Fuel relay wiring explanation please

[StuartH1]

Hi,



I grabbed the above screenshot from a wiring diagram, this isn't my area of expertise but I think I have a basic grasp.
In the fuel pump relay I have an input (86) and output (87) a ground (85). My assumption is that the relay receives an input, which energises and closes a coil between 87 & 30 (direct from 12v source) and sends off power to the fuel pump? But what is the purpose of 87a? The normally closed output for a common relay?

And if my relay were to fail, in an emergency to get home could I short it direct from 30 to 87? Sending power to the pump without the ignition needing to be on or the relay working?

 

[IndischRot]

let me word it a bit differently...
when pin 85 (one side of the coil) is earthed and pin 86 (the other side of the coil) receives 12V then the relay closes and current flows from pin 30 to pin 87.
pin 87a is connected to pin 30 when the relay's coil *isn't* energised. the inclusion of the 5th pin makes the relay more versatile.

if you wanted to hot wire your fuel pump you would indeed put a jumper between pins 30 and 87

 

[StuartH1]

Thanks, that's how I interpreted it.
I'm still not clear on the function of 87a though.

I need to get hold of a proper wiring diagram and see what 87a is connected to, there must be something that needs to receive a permanent 12v connection when the car is off and not running and then lose it when the relay operates the fuel pump and the NC 87a opens.

Failing to an 'open' condition is usually a safety system in my limited experience, so in the event of power loss something will happen even with no power, so for example under normal conditions (closed) it holds contacts open and keeps an alarm from sounding, but once power is cut, the circuit that's holding the contacts apart 'opens', letting the contacts close between a battery and a sounder.

So after all that waffle (lol) I'm just wondering what 87a actually does, other than remain closed from a constant 12v feed.

 

[IndischRot]

As I pretty well said above it's a multi purpose relay; it was used for numerous applications in Porsches of that era. Sometimes you want to switch a load on; sometimes you want to switch a load off. So make one relay and one socket that can be used across the piste.

 

[StuartH1]

As I pretty well said above it's a multi purpose relay; it was used for numerous applications in Porsches of that era. Sometimes you want to switch a load on; sometimes you want to switch a load off. So make one relay and one socket that can be used across the piste.

Yeah I got that, but I'm offshore and the car is in the UK, so I can't go and look and I don't have a wiring diagram, so I wondered what/if 87a is connected to. If it's just a generic relay and all points aren't used then fair enough, I assumed it might be connected to something though and as I've had fuelling issues in the past I thought it would be a good idea to get familiar with it.

I've ordered a new modernised fuse carrier and once I'm home I'll fit it and see what's connected and what isn't.