How does dual voltage fuel pump control works on S5 TII?

I know i should be seeing 9V at idle/cruise and 12V+ on high/load RPM, but how exactly does it work?

What are triggers to switch it (MAP ? RPM? TPS? values?)?
Which part actually does the switch ? (ECU?)

Last year i had S5 TII which was not correctly switching from low to high voltage and we fixed it by wiring 12V constant, but i would rather find out what exactly is cause of that and how to check/fix....

 

ECU governs the voltage output of the fuel pump relay which governs how much voltage is sent to the fuel pump.

 

interesting, the manual is basically no help...

the easy one is that it looks like during cranking, the fuel pump sees 12V, the start pin bypasses the resistor.

when the car is running though, it looks at the CAS, narrow range TPS, the temp sensors, and AFM, but NOT the pressure sensor!

so i'd expect it to trigger over a certain rpm (3500, when the injectors stage maybe?) and then over a certain TPS, the narrow TPS only reads to like 30% throttle opening, so it must be somewhere in there too?

 

I would have guessed just the boost sensor/pressure sensor/MAP all by itself. Wrong according to the FSM and J9FC3S.

Pin 1K on USA ECU outputs a gnd to the relay when the items in my first jpg all get together and lined up right to cause that gnd to the relay.

Maybe check each of those items one at a time till you find out which isn't doing its job???? Make sense?

 

To me it seems likely that fuel pump resistor control is related to the load calculation in the ECU: Airflow and rpm are likely the most important, with those other factors HAILERS pointed out (from the FSM) playing a more minor role.

Are you sure the resistor relay itself didn't fail?

 

i wanna say that there is probably a minimum rpm and a minimum temp that it can turn on, but maybe its 12V when the engine is cold?

the FD is MUCH simpler, it just switches @1psi.

oh BTW the VDI turns on when the AFM is <0.7v and rpm is over 5200

 

I bet during a hot [enough] start it runs at 12V. This makes sense considering there is a hot start solenoid to raise the fuel pressure when manifold air temps are high.

 

agreed, it might run @12v when the engine is cold too, as its running richer

 

Last car with this problem (year ago) i didnt have time to diagnose more and just hardwired it to 12V constant. Swapping resistor relay from 100% working car itself did not helped.

Now i am in Italy and tomorrow i should check Cody's TII which might have same problem.
I have brought few extra parts with me (ECU, TPS, AFM, Boost sensor, Fuel pump resistor relay) which i can replace if necessary.

In worst case, i can probably just ground that 1K wire and it will run 12V constant......

Thanks for input guys!

 

I keep forgetting that Higgi is in Europe and his is a non USA car and that I DON'T KNOW what the wiring is on that particular car.

My way of saying that your car may be wired differently than USA.

 

Its pretty much same, we interchange ECU's from US to EU cars with no problems.......