Engine/Transmission (1998.5 - 2002) ECM to LP circuit questions

[DLausche]

2 questions related to the ECM to LP circuit. I am desiring to wire in a LP relay so my LP is not drawing current from the ECM but from battery circuit instead. I also desire to add a double throw switch under the hood to trigger the relay (and LP) to make priming the system a bit easier.



1) I assume that the ECM controls the + side of the supply to the LP and that the negative side supply to the LP is basically just grounded inside the ECM. If so, I can get by with switching only the + side of things between the ECM and relay, right? Basically, I'm thinking a SPDT switch will suffice here.



2) Is the ECM lift pump output full 12v or is it modulated somehow? I have always assumed it to be full 12 volts at the output of the ECM but I have seen a few posts here refering to "PWM" or Pusle Width Modulation on this ECM output.



Thanks

-Deon

 

[nps]

The + to the LP varies depending on whether you are in cranking mode or running mode. The - side is a ground.



I ran a SPST automotive relay I picked up at radio shack off the battery with a 10A inline fuse, then off to the pusher with 12g wire.



Rather than fooling with the wires at the LP, the relay is controlled by a switched ignition on wire. No +V when in ACC position or cranking - only when on. No hard start problems and my pusher is independent of the ECM.

 

[Blacksdieselhp]

So do I just ask for an spst automotive relay at radio shak and they will know what I need? I have a truck that has no power to the lift pump and was looking for an alternate fix. Does this relay allow the pump to run as it did on the factory wire? So you just wired it up from batt + to the relay and them the pump?

 

[nps]

So do I just ask for an spst automotive relay at radio shak and they will know what I need?

Yes - that should work. All radio shacks have a set of cabinets that hold switches, lamps, and other misc components. These are in the relay drawer. It's model #275-226 and you can see it here.

Does this relay allow the pump to run as it did on the factory wire?

My pusher has terminals for + and - connections. With the factory plug, you have 2 options:

1. Cut the plug off the factory wires leaving about 1" to splice in the new wires.

2. Go to cummins and get the LP adapter harness. Cut off one side and splice the adapter harness into the new wires. This would leave the factory plug intact.

So you just wired it up from batt + to the relay and them the pump?

Yes - piggybacked a terminal onto the +batt terminal, put an inline fuse holder (using the old style cylindrical glass fuses) and ran this to the relay contact. The other side of the contact goes to the + connection on the LP.



The coil of the relay is run off a switched + voltage I took from an add-a-circuit fuse holder adapter. The inline fuse holder and add-a-circuit came from autozone.

 

[Blacksdieselhp]

So once the truck is running does the pump run constantly or does it run like the stock lift pump. And you just ran a switched 12v source to the + on the lp harness, correct? Just wanna make sure I've got everything right.

 

[nps]

So once the truck is running does the pump run constantly or does it run like the stock lift pump. And you just ran a switched 12v source to the + on the lp harness, correct? Just wanna make sure I've got everything right.

Yes - it runs constantly. But so does the stock lift pump. The only time the stock pump isn't going full bore is when the key is on/engine off, and when cranking, when it is turning at half speed.



The + on the LP harness is constant battery power fed through Normally Open relay contacts. The relay coil gets the 12V switched source to engage the relay, close the relay contacts, and cause the pump to run.