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40 amp power relay has a resistor that may prevent blown fuses.

Discussion in '308/328' started by JohnInItaly, Apr 15, 2020.

  1. JohnInItaly

    JohnInItaly Karting

    Feb 5, 2019
    86
    Santa Cruz, California
    Full Name:
    John McCoy
    12 volt 40 Amp relay with a 680ohm resistor added to slow power delivery and may prevent blown fuses when you slightly exceed the amperage upon activating a device, but not upon continued operation. Not a substitute for a major electrical problem, but may prevent annoying fuse changes when the system usually operates just fine. I had these made at the Guilera relay factory in Spain after discussions about Birdman fuse blocks (that I absolutely love) helping to prevent blown fuses on window motors, headlamp activators, and radiator fans that were drawing too much power when first activated (with the old original fuse blocks), but not during normal operation.

    First wiring diagram is the normal # 020308 40Amp relay, and second wiring diagram is this # 020346R 40Amp relay with resistor. Simple plug in option may allow you to delay major electrical system revisions until have time, money, and energy later. Same cost for either at $8.99 each. Search eBay for either part number.




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  3. thorn

    thorn F1 Rookie
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    Aug 7, 2012
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    What is the purpose/benefit of the bracket?
     
  4. JohnInItaly

    JohnInItaly Karting

    Feb 5, 2019
    86
    Santa Cruz, California
    Full Name:
    John McCoy
    This relay fits literally hundreds of applications. While I believe that all Ferrari and BMW relays plug into a solid electrical panel, some motorcycles use a loose plug in a wiring harness and then confine the relay by attaching it to something solid with the bracket, like the plastic battery box. The bracket slides in and out and can be discarded if not needed. The photo attached here shows it halfway in, to indicate movement.
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  5. conan

    conan Formula Junior

    Nov 13, 2011
    375
    I can not see how the resistor can prevent blown fuses.

    It should have been a diode used to limit the generated high voltage when deenergizing the coil. Sometimes a resistor is used since it has no polarity requirement, but a resistor is not as good as a diode.

    The real benefit with a diode/resistor across the coil is that it prevents arcing (and premature failure) of any relay or switch used to energize the coil.
     
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  6. JohnInItaly

    JohnInItaly Karting

    Feb 5, 2019
    86
    Santa Cruz, California
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    John McCoy
    A more complicated diode would be better than a simple resistor, in theory. Diodes that I have seen are much larger than resistors and would not fit into the same small space of the original relay, and I cannot imagine how someone would make a larger diode based system work without a total electrical redesign, and there is nothing available to plug into the relay position directly. On my own car, I spent a lot of time and money changing the fuse blocks, a complete redesign of the cooling fan system, and I took the doors apart to work on my window lift motors. I no longer have a need for help with my own fuses but for EIGHT DOLLARS AND NINTY-NINE CENTS, a simple plug in relay swap could be either a convenient trouble shooting tool or short term solution, with no chance of harmimg anything.
     
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  8. johnk...

    johnk... F1 Veteran
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    Jun 11, 2004
    8,961
    Am I reading this wrong? The resistor is across the coil in the relay and has nothing to do with the circuit that is activated by the relay closing?
     
  9. mike996

    mike996 F1 Veteran

    Jun 14, 2008
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    From what I see in the diagram, that is correct - the resister has nothing to do with the load in the circuit being activated. Therefore, if you are using this relay where the circuit involved still passes through the fuse, there wouldn't be any difference in the instantaneous load across the fuse when the relay is activated.
     
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  10. conan

    conan Formula Junior

    Nov 13, 2011
    375
    Just to answer on the diode thing, it does not require a large diode to clamp the excess voltage across the coil. But, a resistor is cheap and may have a longer service life than a diode.
     
  11. mike996

    mike996 F1 Veteran

    Jun 14, 2008
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    Since the resistor only reduces the power going to the coil, adding this device is increasing the electrical load on the system. Where you used to have, say, a motor that when turned on, drew X watts of power through a fuse, now you have that motor AND the relay's coil drawing power through that fuse. So you have added the power drawn by the relay coil to the circuit load. Frankly, it increases the chance of blowing a fuse! ;) Of course, it is entirely possible that installing one of these reduced blown fuses because in the process, wiring/fuses/contacts/whatever where cleaned up. Did the OEM fuses create problems when these cars were new? It seems like they are a problem now, 30+ years later but were folks constantly taking their new 308's back to the dealer for electrical work?

    FWIW, if you add the relay coil to the original circuit but remove the motor's load and place it (through the relay) on a new, properly sized/fused wired circuit, that's a whole different deal.
     
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  13. conan

    conan Formula Junior

    Nov 13, 2011
    375
    Yeah, on our old cars, this resistor thing is too late to add anyways. Years of arcing in the headlamp switch has already happened ...

    I noticed on one relay that it was 680 ohms in parallell which would add as much as 18 mA of current when the relay is active. I would not worry about it.
     
  14. Steve Magnusson

    Steve Magnusson Two Time F1 World Champ
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    +1 -- Keeping careful, and consistent, control of what's on terminals 85 and 86 would be a real challenge -- IME, Ferrari often even has the wires physically switched on those two terminals in the as-built condition vs what's shown on the schematic ;)
     
  15. JohnInItaly

    JohnInItaly Karting

    Feb 5, 2019
    86
    Santa Cruz, California
    Full Name:
    John McCoy
    I have opened a can of worms!
    From the internet:
    Relays can feature a built-in resistor and will fit within a standard fuse box. While the relay permits a low current flow circuit to control a high current flow circuit, the internal resistor protects sensitive equipment against voltage spikes. The best solution for full voltage applicable to head lamps, auxiliary lamps, fog horns, motors for fans, window lifters, air conditioners, heated rear windows, or the myriad of other equipment existing on vehicles today.
    I did find some relays with diodes small enough to plug into the original socket, BUT (from the internet): When connecting a coil surge protection circuit to these relays, we recommend a zener diode with a zener voltage of 24 V or higher, or a resistor (680Ω to 1,000Ω). When a diode is connected to the coil in parallel, the release time will slow down and working life may shorten.
     
  16. thorn

    thorn F1 Rookie
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    Aug 7, 2012
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    I fail to see how a resistor on the solenoid voltage circuit has any effect whatsoever on the current flow for the load itself. At best, it's only going to protect the relay from damage.
     
  17. Steve Magnusson

    Steve Magnusson Two Time F1 World Champ
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    +1 - this sentence: "the internal resistor protects sensitive equipment against voltage spikes" is very misleading. It means: "the internal resistor protects sensitive equipment actuating the relay against voltage spikes that occur when the relay un-actuates" -- not that the resistor is beneficial for the load side equipment in any way.
     
  18. mike996

    mike996 F1 Veteran

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