Fuel Pump Control Unit - what does it do?

[ilib11]

Hi All,

There is a FUEL PUMP CONTROL UNIT - exists for turbo & 3.6L . Does anyone know what does it do exactly?
PN 22648AA180, see it marked on the pics attached.

Thanks and regards, ilib11

[RX25SE]

It controls the output of the fuel pump.

Basically it receives serial data from the ECM about how much output is required from the pump. The module controls the output using Pulse Width Modulation (PWM) with is a high current transistor (switch) running at a fixed frequency (speed) and the duty cycle (%) determines the pumps output.

Imagine a switch being flicked on and off once per second (1Hz frequency) and it spends half it's time 'on' and half its time 'off', thus the duty cycle is 50% and the pump output is half.
The switch remains at 1Hz frequency, but now spends 3/4 of it's time 'on' and 1/4 of it's time 'off' then the duty cycle is now 75% and the pumps output is raised to 75%.

It essentially controls the pumps current (Amps).

IIRC the 3.0R has three output levels (can be seen with a scan tool) 33%, 66% and 100%. I can't remember the frequency but they are usually in the KHz.

[ilib11]

Thank you, RX25SE for your detailed explanation!

I would suspect that module changes the volume but not the fuel pressure, fuel pressure is controlled by a purely mechanical fuel pressure regulator, correct?

If so, it is still puzzling why Subaru would need to change fuel flow on turbo 2.5 and 3.6L but not on an ordinary NA 2.5L

(P.S. I suspect I may potentially have a low fuel pressure on idle and try to figure out if this device may have its own logic and can contribute to that while I am waiting for a manometer to arrive and measure the actual pressure)

[RX25SE]

Generally speaking the larger engines require a higher output pump at the top end of the engines power, but this is not necessary at lower engine power outputs. Saving electrical energy is helpful towards emission targets, as essentially the alternator is petrol powered.

The n/a could get away with a lower output pump running at 100% all the time.

[bigBADbenny]

In addition to the above, from memory the pwm is applied to the pump negative circuit.

[ilib11]

This topic is becoming more and more interesting - what is the pump negative circuit? Am I right thinking that it means the fuel pump has direct +12v from the fuel pump fuse and the "pwm transistor" is between the fuel pump and the ground?

I have measured my car today on idle:
a)with vacuum hose connected to the fuel pressure reg: 41psi, 42psi with AC on
b)w/o vacuum hose: 51 psi
c)return line blocked by pliers: 55psi, fluctuating
d)engine shutdown: increasing pressure 39psi->40->41

Pump "voltage" on idle: 7.4 v
Seems to be as per the specs, however I expected more pressure with the return line blocked.

[RX25SE]

You are correct, the ground circuit is just the negative terminal of the pump. Battery voltage is applied to the positive terminal whenever the ignition is on and the negative terminal (ground side) is switched on and off to control the pump. It's very common, almost always really, that PWM uses ground switching as the internal resistance of the load, in this case the pump, helps with lowering voltage spikes across the switch (transistor) when the load is turned off.

With a ground switched device, if you measure the ground side with a volt meter, battery voltage will be 'off' and 0v will be 'on'.

A voltmeter across the pump will display the average voltage, as the PWM frequency will be too high for the voltmeter to pick up. Basically it can't keep up with a switch flicking on and off thousands of times a second so this would be why your voltmeter shows 7.4 volts.

The best way to see what the pump is doing is to connect a oscilloscope from the negative side of the pump to ground. This will clearly show that the pump only has two states, OFF (12v) and ON (0v). A good scope will pick up a switching voltage down to micro seconds (one thousandth of a millisecond) which is the speed CANBUS voltages switch at.

I wouldn't recommend clamping a fuel line when the pump is running. Most pumps have an internal pressure relief valve (ball and spring) that opens to bleed off excess pressure and prevent the pump stalling in the event a line becomes blocked. This is the small hole near the outlet port. If the valve is forced open, there is a chance it may not return to its seat correctly, or if a small bit of debris gets onto the valve seat, causing the valve to bleed a small amount of fuel which lowers the output pressure slightly.

[bigBADbenny]

Additionally logging fuel pump duty cycle vs tuning parameters to see if lean/rich conditions are related to the three fuel pump duty states, 30, 60 & 100%.

School is in, I love school

Fun factoid: Fcpm’s are known to fail, one failure mode is insects eating the potting on the circuit board.

[ilib11]

Thank you, RX25SE and bigBADbenny ! Your explanations are very detailed.

My initial hypothesis was that a dying fuel pump with low duty cycle from the pump controller may result in a low fuel pressure on idle and that may cause my problems with idle shaking under the load that is described in my another tread.

From the experiment I can see that fuel pressure is in the spec range and unlikely causing problems.

Re internal pressure relief valve and a pressure with return line blocked - I would expect this pressure where the relief valve would open to be high enough to cover boost - i.e. 51psi + max boost pressure , i.e. in 60+ psi range. The engine control module knows the manifold pressure and it simply may not engage a high duty cycle until it sees a boost. Probably with low duty cycle on idle the output pressure simply did not reach 60+ psi range.

Anyway, it looks like I have enough pressure on idle, so the hypothesis is yet another miss and I need to keep digging.

Re Fcpm’s are known to fail - if I see a low pressure /pump degraded performance, I had a plan to completely disconnect the fuel pump from it ( both wires) and use plain 12v source to feed the pump. Have not done that yet. And Fcpm may report to the engine control module a disconnected fuel pump as a failure due to zero current.

[RX25SE]

Additionally logging fuel pump duty cycle vs tuning parameters to see if lean/rich conditions are related to the three fuel pump duty states, 30, 60 & 100%.

I'm not sure what this means?

My initial hypothesis was that a dying fuel pump with low duty cycle from the pump controller may result in a low fuel pressure on idle and that may cause my problems with idle shaking under the load that is described in my another tread.

From the experiment I can see that fuel pressure is in the spec range and unlikely causing problems.

A dying pump will usually draw less current. As this pump's current is PWM controlled, you would need to know the current of a 'known good' at a particular PWM duty cycle.

Re internal pressure relief valve and a pressure with return line blocked - I would expect this pressure where the relief valve would open to be high enough to cover boost - i.e. 51psi + max boost pressure , i.e. in 60+ psi range. The engine control module knows the manifold pressure and it simply may not engage a high duty cycle until it sees a boost. Probably with low duty cycle on idle the output pressure simply did not reach 60+ psi range.

The internal pressure relief valve only opens at a pressure well above the pumps maximum operating pressure, it's purely a mechanical safety release and is not boost related.