PDM or ECU driven outputs

Hi,

I am struggling a bit with deciding whether the control functions for the likes of the fuel pump, Electric water pump, fan control should be driven and controlled by the ECU (in my case a life racing F88) which would then send the signal via CAN to the PDM (in my case an AIM PDM32) to tell it to activate the output or whether its best to control these functions from logic within the PDM.

What are the pros and cons of each option?

Generally the ECU has features designed to control the pump (like priming for X seconds when the ignition is switched on). By letting the ECU do the logic, you don't have to develop the code/conditions in the PDM. So I think Fuel Pumps, and Cooling fans are best done in the ECU.

However, you can have overriding logic (like turn off the cooling fans and high-power lights when cranking to provide max power to the starter) in the PDM as well. So the PDM would have a manual fuel pump override, to allow pumping out the fuel cell (a task usually difficult with the ECU).

Another good PDM use is a backup pump that is controlled by a switch / keypad -- the ECU just thinks there is one pump, the PDM does the logic for switching the selected one.

Thanks David,

I am trying to work out how best to control the fuel pumps in my racecar with the Life Racing ECU and AIM PDM32.

I have a fuel cell that has 2 high flow low pressure lift pumps at the rear corners of the tank that fill an internal surge tank. On top of that surge tank I have a float switch that can be use to indicate that this surge tank is full. Then from the bottom of the surge tank a have the high pressure fuel pump to go to fuel rail then returns back to the surge tank which also has a hole to allow excess fuel to escape back to the fuel cell.

What I am wondering is if this can be controlled through the LR88RS6 ECU or would it have to be done through logic in the PDM?

My Thoughts were the following: upon engine enable prime lift pumps until float switch is activated (and keep them running) then prime the high pressure circuit until "x" fuel pressure is reached and held for "y" seconds then send signal to ECU to allow engine to be started upon which power to ignition and injectors will be activated.

I also intend on having manual fuel pump overides for both the High Press and Low Press circuits.

Does this logic sound correct and can this be done through the ECU or is it best handled by the PDM?

Thanks

Karl

I would say your Life ECU requests when the High Pressure (HP) pumps are to be turned on. You use the PDM logic to turn on the lift pumps (possibly considering the float switch -- I've never needed that). If the float switch indicates low -- maybe they stay on a bit longer after the HP pump is turned off.

I think you are overthinking the "wait for fuel pressure then let the ECU start". Once initially filled, your surge tank should have fuel until the tank is empty. So turning on the HP fuel pump should almost immediately build fuel pressure and the engine will be ready to run.

If you are concerned, then just program the Life ECU for a long fuel prime (5 -10 sec), so the lift pumps can get the surge tank mostly filled.

I'm currently doing something similar for a client who wants state of the art gear to act like a race car from the 80s but also have twin side tanks that auto switch over.

My thought process is usually "How will I have to diagnose and support this" - because of this I try to keep solutions isolated to as few pieces of gear (read: software and interface cables) as possible, so in my case I'm only using pump control logic on the PDM with the only real feedback loop the ECU cares about being pressure at the rail.

That said, I'm sure there will be some quirks introduced (this is OK in my case, 'act like a race car from the 80s' and all that) so you'll have to factor that in to your choice.

*edited to clarify that I actually removed fuel pump logic from the ECU, not the PDM. My brain was going in 2 different directions.*