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Blowing 20 amp fuse on my Ac/heater blower

Discussion in 'Boxers/TR/M' started by silvergts1998, Apr 1, 2008.

  1. silvergts1998

    silvergts1998 Formula 3

    Apr 10, 2005
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    Adam
    I figure I would start a different thread on this. When I start my AC/Heater blower, I keep blowing the 20 amp fuse. I looked at the fuse panel behind the fuses and relays and everything behind it seems to be okay, but not totally sure. I figure if there was a short somewhere, it would blow the fuse immediately, but it doesn's so what else could cause my ac/heater blower 20 amp fuse to blow? Any leads where to look or how to test? When it comes to electricity...I can be lost.

    Thanks in advance.
     
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  3. vincenzo

    vincenzo F1 Rookie

    Nov 2, 2003
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  4. samsaprunoff

    samsaprunoff F1 Rookie
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    Jun 8, 2004
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    Good day Adam,

    You are correct in that if there was a short, then the fuse would blow immediately (infinite current). If it blows sometime later after switching on the circuit, then most likely the current being drawn is higher than the rated fuse size. How much current drawn over the rated fuse size determines how fast the fuse blows. So, a current that is much, much larger than the fuse size, will cause the fuse to blow very quickly. If the current drawn is just slightly over the fuse size, then the fuse will blow sometime later. The exact time is dependent upon the fuse material, the rated fuse size, and the current being drawn.

    Just to rule out an intermittent short, I would measure the current being drawn on the fully loaded circuit (i.e. fans on, etc). If this current is well below the rated fuse size (usually 1/2 the rated fuse size), then you probably have an intermittent short. If the current is quite close to or greater than the fuse size, then you have a component that is potentially failing... perhaps a (dragging) fan motor, switch, etc that is on its way out. It is a difficult diagnoses, as one would need to know all of the components that are fed from this cranky fuse circuit and also what their respective "normal" current draw is. It is indeed a time consuming task, but would be necessary to resolve your problem.

    Cheers,

    Sam
     
  5. Steve Magnusson

    Steve Magnusson F1 World Champ
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    Jan 11, 2001
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    That's a tough one because most handheld multimeter can only measure up to 10A (and it would be best to measure just what the current is for the brief period that it works as Sam suggested -- i.e., 14~15A might not be unreasonable for a 20A fuse, but 19A would be unreasonable).

    One factor that can be in play here is I^2*R heating at the fuse terminal connections. Because the current is so high, if the connections to the fuse terminals have even just a tenth of an ohm of contact resistance, there will be significant heating (like 10~20W). This heat gets conducted down the metal terminals and can soften the fusible element so that it blows at less than the rated current. For a test, one thing you might try is adding say two 6" long jumper wires (where each jumper wire has a male spade terminal to go into the fuse socket and the other end has a female spade terminal to go onto the fuse terminals) so the fuse is hanging out in free air. If the male spade connection(s) into the fuse socket are still crappy (i.e., a tenth of an ohm resistance), the I^2*R heat will still be generated there, but the extra length of wire in between should help the fuse run cooler (assuming the connection from the female spades to the fuse terminals are good). If this extended the time to the fuse blowing (or solved the problem) that would be a sign that heating at the stock connections is the trouble.

    Good hunting!
     
  6. silvergts1998

    silvergts1998 Formula 3

    Apr 10, 2005
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    I will have to give this a shot. Knowing myself I will figure it out one way or another because it bugs the crap out of me.

    Thanks for advice and keep it coming. I will take pics of the progress to help others down the road.
     
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  8. johnvwatts

    johnvwatts Formula Junior

    Jul 7, 2007
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    I had this problem on my 365BB. Turned out that the fan was seized. If its in the same place as a 365BB and I think it is, brake fluid can drip out of the top up pots and causes it to rust. It was only £50 anyway so worth a go - if its the same type.
     
  9. silvergts1998

    silvergts1998 Formula 3

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    I am wondering if it's my blower too. It was making a funny noise which could mean that the motor was going bad and causing the fuse to blow. Infact I can have the blower not blowing any air, but when I turn it up I don't get any air and then a few seconds later it blows the fuse. If I leave the blower off but turn on the floor vents which is desplayed by LEDs, it doesn't blow the fuse....so maybe my dash is coming apart here soon.
     
  10. johnvwatts

    johnvwatts Formula Junior

    Jul 7, 2007
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    If its the same as a 365BB you get to it in the luggage area at the front - four screws easy to remove the panel that covers it just below the screen.
     
  11. Shamile

    Shamile F1 Veteran

    Dec 31, 2002
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    Shamile
    Dear Ferraristi,

    Ahhh, if only it were so.

    Unfortunately, I've got the same problem...seized blower motor. My AC is really cold but no blower to blow it unless I'm moving.


    The blower motor is in a sealed black evap box know by the brand name boraletti. The box is sealed and to get the motor out you have to remove the dash, coolant lines, heater lines and lower portion of the dash / shifter center console.

    The part is no longer available and at last price was $3500.00 ( Ricambi) Ferrari considered it a single part #, so no explosion view in the parts manual from Ferrari is available. Also, since Ferrari considers it a single part, Ferrari will remove it from my dash but will not open the box, repair the motor or warranty the repair.

    So....it's a do it yourself. I just have to get around to it...or as soon as my Lambo is back on the road, I can pull the dash. I'm sure once you do all the work, the blower motor is probably just a $100.00 rebuild. To get to the motor.....ahh, priceless !

    Adam, this task is just up your alley!

    ....and you thought you only had to do stupid stuff with your Lambo. :D


    Shamile

    Freeze...Miami Vice !
     
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  13. silvergts1998

    silvergts1998 Formula 3

    Apr 10, 2005
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    #10 silvergts1998, Apr 10, 2008
    Last edited by a moderator: Sep 7, 2017
    Does the blower share the same part number as the 512 TR? I think my blower motor is on it's way out. Do you see it in this exploded view? Kinda hard to recognize the blower.
    Image Unavailable, Please Login
     
  14. Shamile

    Shamile F1 Veteran

    Dec 31, 2002
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    Dear Ferraristi,

    Yes, that is the evap box that Ferrari used to sell as a single part #. This is the only pix from the parts manual. When I was at dealer, we went through all the TR, 512, M manuals and this was the only pix describing the HVAC.

    The blower is sealed inside the box. You can't see it from the diagram.


    Shamile

    Freeze...Miami Vice !
     

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