Confusion about replacing relays (ECU and fuel pump questions)

I'm in the process of installing an AiM PDM32 in my car, that currently has the stock ECU and stock harness in there, including all of the original fuses and relays.

I wanted to go through each collection of fuses and relays in the car and start removing them, and power from PDM instead, and after watching so many videos/reading up on it I thought it looked quite straight forward but now I'm getting very confused when it comes to practically doing it.

If I take the first group of relays...

* Starter (Lotus B116M0032)

* ECU (Siemens AMR 1088)

* Fuel pump (Bosch 0332209159)

Starter

The starter seems the most straightforward. I can wire it to a power output on the PDM (specifically High PO1 on the PDM32) and manage switching on/off via an input the PDM is aware of (i.e. a switch or button on CAN keypad). I can add in some logic in the PDM to not output power if the RPM is over some value (e.g. 250rpm) to avoid running the starter once the engine is started.

The original relay has an ECU connection, but this is just to kill power once the engine is started. This functionality is replicated by my RPM check in the PDM logic, so I don't need to worry about that. Conclusion, power from PDM, some internal logic, nothing complicated.

ECU

I want the PDM to control the ECU power so I can kill power to it when the PDM receives an engine-kill signal from a battery isolator (Cartek XR).

I thought this would be a fairly straightforward one also. I have my PDM32 safe-ignition input wired to an ignition switch (so turn ignition switch on, 12V goes to PDM and it starts up), and also the kill signal from the battery isolator wired to channel 11 (which also powers the PDM).

I figured I would just replace the ECU relay with a direct connection to one of the PDM power outputs, and have the power output from the ECU always on unless we get the kill signal from the battery isolator, then we'll shut down the ECU.

I'm using the existing fuse values as a guide for what size power output I should be using, and looking at the existing ECU relay, it's protected by a 40A fuse. This is much larger than I was expecting and much higher than any of the power outputs available on the PDM.

Is that normal? I'm thinking I could group multiple outputs to power the ECU up to 40A, but I'm confused why it's so high as other examples I've seen of wiring in an ECU to PDM has seen values around 5-10A. I feel like there's something I'm just not understanding here, as it doesn't seem like it should be that high.

I guess one option is to retain the fuse and relay for the ECU, and instead use the PDM to handle the switching of it. Downside is it's a fuse and relay I've not got rid of, but at least sidestep the 40A issue...

Fuel pump

Am I right in thinking I'd want both an input and output on the PDM to handle the fuel pump? i.e. a power output connected directly to the fuel pump from PDM, but the ECU still needs to control when the fuel pump runs. So I'd take the control wire that currently goes to the fuel pump relay from the ECU, and run that as a digital input into the PDM? Then when that is active (i.e. ECU is requesting the fuel pump to run), I enable power output to the pump?

If so, I think I'm good, but I've not seen that input from the ECU mentioned in other examples so I want to check I've not misunderstood something. Is that a typical setup for handling the fuel pump?

Edit: Had a moment of clarity after posting this. I guess the ECU connection here is just so the fuel pump is not running when the engine isn't running (outside of priming it when ignition is switched on). So I think this means I can wire the pump directly to the PDM, ignore the ECU connection completely, and instead add some PDM logic that powers it on when:

- Ignition is switched on (power for x seconds)

- Starter power output is active

- Engine RPM is over some value (e.g. 250rpm, so we know it's running)

Once the engine is running, we'd run the pump continuously until the engine stops.

Tom,

Measuring amp draw over varied conditions will inform what the circuit needs to handle. You can hook the ECU to two PDM circuits to play it safe, monitor the total draw, and then drop down to one circuit if you're confident it can handle it.

Yes you can set up a series of logic on the PDM to control the fuel pump. That said, the ECU already has all that logic built in so you can also let it do the job. With a stock ECU that would mean running the fuel pump output to a PDM input to trigger things, or with a standalone I have the ECU send the fuel pump signal over CAN.