ago, Tractorman said:

@WesHawkins, the 10 amp draw sounds like a healthy lift pump.  You are probably right regarding an overlooked fuse holder condition. 

 

- John

  I’m hoping, fingers crossed.  Of course I’m the third guy in line that has to fix it right this time.

[Mopar1973Man]

Just keep in mind you want 14 PSI as a minimum fuel pressure at WOT. This will help with cooling and lubing the VP44. The bad side is never pull fuel from the sender basket being that is the hottest fuel return from the engine and can cause VP44 damage to the PSG module. Being my fuel system I use an old school draw straw in the tank my fuel temperature has are considerably lower than most others on the forum here.