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Phill

Fuel pump relay signal.

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The last thing I need to do before start up is to connect the ECU signal to the relay. Which is the correct terminal on the relay?

Also, I want to run the pump to check for leaks while not starting the car. What would the best way to do this?

Thanks 

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If you want the ECU to drive the fuel pump relay coil, that would be pin 86.  

It might not be that simple unless you also defeat or rewire the CIS air flap sensor. 

The operation of the fuel pump relay is often misunderstood. Essentially the flap must be lifted to run the pump when the engine is running. If however, the engine is cranking, the pump will run irrespective of the flap position. 

Edited by Jonny Hart

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Thanks Jonny,

 I no longer have CIS so there is no sensor. Does that make it more straight forward?

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Yes I realise you no longer have CIS but the sensor wiring is still wired to the fuel pump relay unless you have removed it.

The original sensor works by grounding the low side of the coil when the flap is lifted.  If you stil have the plug in the engine bay I would suggest you connect the pins together.  A better long term solution would be to ground terminal 85 of the relay. 

Can your ECU drive a relay coil? Is it low side or high side switching?

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 I no longer have the plug in the engine bay. 

The ECU has a wire to drive the relay but I don't know if it is high or low switching, I will find out. What is the difference?

Grounding terminal 85 sounds the most straight forward solution. Connect the existing wire from the terminal to ground?

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30 to 87a will run the pump (this will bypass the 85 to 86), for 76 to 83 CIS which you can use to check for leaks without starting the car. Alternatively you can put power directly to the fuse pump fuse to run the pump. Check your diagram for your year, its preety easy to work out

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OK, so ignition on, stick a wire into holes 30 and 87a and then pump will run and I can check for leaks? That's great. Then signal wire connected to 86 and 85 grounded. Tell me if I'm wrong please.

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Hi Phil

Depends on whether your ECU provides a ground or a 12V signal for the fuel pump relay - I've only used Emerald, but would expect the ECU to provide a ground rather than 12V.  In which case, provide a fused 12V supply to relay pin 86 (live only while the ignition is on, not permanent), remove any original wires from relay pin 85 (see Jonny's comments) and connect the ECU trigger wire to relay pin 85.

Alternatively, if the ECU does provide 12V, then earth relay pin 85, remove all other wires from relay pin 86 and connect your ECU signal to relay pin 86

Peter

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Thank you all for your excellent help as always. Onwards and upwards. More questions will undoubtedly follow :D

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^. What Peter said. 

For information and understanding only:

ECU provides 12v = high side switching.  It is a power source.  The ECU provides the switched power and the other side of the relay coil (85) is grounded elsewhere (chassis).

ECU provides GND = low side switching.  It is a power sink.  The power comes from elsewhere (connected to 86) and the ECU switches the ground (85).

The output from the ECU needs to be able to source or sink at least 300mA to switch a relay. I’m sure it is more than capable of that.  Have a look in the manual, there should be a section describing the output capabiliy.  

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Thank you all.

As promised, more questions for clarification if you don't mind.

IMG_20190109_092156.thumb.jpg.352c5ee704adc848450a99f103c1cab0.jpg

At the moment pin 85 has a brown and red wire running to the airflow sensor (presumably the signal from the CIS as Jonny mentioned) and the RPM limiter switch. Is the RPM limiter switch part of the dizzy which I no longer have? And as such now defunct so I can cut it?

Pin 86 currently has a feed from 87a which, in turn, is fed from fuse 16. The ECU signal is 0v, confirmed. So does that have mean I do not need to provide a new switched live to pin 86 as it already has one?

Pin 86 also has a wire to the RPM limiter switch which, in my mind, is now defunct.

In summary I think all I need to do is cut the brown and red wire and replace with the ECU signal. Am I correct? :D

 

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Yes 😁

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Thanks Peter :)

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Hi Phil. 

I would guess you have this sorted by now but the MS switches low side, "Ground" If you look at any of the MS diagrams you can see the normal switching. 

I would think you have removed ALL the CIS connections/sensors as you are using the T Bodies so the fuel pump relay only needs to control the fuel pump (and if you have wired like my diagram the wideband). as Jonny says this would usually have power 12v via ignition to term 86 and ground to term 85, This would be the black wire from pin 37 on the MS. 

Do you have any other thing to do before starting it up.

Mike

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Well, after all that the MS needs different wiring. The ECU needs to power the pump straight away (as it primes for 3 seconds and then  stays on once crank signal is received) and can't do that if the relay is only energised in start and run. So I need to power it at the same time as the ECU. Shaun is on the case!

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Phil

The ECU should only be powered in run (and start), so where's the problem.  When the ignition switch goes to run, the ECU is energised and earths (or powers) the fuel pump relay ??

Peter

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To me there are 3 ignition points. On, start and run. The FP relay currently is only energised in start and run but the ECU needs it to be energised in "on" so it can prime the pump before the starter engages. If you see what I mean.

So the new wiring will be 85 from ECU, 86 switching live at "on", 87 live from battery, 87a disconnected and 30 remains as is.

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OK, I thought ignition switches were normally accessories (things you may need to operate such as windows, but not powering ignition), on / run (which adds ignition, etc) and start (operates starter motor - and effectively bypasses air flap in CIS system to operate fuel pump - and the key springs back to run when released).

Peter

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Given that the ECU switches ground, I’m fairly certain all you need to do is wire the ECU to 85 and leave everything else as Porsche intended. 

Effectively the ECU output is the same as lifting the CIS airflow plate.  This causes the pump to run with the ignition in the ON position which is what you want isn’t it?

Edited by Jonny Hart

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I tried that and it doesn't work. The MS has a fuel pump test function which is very handy but it only works if the relay is energised, obviously. The ECU is on when you turn the ignition to the first position. It is at that point it will run the fuel pump for 3 seconds while it looks for the crank signal. However, as above, the standard Porsche wiring doesn't run the pump until the second ignition point (start). Yes, it will probably work as standard but then the test functions won't work properly and to run the pump from the ignition I would also haven't I be cranking the engine . It's much easier to run the pump from the laptop than to run a wire from the battery to 30 and keep walking backwards and forwards. I'd rather have the ECU fully in control, as designed,  than not.

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I’m surprised that the ECU powers on the first key position. I wired my (Lotus) ECU to the run position (via a slack handfull of relays in the fuse box).
I would wire the MS from position 2 and all would then work as desired.
Peter


Sent from my iPhone using Tapatalk

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Fault finding telephone conversation with Peter and we have hopefully sorted the conundrum - immobiliser!!!!!!!! Full update tomorrow. Thanks Peter :signs85:

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Things did not go according to plan this morning so another telephone diagnosis with Peter established that the relay was working in the opposite way to the Bentley diagram. Once that was established this is the way it is wired now. 

85 signal from ECU.   86 fed from 87.   30 to fuel pump as original.    Two reds moved from 87a to 87.   87a no longer has a connection.

Now it works as it should with the ECU triggering the pump.

To make life easier I have cut all the wires and put space terminals on them and connected them to a normal square relay which will have to be mounted elsewhere. So no round red relay anymore :(

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