View Single Post
full quack full quack is offline
Registered User
full quack's Avatar
Join Date: Apr 2007
Location: Seattle-ish
Posts: 1,169
Send a message via Yahoo to full quack
Just FYI for all you folks that were wondering WTF is with the yellow relay.

Copied from over on Rennlist by PorscheMD....

Okay I have the answer to "why Porsche put the magic Yellow relay in the fuel pump circuit instead of a regular red relay". For starters the red relay and the yellow relay pin connections are the same so a red relay will work. As you know, in the yellow relay there is a circuit board with diodes and transistors on it. I enlisted the help of a great friend who is an EE to tell me what the circuit board does. Between my understanding of the fuel pump circuit and his understanding of the circuit board I think I have the answer. As many of you know when you have the key in the on position and the engine off, if you push the sensor plate down or disconnect the plug at the plate the pumps will run. This switch on the plate is a very unstable "switch" in the world of electronics being it is mechanical and moves very sudden given its environment. So, there is potential for the plate to make contact or bounce against the contact and very momentarily cut the fuel pump off if the plate is swung violently up. (high revs, high engine load, quick shift type of scenario). Within the circuitry of the yellow relay there is a de-bounce function on pin 85 which is the ground potential coming from the plate switch to the tune of about 282milisec. What that means is if the plate for some reason is swung up and makes contact with the switch and completes the ground path to pin 85 of the yellow relay while the engine is running there is a delay of approx. 282milisec. (which is about two blinks of the eye) before it completes the circuit and cuts the fuel pump off. This potential for cutting the pump off in a turbo motor even for a short amount of time or if done repeatedly could theoretically be very damaging to the engine as we all know. So, the red relay will work but you run the risk of cutting the fuel pump off momentarily if you use one. Perhaps its a little overkill on Porsches end but very well thought out I think...

Old 09-28-2017, 12:45 PM
  Pelican Parts Catalog | Tech Articles | Promos & Specials    Reply With Quote #13 (permalink)