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Join Date: Sep 2002
Posts: 3,580
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The Special Red Relay...what's it protecting?
Arcane electrical question ahead!
For the EEs in the audience, last night I woke up wondering about the following: We know that the Red Relay, used in the fuel pump circuit, differs from the Black Relay, used everywhere else, in that it has a diode between pins 85 and 86. From searching (thanks Jim Sims) I undertand that this diode reduces the power spike through the circuit when it's engaged and/or disengaged. What I don't understand is whether the goal of reducing the spike is to protect a) the fuel pump b) any other devices that are in that circuit, or c) the relay itself. It looks like the 914 actually uses a Black Relay for the fuel pump, and I'm wondering why. Doesn't this question keep YOU up at night too? 
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08-13-2003, 11:43 AM
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Join Date: Dec 1999
Location: TX USA
Posts: 9,804
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Colin,
The quench diode is to protect the air flow switch in the CIS system, specifically, though reduction in system noise is a good idea for the health of the factory alarm control module, too! If the alarm unit dies ... it disables the ignition and you can't start your car!
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Warren Hall, Jr. 1973 911S Targa ... 'Annie' 1968 340S Barracuda ... 'Rolling Thunder' |
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08-13-2003, 12:06 PM
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Thanks for the information, Warren! Maybe the 914 gets away with the black relay because it doesn't have that air plate switch...my '75 doesn't have one either.
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08-13-2003, 01:17 PM
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