What size relay to bypass fuel pump circuit? | FerrariChat
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What size relay to bypass fuel pump circuit?

Discussion in 'Technical Q&A' started by nathandarby67, Oct 29, 2007.

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  1. nathandarby67

    nathandarby67 F1 Veteran
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    Feb 1, 2005
    8,349
    Mississippi
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    Nathan
    #1 nathandarby67, Oct 29, 2007
    As so many others have done, I am wanting to add a relay to my fuel pump circuit to take the full amperage load off my crappy Mondial fuse box. The fuel pump pin on the wiring harness connector is just starting to turn brown, not nearly as bad as some I have seen pictures of, but still pointing to too much heat. Anyhow, I have a few questions.

    First, what amp rating do I need on the relay? Is a 30 or 40 amp relay sufficient?

    Second, Where is the best place to tap into for the new +12V power source? Do people usually run a new hot wire all the way from the battery, or would it be ok to run the hot wire over to the positive post on the starter where the big positive line from the battery is, and use that line as a 12V source? This would save having to thread a long wire all the way up to the battery, and keep all the new wiring back in the same area of the engine bay. I have already tapped onto this post to power a relay I installed on the starter solenoid circuit. Basically I just want to make sure that having this large cable feed power to the fuel pump, starter solenoid, and starter motor at the same time when cranking the car would be alright.

    Third, I want to run an in-line fuse in this circuit. Which wire should it be on and what size fuse?

    Thanks guys!
     
  2. Steve Magnusson

    Steve Magnusson Two Time F1 World Champ
    Lifetime Rossa

    Jan 11, 2001
    26,506
    30°30'40" N 97°35'41" W (Texas)
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    Steve Magnusson
    #2 Steve Magnusson, Oct 29, 2007
    Yes, either is OK (your stock FP relay is the 30A ...113 type)

    You've got all the right issues in mind here -- it needs to be able to carry a big current (IME the CIS fuel pump draws 9~11A steady-state depending on age -- with newer/tighter = higher). IMO, you probably could use the big +12V wire going to the starter motor especially if you push the clutch in and select N during restarts which reduces the starter motor current from something like ~220A (if you leave the clutch out and select N during restarts) to ~150A -- i.e., your mod for the solenoid + the FP mod won't draw as much current as the ~70A difference between clutch out vs clutch in (and clutch out restarts usually work OK).

    You could probably use either a 16A (same as Mondial8 stock) or a 15A fuse (used on some other CIS-equipped F models). The food chain you want is:

    big +12V source -to- fuse -to- new relay terminal 30

    and terminal 87 of the new relay is connected to the +12V terminal on the fuel pump

    (with terminal 85 of the new relay connected to existing +12V wire going to the fuel pump, and terminal 86 of the new relay connected to ground -- or you can swap 85 and 86 -- doesn't matter).

    Good luck with the mod...
     
  3. nathandarby67

    nathandarby67 F1 Veteran
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    Feb 1, 2005
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    Nathan
    #3 nathandarby67, Oct 29, 2007
    Very thorough information from you as always. Thanks! I now have my "project of the week" all lined up.

    I had NO IDEA that pushing the clutch pedal in reduced the load on the starter motor by such a large amount. That is truly impressive. So I guess "clutch in, stick in neutral" starts are the way to go for these cars, huh?

    Nathan
     
  4. 350HPMondial

    350HPMondial F1 Veteran
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    Feb 1, 2002
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    18 mi from the surf,, close to Pismo, CA
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    Edwardo
    #4 350HPMondial, Oct 30, 2007
    Just use a 12ga wire and a Big switch.
    (20 amp fuse)
     
  5. ckracing

    ckracing Formula Junior

    May 20, 2006
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    Jacksonville,Florida
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    Charles
    #5 ckracing, Nov 5, 2007
    All the above is true. Some of the Guys used connections from the original fuse box relay socket using jumper wires.

    I did it that way at first. Then that part of the fuse box failed.
    I was forced to cut the fuel pump wires(brown). Run a new 12+ with 30amp fuse. New ground wire and find a NEW ignition switch ON wire to attach to the removed relay. Now my fuel pump relay and all the wiring is now bypassed from the fuse box.

    Charles
     

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