Fuel Pump Relay Failure

[Mark Salvetti]

Originally Posted by proporsche

Mark.. it is a factory relay set up for checking the idle on Carrera models.Some people use a wire i use this tool.

Ah, interesting. Thanks for the explanation.

Mark

[wilsonlaidlaw]

I have found my official Porsche GMBH workshop manual for the original 1977 911S and it looks on its circuit diagram (helpfully in black and white), as if it is the same as Timmy posted above, so requiring a five pole relay for the fuel pump. I have found what I think is a 5 pole SS relay from a competition parts seller in the UK at the Brands Hatch Circuit. However, it is only 17A and for starting overload current flow, I would have preferred 25A. I am waiting for confirmation that it is a non-latching changeover relay and not just a 5 pole with one pole not connected. Strangely if I wanted a latching SS 25A 12V relay, they are easy to find.

Wilson

[tirwin]

Originally Posted by wilsonlaidlaw

My car was originally a 1977 911S not an SC, albeit I am now running gas flowed Motronic fuel injection on a big bore block but with 3.3 Turbo crankshaft. I wonder if that makes a difference? Is the diode inside the standard red 5 pin relay or external? The black relay from the horn circuit, which is I believe a simple make and break relay not a changeover, works OK (at least in the short term) on my car in the fuel injection relay socket.

Wilson

The diode is internal to the relay. If you look at the wiring diagram for the SC, it includes as over-rev protection circuit (rpm limiter). It is one of 3 things that can put a ground on pin 85 (the other 2 being an alarm system, if installed, and the air pressure plate sensor at the engine). When the rpm limit is tripped it puts a ground on pin 85, which effectively shuts off power to the fuel pump.

I believe the purpose of the diode is to act as a "flyback diode". It is the presence of this diode that makes the red relay different from the black ones. Porsche engineers believed it needed to be there. I would not omit it.

[tirwin]

Here is an interesting article on flyback diodes.

https://electronics.stackexchange.com/questions/56322/do-i-need-a-flyback-diode-with-an-automotive-relay

[wilsonlaidlaw]

If I replace the relay with a solid state one, there should not be any requirement for a diode as there is no arc burn on a solid state relay, so no spike to affect the ECU/DEM. My 911 has a rev limiter but it feels like ignition to me but I suppose might be fuel. It cuts in just above my 7200 RPM red line.

Wilson

[uwanna]

Originally Posted by wilsonlaidlaw

If I replace the relay with a solid state one, there should not be any requirement for a diode as there is no arc burn on a solid state relay, so no spike to affect the ECU/DEM. My 911 has a rev limiter but it feels like ignition to me but I suppose might be fuel. It cuts in just above my 7200 RPM red line.

Wilson

The diode is not protection from "arc burn" but rather protection from
back EMF from the collapse of the de-energizing relay coil. It's meant to protect the solid state over rev protection ckt. Therefore, you are correct in that a diode would probably not be necessary as there is no relay coil with a SS relay.

[tirwin]

Ok, I see what you are doing now. I initially thought you were just looking for a different supplier of the same type of relay. Interesting. Keep us posted on what you find.

I had a coil go bad the other day. It was 3, maybe 4 years old. The car had the original black Bosch coil for 30+ years until it died. They definitely do not make ‘em like they used to!

[Dr J]

Originally Posted by uwanna

The diode is not protection from "arc burn" but rather protection from
back EMF from the collapse of the de-energizing relay coil. It's meant to protect the solid state over rev protection ckt. Therefore, you are correct in that a diode would probably not be necessary as there is no relay coil with a SS relay.

A point of clarification. The flyback diode is not there for the relay coil; it is for whatever inductive load the relay is powering. Thus, a flyback diode is definitely recommended even with SS relays.

[wilsonlaidlaw]

Originally Posted by Dr J

A point of clarification. The flyback diode is not there for the relay coil; it is for whatever inductive load the relay is powering. Thus, a flyback diode is definitely recommended even with SS relays.

I am not sure you are correct there. If you look at the wiring diagram, the diode is connected across the primary coil of the relay not the contact side. I think therefore as others have said, it is there in order to short out the collapsing reverse surge current from the collapsing magnetic field of the primary. I have a high current diode from the car side of the battery master switch to earth, in case a rally marshal switches it off after an accident, without switching off the ignition first, to isolate the alternator and so that the master switch will also stop the engine. Again any reverse voltage from the alternator could in theory, damage various solid state devices like the DEM, noise reduction intercom and the digital rally navigation equipment.

Wilson

[proporsche]

all i can add to this is..some of you should send this conversation to Porsche in Stuttgart so they know and tell their people in China how to make these relays reliable again;-)

just my 02 cents....

Ivan

[Dr J]

Originally Posted by wilsonlaidlaw

I am not sure you are correct there. If you look at the wiring diagram, the diode is connected across the primary coil of the relay not the contact side. I think therefore as others have said, it is there in order to short out the collapsing reverse surge current from the collapsing magnetic field of the primary. I have a high current diode from the car side of the battery master switch to earth, in case a rally marshal switches it off after an accident, without switching off the ignition first, to isolate the alternator and so that the master switch will also stop the engine. Again any reverse voltage from the alternator could in theory, damage various solid state devices like the DEM, noise reduction intercom and the digital rally navigation equipment.

Wilson

Ahhh yes. I see it in the diagram now. Interesting since most applications have it on the load side. In that case it seems you don't need the diode for SS relay.

[Avanti]

Originally Posted by proporsche

Hi..i have been working on 911 for many many years ..never had a problem with burning out relays, but as you can experience the quality of the relays is gone..i suggest to find and original Wehrle relay..that will solve your problem.Even new relays are made in China unfortunately so find an old stock..i have plenty of them but am over the pond.....

Ivan

Such a tease showing us all those German relays.

[wilsonlaidlaw]

Originally Posted by proporsche

all i can add to this is..some of you should send this conversation to Porsche in Stuttgart so they know and tell their people in China how to make these relays reliable again;-)

just my 02 cents....

Ivan

My son was sent out to China by a large UK engineering company to try and improve the quality control at their Chinese factory making hydraulic control components. After 6 months he recommended either selling the factory to a local combine, who would then have to take responsibility for the reject rate or simply just shut it down. Getting consistent really high quality appeared to be impossible and having to work via an interpreter did not help either. My father used to say you can have cheap or good but both is very unlikely.

Given that after 7 months, I am still waiting for a software update for my 2018 model Panamera, to correct instrumentation errors (speedometer does not change from MPH to KPH), which Porsche admit was their fault, I suspect getting them to repatriate production of relays for old cars is a non-starter.

Wilson

[proporsche]

hi Wilson...that says it all ..btw i used to live near your Tourtour...do you know there lady, Sears??
I was in Bargemon..............
Ivan

[timmy2]

Perhaps this one?
https://www.waytekwire.com/item/75532/InPower-VCM-10-Dual-Input-Solid-State-Relay-/

Never mind...
http://www.inpowerdirect.com/docs/PDS-88C.pdf

[wilsonlaidlaw]

Originally Posted by proporsche

hi Wilson...that says it all ..btw i used to live near your Tourtour...do you know there lady, Sears??
I was in Bargemon..............
Ivan

Hi Ivan,

I don't know Lady Sears but if you lived in Bargemon, you must have come across the very pleasant and helpful Dutch estate agent, Theo Punt of France-Sud Immobilier in Bargemon.

Wilson

[wilsonlaidlaw]

Originally Posted by timmy2

Perhaps this one?
https://www.waytekwire.com/item/75532/InPower-VCM-10-Dual-Input-Solid-State-Relay-/

Never mind...
http://www.inpowerdirect.com/docs/PDS-88C.pdf

Hi Dennis,

I fear neither of those are suitable. They are dual input and single pole control. What I need is a single input but dual pole changeover output i.e two input terminals and three output. These SS relays seem to be as rare as rocking horse poo in DC in/DC changeover and I am having difficulty tracking one down. If I wanted DC/AC they are easily available.

I have now also received two genuine new, not Üro but Porsche red relays (albeit made in China). So I now have the one plugged in, that I know is a Üro as I found the box and two OEM spare ones. I also have the query in to Wehrle to ask if they might still have some original stock left that they could sell me.

Wilson

[proporsche]

yup i know of him right at the corner in down town;-)

[targa80]

Originally Posted by uwanna

The diode is not protection from "arc burn" but rather protection from
back EMF from the collapse of the de-energizing relay coil. It's meant to protect the solid state over rev protection ckt. Therefore, you are correct in that a diode would probably not be necessary as there is no relay coil with a SS relay.

In my own work experience the first time I had a relay with a diode across the coil was for one purpose of protecting a high current circuit. The diode was installed to prevent relay chatter causing a momentary closing of contacts when the coil field collapsed and created a reverse current flow. The diode prevents the reverse current and preventing the momentary closing of the contacts. Under high current applications the non diode relay would arc the contacts and cause the relay to fail prematurely. Does this theory apply to the relay for the fuel pump? I believe that it does, a diode relay can be used in any circuit but a non diode relay in a high current circuit will suffer premature failure.

[wilsonlaidlaw]

It will be interesting as and when the Üro relay fails and I take it apart, to see if the diode has been omitted and that is what the problem is. I would not be totally surprised.

Wilson