Menu toggle

Fuel relay wiring explanation please

StuartH1

New member
Member
Hi,

Screenshot 2025-07-09 at 6.37.30 PM.jpg

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?
 
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
 
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.
 

Posts made and opinions expressed are those of the individual forum members

Use of the Forum is subject to the Terms and Conditions

Disclaimer

The opinions expressed on this site are not necessarily those of the Club, who shall have no liability in respect of them or the accuracy of the content. The Club assumes no responsibility for any effects arising from errors or omissions.

Porsche Club Great Britain gives no warranties, guarantees or assurances and makes no representations or recommendations regarding any goods or services advertised on this site. It is the responsibility of visitors to satisfy themselves that goods and/or services supplied by any advertiser are bona fide and in no instance can the Porsche Club Great Britain be held responsible.

When responding to advertisements please ensure that you satisfy yourself of any applicable call charges on numbers not prefixed by usual "landline" STD Codes. Information can be obtained from the operator or the white pages. Before giving out ANY information regarding cars, or any other items for sale, please satisfy yourself that any potential purchaser is bona fide.

Directors of the Board of Porsche Club GB, Club Office Staff, Register Secretaries and Regional Organisers are often requested by Club members to provide information on matters connected with their cars and other matters referred to in the Club Rules. Such information, advice and assistance provided by such persons is given in good faith and is based on the personal experience and knowledge of the individual concerned.

Neither Porsche Club GB, nor any of the aforementioned, shall be under any liability in respect of any such information, advice or assistance given to members. Members are advised to consult qualified specialists for information, advice and assistance on matters connected with their cars at all times.

Back
Top