Fuel system issue with lift pump

[Schlickenmeyer]

2001.5 with an Airdog that's been in place for a number of years. I cannot get the lift pump to run. I have confirmed 12v at the connector for the pump, voltage for 20-25 seconds on key bump as she should. But no pump activity, new pump motor, when connected to a spare battery she spins just fine. I'm quite at a loss, and need to get her up and running. Thanks in advance.

 

[petersonj]

I have confirmed 12v at the connector for the pump, voltage for 20-25 seconds on key bump as she should.

Are you checking the voltage at this location with the wire to the pump connected or disconnected? If you are checking with the wire disconnected you may be drawing an incorrect conclusion because the multimeter will not impose a load on the circuit. Think of it this way - one strand of wire will show proper voltage under no load, but hook the wire to a load and the voltage will fall on its face.

The fact that the pump will run on a separate power source indicates that something like this is going on. Also, be sure you have a proper ground.

- John

 

[Schlickenmeyer]

Thanks, yeah it was disconnected, and I was thinking voltage but no amps, maybe a bad relay? I will recheck the ground tomorrow when I get under the hood as well as all connectors tight

 

[petersonj]

maybe a bad relay?

If the fuel system for the lift pump circuit has not been modified, the power to the lift pump is provided directly from the ECU - no relay. If the lift pump system has been modified, then a relay has probably been added and the power from the ECU will operate the coil part of the relay.

- John

 

[Schlickenmeyer]

Used the harness from AirDog, and don't recall anything other than plug and play, but it was years ago. Will trace the harness and see what I find.

 

[p-Bar]

Are you sure that there is no relay in that harness from airdog?

 

[Schlickenmeyer]

Looking at the current manual there appears to be, not sure if it had one years ago when I installed it, but will trace the harness back from the pump and see if I can find one. Thanks for the tip.

 

[JR]

If it is an Air Dog 100/150 with the dual filters there will be both a 20 amp fuse and a relay. The fuse should be off the battery lead wire. I always carried spares of both in my glove box when I had one.

 

[DavidC]

Are you checking the voltage at this location with the wire to the pump connected or disconnected? If you are checking with the wire disconnected you may be drawing an incorrect conclusion because the multimeter will not impose a load on the circuit. Think of it this way - one strand of wire will show proper voltage under no load, but hook the wire to a load and the voltage will fall on its face.

The fact that the pump will run on a separate power source indicates that something like this is going on. Also, be sure you have a proper ground.

- John

Badly arced relay contacts will do the same along with loose connections.

 

[Schlickenmeyer]

Alright, found the relay, took a spare from the fuse box and it worked backwards, pump ran with no key, bump starter it stopped for 20-25 seconds. Tried a new one a pal brought and it did the same, cleaned up old one and it acted normally. I'm at a total loss here why this would happen, schematic on the other relay looked the same. Now to get fuel to flow, either it's running in reverse, or the little ball got stuck installing it, which I can tell with a chopstick easily enough.

 

[petersonj]

Alright, found the relay, took a spare from the fuse box and it worked backwards, pump ran with no key, bump starter it stopped for 20-25 seconds

I'm at a total loss here why this would happen, schematic on the other relay looked the same

I would take a closer look at the relays you using to perform the test. Relays may fit the receptacle, but they could function differently internally.

Some examples below:

- John

 

[p-Bar]

Good post John

 

[Schlickenmeyer]

That's what I was looking at when I referenced schematic of the relay, the numbering and the layout seemed the same with only one minor difference, one had 87 87 and one had 87a 87 on the switched side. Will get one with that exact bosch part number next. I never did like vehicle electrics. Thanks!!!

 

[Schlickenmeyer]

No idea why that double posted, can't seem to find delete.