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Registered User
Join Date: Apr 2007
Location: South Africa
Posts: 83
Fuel Pump relays- differences

I am trying to ascertain whether I have the correct relays in the correct places in my 77 930. I have one red relay in the front fuse box, right at the back of the fuse box, close to the windscreen. In the rear fuse box, only 1 black relay. In photos of various other Porsche's, including Turbo 3.3s, I see some have a red relay in the rear, sometimes with a black one, sometimes not, and some have 2 in the front box, again others not.

What is the difference in function between red and black relays? Should I be running another red in the front or the rear?
Old 07-30-2007, 01:19 PM
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Forced Induction Junkie
WERK I's Avatar
Join Date: Jan 2003
Location: Wisconsin
Posts: 3,291
Aren't the relay positions marked out on the inside of the fuse panel cover for your car?

There are two red relays on the 930's. One for each fuel pump, front and rear. The differences between the red and black relays is there is a diode in the red relay that reduces the EMF "kick" that occurs when the fuel pumps are shut off. The diode provides a path for the current flow instead of arcing through the points of the relay. This is what would happen if the black relay is used instead of the red. The black will do in a pinch, but won't last long.
'85 930 Factory Special Wishes Flachbau
Werk I Zuffenhausen 3.3l/330BHP Engine with Sonderwunsch Cams, FabSpeed Headers, Kokeln IC, Twin Plugged Electromotive Crankfire, Tial Wastegate(0.8 Bar), K27 Hybrid Turbo, Ruf Twin-tip Muffler, Fikse FM-5's 8&10x17, 8:41 R&P
Old 07-31-2007, 05:39 AM
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Registered User
Join Date: Apr 2007
Location: South Africa
Posts: 83
Appreciate your input, indeed there are markings showing the position of the relays, but they do not indicate the type. What was making me consider the positioning of the relays and type was looking at various 3 and 3.3 litres at a concourse, where it seemed some had a red relay in the rear, and those that had red relays in the front had them in different locations in the fuse box, I was trying to figure out if some circuits( other than fuel) needed a red relay and if I had a red instead of a black and if the black relay i have in the rear of my car should in fact be a red one- someone mentioned that the red fuel relay in fact controlled the black relay.

In the rear I was told the black relay was variously- a safety or roll over relay, rear window demister relay, I understand that the small silver rectangular relay with 4 pin connector ( marked I in atatched photo) is in fact the so called safety over boost relay that cuts the fuel pump if things get to hectic on the boost, and also switches the CDI and fuel pumps off sequentially when shutting down.
Old 07-31-2007, 10:54 AM
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