Details on fuel pump relay in 1978 to 1983 SC

[PeteKz]

Gents: I've searched though several past threads about SC fuel pump relays, and I think I have a pretty good understanding of them and how they are wired, so let me confirm that.

Project: I have a CIS 1980 SC engine in my CIS 1973T car. Years ago when I replaced the engine, I swapped around some of the pins in the connector between the engine harness and the circuit panel at the left rear of the engine bay to get it to work and power the gauges correctly. Several years ago I switched from an externally regulated alternator to as WOSP high-amp alternator with internal regulator. So I've had the VR, its connectors ,and other wires hanging out that didn't do anything. A few days ago I decided to clean up the wiring by eliminating most of the extra connectors and wires, and to either repurpose some of the wiring in the chassis harness or add a wire to add an oil pressure light and buzzer to my instrument panel (the 1973 cars did not have an oil pressure light or buzzer, but the later cars did have the light).

I also decided to add a fuel pump relay, because the 1973 cars did not have one--they took power straight from the ignition switch. I also thought this would be a good point to add the safety cutoff function that comes from the switch on the CIS air metering unit. Fortunately, there is an extra round relay socket on the circuit panel next to the CIS relay socket (also round). That extra socket was used for the Sportomatic or MFI system on the 1973 and earlier cars, so I can repurpose it and rewire it for the a fuel pump relay.

The safety switch on the airflow sensor is closed to ground when the air metering arm is at the rest position (all the way down). This prevents the pump from running when the engine is not running. It opens when the air metering arm lifts. The fuel pump runs when the pump relay is normally closed from pin 87a to pin 30. The pump stops when that ground through the safety switch is completed (closed), which occurs when the sensor arm drops to the rest position. That moves the relay contact from pin 87a to pin 87. Therefore, the pump needs to be wired "backwards" from the usual relay mode of operation (pin 87 to pin 30 for applying power to a device).

Is this a correct understanding of the wiring and operation of the fuel pump relay in SC's?

And I know I should get a red relay with the internal diode, but I have several black ones handy, so that's what I will use until I have it working correctly.

Thanks.

EDIT: I crossed up the cut and paste in my original message swhich mangled the meaning, so I have corrected that above.

[ahh911]

I like the diode in place to help protect the over-rev electronics box mostly and looks like to also protect the alarm system

Phil

[AndrewCologne]

Originally Posted by PeteKz

Gents: I've searched though several past threads about SC fuel pump relays, and I think I have a pretty good understanding of them and how they are wired, so let me confirm that.

Project: I have a CIS 1980 SC engine in my CIS 1973T car. Years ago when I replaced the engine, I swapped around some of the pins in the connector between the engine harness and the circuit panel at the left rear of the engine bay to get it to work and power the gauges correctly. Several years ago I switched from an externally regulated alternator to as WOSP high-amp alternator with internal regulator. So I've had the VR, its connectors ,and other wires hanging out that didn't do anything. A few days ago I decided to clean up the wiring by eliminating most of the extra connectors and wires, and to either repurpose some of the wiring in the chassis harness or add a wire to add an oil pressure light and buzzer to my instrument panel (the 1973 cars did not have an oil pressure light or buzzer, but the later cars did have the light).

I also decided to add a fuel pump relay, because the 1973 cars did not have one--they took power straight from the ignition switch. I also thought this would be a good point to add the safety cutoff function that comes from the switch on the CIS air metering unit. Fortunately, there is an extra round relay socket on the circuit panel next to the CIS relay socket (also round). That extra socket was used for the Sportomatic or MFI system on the 1973 and earlier cars, so I can repurpose it and rewire it for the a fuel pump relay.

The safety cutoff switch is closed to ground when the air metering arm is at the rest position (all the way sown). It opens when the air metering arm lifts. The fuel pump runs when that relay is closed, powering the pump, and stops when that ground through the switch is interrupted (opened). Therefore, the pump needs to be wired "backwards" from the usual relay mode of operation. The power wire to the pump must connect to the relay's normally closed position, so that when power is applied to the relay's internal coil, it opens and stops the pump.

Is this a correct understanding of the wiring and operation of the fuel pump relay in SC's?

https://nineelevenheaven.wordpress.com/the-protection-circuit-of-the-fuel-pump/

And I know I should get a red relay with the internal diode, but I have several black ones handy, so that's what I will use until I have ti working correctly.

Thanks.

The internal diode is needed as flyback for inductive spikes coming from the coil when its getting switched off.
You can easily solder/add that diode to a black relay by yourself.






Here's how it should be soldered:


86 = 12v and 85 = GND

The flyback diode in the fuel pump relay is primary used to protect the electronics in the Rev limiter switch (if existing the car) as its directly connected to the terminal 85 and 86 of the FP relay.
EDIT: I See now its exactly what Phil meant

[PeteKz]

Phil and Andrew, thanks for the quick responses. Upon further investigation, I have an additional problem. The extra socket does not have 5 pins. It only has 4. The fuel pump relay uses the 5th pin (87a) for its normally closed operation of the pump. So I have to get a socket that has 5 pins. Does anyone have a preferred source?

As for the diode, my car has no alarm, etc. So momentary voltage spikes would only hurt my tach or radio, which is a newish Sony. I doubt it would have any effect on the CDI box. certainly not any more than when I switch the ignition on and off. already But I will look for a red relay, or modify the black relay.

[AndrewCologne]

Pete, maybe you should take into consideration a 15v TVS diode in combination with a 47 µH Inductor to protect further sensible electronics in your circuit – a fuse like 1a (depending on the load/ampere the target device(s) draw and the inductor/TVS capacity in watts) in front of the inductor/TVS is mandatory as the TVS diode on spikes simply does a short cut where the fuse here protects the components from further damage.
A rectifier diode additionally in series with proper amp capacity could be an option as well as its a plus for preventing from negative voltage spikes, but on the other hand this reduces the voltage by approx 0.5v!

https://www.mikrocontroller.net/articles/Kfz_Spannungsspitzenkiller_/_Transientenschutz
I guess you can use an online translator to switch it to your language.

[proporsche]

i wonder what is inside these aluminium relays inside(my little stash);-)?Never took one apart

[spuggy]

Originally Posted by PeteKz

So I have to get a socket that has 5 pins. Does anyone have a preferred source?

Someone complained in this thread 911 sc Wiring in process that he kept ordering the correct pigtail - but was always sent replacement pigtails with light gauge wire. Rather than the heavier gauge you want for the fuel pump.

Dennis replied in permalink saying

Originally Posted by timmy2

They only make one type of relay base pigtail now.
I can make you the correct one.
PM me if interested.

I'd ask him, I think, if that's all you want, it had to be the correct spec and look like it belonged there.

Kroon make complete fuel pump harnesses for middies and/or SCs, that might also be of interest:

Fuel pump harness for Porsche 911 / 911S / Carrera 3.0 1977 / 911 SC ROW 78- art.no: 911.612.072.14
Fuel pump harness for Porsche 911 SC US art.no: 911.612.072.01

No personal experience with either - but others have spoken in glowing terms of both.

[ahh911]

Hi Pete,

Can't answer 4 or 5 pins, but what I was trying to say earlier is twofold.
You could add the rev limiter that uses the relay.
Or / if you don't attach the electronic rev limiter then it probably won't matter if you have a diode or not as there is nothing for the voltage spike to damage.

Phil

[boyt911sc]

Originally Posted by PeteKz

Gents: I've searched though several past threads about SC fuel pump relays, and I think I have a pretty good understanding of them and how they are wired, so let me confirm that.

Project: I have a CIS 1980 SC engine in my CIS 1973T car. Years ago when I replaced the engine, I swapped around some of the pins in the connector between the engine harness and the circuit panel at the left rear of the engine bay to get it to work and power the gauges correctly. Several years ago I switched from an externally regulated alternator to as WOSP high-amp alternator with internal regulator. So I've had the VR, its connectors ,and other wires hanging out that didn't do anything. A few days ago I decided to clean up the wiring by eliminating most of the extra connectors and wires, and to either repurpose some of the wiring in the chassis harness or add a wire to add an oil pressure light and buzzer to my instrument panel (the 1973 cars did not have an oil pressure light or buzzer, but the later cars did have the light).

I also decided to add a fuel pump relay, because the 1973 cars did not have one--they took power straight from the ignition switch. I also thought this would be a good point to add the safety cutoff function that comes from the switch on the CIS air metering unit. Fortunately, there is an extra round relay socket on the circuit panel next to the CIS relay socket (also round). That extra socket was used for the Sportomatic or MFI system on the 1973 and earlier cars, so I can repurpose it and rewire it for the a fuel pump relay.

The safety cutoff switch is closed to ground when the air metering arm is at the rest position (all the way sown). It opens when the air metering arm lifts. The fuel pump runs when that relay is closed, powering the pump, and stops when that ground through the switch is interrupted (opened). Therefore, the pump needs to be wired "backwards" from the usual relay mode of operation. The power wire to the pump must connect to the relay's normally closed position, so that when power is applied to the relay's internal coil, it opens and stops the pump.

Is this a correct understanding of the wiring and operation of the fuel pump relay in SC's?

And I know I should get a red relay with the internal diode, but I have several black ones handy, so that's what I will use until I have ti working correctly.

Thanks.

Peter,

Sorry to disagree with your post. But your understanding how the FP relay works is FLAWED. Why? With power applied to the relay, the FP will NOT RUN as long as terminal #85 is GROUNDED (controlled by the safety switch). Your understanding how the air flow switch operates is BACKWARD. So the answer to your question is NO.

Tony

[PeteKz]

Spuggy, thanks for the reference to timmy2.

I was thinking about this as I was laying in bed last night (hey, Porsche wiring diagrams are better than counting sheep), and upon further reflection, I think I can split open the hole where the 5th socket should go, and insert a socket and wire in there. If I cut down from the top of the socket, then when I stuff the whole plug back into the circuit board, the metal board will hold it together effectively. That way, I don't have to wait for one to arrive.

In cutting and pasting the text to edit my original message, I made a mistake and mangled it. I have corrected that in the original message.

For completeness in this thread, here is the section of the SC wiring diagram that shows the fuel pump relay and pump circuit.

[pmax]

Originally Posted by proporsche

i wonder what is inside these aluminium relays inside(my little stash);-)?Never took one apart

The relays with the toggle and lights, anyone still sell them ?

[Schulisco]

Originally Posted by PeteKz

Spuggy, thanks for the reference to timmy2.

I was thinking about this as I was laying in bed last night (hey, Porsche wiring diagrams are better than counting sheep), and upon further reflection, I think I can split open the hole where the 5th socket should go, and insert a socket and wire in there. If I cut down from the top of the socket, then when I stuff the whole plug back into the circuit board, the metal board will hold it together effectively. That way, I don't have to wait for one to arrive.

In cutting and pasting the text to edit my original message, I made a mistake and mangled it. I have corrected that in the original message.

For completeness in this thread, here is the section of the SC wiring diagram that shows the fuel pump relay and pump circuit.

See on Andrews Website all states of operation of the FP circuit in pictures:

https://nineelevenheaven.wordpress.com/the-protection-circuit-of-the-fuel-pump/

Browse all states by clicking on the arrows left and right of the circuit picture...then it should be pretty clear how it works.

Thomas

[PeteKz]

Thomas, thanks for the reference. I did look at Andrew's web site too. I understand how the relay works. The interesting task is adapting it to the wiring in my 1973 car with the minimum of electrical surgery.

By the way, my car has a brown/white stripe wire that goes from the Auxiliary Starting Relay connector, on the engine bay circuit panel (for cars with MFI), to the chassis wiring loom. That relay was not installed in my 1973.5T because it had CIS originally, but the connector and wiring was built into the harness. I do not see it on the several wiring diagrams I have consulted. Does anyone know where the other end of this wire comes out of the chassis wire harness?

[AndrewCologne]

Originally Posted by PeteKz

Thomas, thanks for the reference. I did look at Andrew's web site too. I understand how the relay works. The interesting task is adapting it to the wiring in my 1973 car with the minimum of electrical surgery.

Well at least you must install the wire from the air plate sensor to the FP Relay, if your protection circuit should still be driven by that sensor. So ist very simple: recreate the circuit as it is in the diagram from the year your engine belongs to.

Even I do not recommend the follwoing, ... but you also could do it the classic common way and use the oil pressure lamp signal to drive your FP protection circuit, there are several approaches shown in the web.

[proporsche]

Originally Posted by pmax

The relays with the toggle and lights, anyone still sell them ?

I think you can get it at the Porsche ,maybe even our host..it is a tool for 944
I use it for idle set up instead of the wire clip....;-)

[3rd_gear_Ted]

Basic design premise of electrical safety circuits is to use "negative logic".
Switch the ground instead of the power which might not be available in all cases after an incident.

[proporsche]

since we are talking relay ..i have forgotten what is this one for? if i remember it was on 1970-73?
Ivan

[boyt911sc]

Ivan,

The relay has the same 5-pin configuration as the RED RELAY for FP except for the colored pins. It should work for switching terminals #87A & #87 as the Red Relay. Have you decided to come back to US? Thanks.

Tony

[proporsche]

Originally Posted by boyt911sc

Ivan,

The relay has the same 5-pin configuration as the RED RELAY for FP except for the colored pins. It should work for switching terminals #87A & #87 as the Red Relay. Have you decided to come back to US? Thanks.

Tony

Hi Tony.yes thanx ....going back to old US of A ,i am working on it....but waiting for the election results;-)meanwhile i have plenty of work...here...one engine coming from Vienna and one from Brussel...;-)