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Old 07-22-2015, 09:05 AM   #1
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Default Fuel pump operation/logic/wiring

Hope someone can help with this.

1993 1.6 UK car.

Before situation:

Car runs with OEM ECU, with AFM, with emanage/bandaids. Would hold 11.5AFR at 8 psi no problem. Ergo - fuel pump 'OK' (if not necessarily great, aware of the voltage drop issues).

After -

New aftermarket ECU fitted, AFM removed, double injector size. Car would start and immediately stall. No fuel pressure once running. Bridging to gnd at diag port enables car to run.

Ergo - something tells the pump to prime, but then doesn't keep it live when engine running.

What energises the pump to prime when you turn ignition on? And is this separate to what energises the pump once engine is running? How are these both switched on a standard car (I believe by the AFM sending a signal to the ECU and the ECU switching an output...?)
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Old 07-22-2015, 09:26 AM   #2
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The AFM flapper door has a fuel pump switch in it that shorts with engine airflow (fuel pump on) and opens/cuts fuel when the engine dies (fuel pump off). Your EM needs to perform this function.

This is illustrated in the factory wiring diagram, and is described in the FSM and Miata enthusiast manual IIRC.
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Old 07-22-2015, 09:56 AM   #3
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Perfect, many thanks. Suggested reading below also explains it well.

https://www.miataturbo.net/megasquirt-18/fuel-pump-only-when-cranking-ms3x-issue-84640/
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Old 07-22-2015, 09:58 AM   #4
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^ What he said.

No idea if the wiring is the same in the UK cars as the US, but here's what the circuit looks like on a US-spec car:











In the MSPnP, they feed +12 from the ECU into the Violet wire on ECU pin 1C, and remove the STS IGN fuse (not shown above) to protect the ECU from the starter.

You can also provide a closure to ground on the light green wire which goes from pin A of the AFM to the fuel pump relay as well as to the F/P terminal of the diagnostic connector. This wire doesn't show up at the ECU, so you have to hunt for it.
Attached Thumbnails
Fuel pump operation/logic/wiring-80-b1b_64732481489fa8eb004ba77aefd49c502eed89cd.gif   Fuel pump operation/logic/wiring-80-b2_6278bb709c75661f29d6a7da043562b335724021.gif   Fuel pump operation/logic/wiring-80-b1a_e1003cef9d14ddf7ffc4918249a380246eb30c72.gif  
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Old 07-22-2015, 09:58 AM   #5
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^ What he said.

No idea if the wiring is the same in the UK cars as the US, but here's what the circuit looks like on a US-spec car:











In the MSPnP, they feed +12 from the ECU into the Violet wire on ECU pin 1C, and remove the STS IGN fuse (not shown above) to protect the ECU from the starter.

You can also provide a closure to ground on the light green wire which goes from pin A of the AFM to the fuel pump relay as well as to the F/P terminal of the diagnostic connector. This wire doesn't show up at the ECU, so you have to hunt for it.
Attached Thumbnails
Fuel pump operation/logic/wiring-80-b1a_e1003cef9d14ddf7ffc4918249a380246eb30c72.gif  
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