I just spent the weekend tracking down a short, and, 8 fuses later, I am
happy to report it is solved...but the fuse discussion is interesting none
the less.
Here's something I found interesting regarding the differing theories
manufacturers use with wiring: My daily driver, a 98 Jetta turbo diesel,
has an individual fuse for just about every bulb on the car! in this case,
and without a great deal of study, I would think the devices are
individually protected!
Mike
72 spit (my first lbc, lots of learning to do!)
-----Original Message-----
From: Peter S. <alfapete@pacbell.net>
To: DANMAS@aol.com <DANMAS@aol.com>; spitfires@autox.team.net
<spitfires@autox.team.net>
Date: Wednesday, July 21, 1999 11:56 PM
Subject: Re: Spit fuses
>
>Thanks for the run down on your theory. It makes sense. Also sounds like
>one single fuse to protect many devices is foolish. In that scenario it
>clearly is only to protect the wiring.
>On the issue of circuit breakers (especially those that are thermal
>sensitive and reset themselves when cool) I have this story. An
>acquaintance had a problem where the lights on the car would go out after a
>55 minute drive. His nightly drive home from work was 60 minutes. Each
>night for a few weeks he'd have the lights go out on an unlit country road.
>His wife would get out with a flashlight and walk in front the last mile or
>so. Drove them nuts of course. By the time they got to the house on a
>gravel road the lights would suddenly come back on. I was not aware there
>were breakers of this sort on a car (80-something Mustang) and couldn't
help
>them. I think a fuse would have been a better idea here.
>Peter S
>
>----- Original Message -----
>From: <DANMAS@aol.com>
>To: <alfapete@pacbell.net>; <spitfires@autox.team.net>
>Sent: Wednesday, July 21, 1999 6:51 PM
>Subject: Re: Spit fuses
>
>
>>
>> In a message dated 7/21/99 1:22:25 PM Eastern Daylight Time,
>> alfapete@pacbell.net writes:
>>
>> > Certainly the first function is to protect the wiring. I don't dispute
>> > that. But devices such as a wiper motor which requires a fairly high
>> > current draw / high amp fuse has internal motor windings which are
>smaller
>> > than the wiring going to it. If the wiper is stalled in one position
>(by
>> > snow or something placed above the wipers) the amp draw goes up until
>> > someting gives. Wiring, with its length will disipate heat up before
>> > finally melting. The windings in the motor will cook and you ruin
it -
>> > potentionaly faster than damaging the wire. If you put in a fuse
which
>is
>> > rated higher than normal you certainly could/will ruin the motor
before
>the
>> > wiring has any ill effects. IMHO.
>>
>> Peter,
>>
>> I understand what you're saying, but I must still respectfully disagree
>with
>> you. The wiper motors in our Triumphs only pull 1.2 amps under normal
>usage.
>> Even if the current is doubled under locked rotor conditions, it would
>still
>> only be 2.4 amps, and I think the motor windings can safely carry that
for
>a
>> reasonably long period of time.
>>
>> Larger motors that can be damaged by locked rotor current are usually
>> protected against damage from long term operation in this condition by
>> thermal breakers rather than overcurrent trip devices. That is, they are
>> tripped on high winding temperature rather than over current. There is a
>good
>> reason for this. Most motors will, from time to time, experience brief
>> moments of locked rotor current during the course of normal operation.
For
>> example, a temporary jam on a conveyor belt. Hopefully, the jam will
break
>up
>> by itself, and the operation can continue uninterupted. If not, the
>thermal
>> breakers will trip after a period of time before any damage is done to
the
>> motor. If over current devices were to trip the motor on overcurrent, it
>> would be an instantaneous trip, and it would be a big nuisance having to
>> reset the breaker or install a new fuse unnecessarily for a temporary
>jammed
>> condition that would have cleared by itself if left alone.
>>
>> Taking the wipers and snow example you gave, you can see where this would
>be
>> a nuisance also. If you turned on the wipers, hoping it was a soft snow
>and
>> not hard ice, the wiper motor would instantly blow the fuse, and you
would
>be
>> faced not only with having to clear the snow to get the wipers freed up,
>you
>> would also have the task of getting the snow and ice cleared from the
hood
>so
>> you could open it and replace the fuse.
>>
>> Another thing to consider is how you would need to fuse it. If you put a
>two
>> amp fuse on it, it would have to be the only thing on that fuse. As
>presently
>> configured (Triumphs), the fuse feeding the wiper motor also has about 15
>> amps of other loads on it - heater fan, gauges, turn signals, etc. A two
>amp
>> fuse just wouldn't carry the normal load of the wipers and ANY thing
else.
>>
>> You may be right, the wiper motor may burn up if left on in a stuck
>condition
>> for an extended period of time, but I'm sure it would take much longer
>than
>> the time the average driver would leave the switch on if the wiper blades
>> were not moving. If a driver leaves the switch on that long, as far as
I'm
>> concerned that is the same as putting aluminum foil in the fuse holder to
>> replace a blown fuse. Not something to design for. I know in the times
>I've
>> had wiper blades stuck from ice and snow (I grew up in Indiana, where
>winters
>> were a bit harsher than here in Tennessee), no damage was done to the
>motor
>> even after several tries, hoping the wipers would break free on their own
>and
>> I wouldn't have to get out and remove the ice.
>>
>> There are some situations where the fuse protects not only the wiring,
but
>> the device as well, but these cases are few and far between. I don't know
>of
>> any in a car of the era we're interested in. Power windows might be an
>> example in a modern car, but there the makers use torque switches (I
>think -
>> anyone know for sure?) to shut off the motors when the window travel is
>> obstructed. Having to replace a fuse everytime one of the kids hung out
>the
>> window as it was going up would be a pain (and you certainly don't want
>the
>> motors powerful enough to cut the kid in two!).
>>
>> My purpose in posting what I did was not to correct you, but to get
people
>> thinking about how to use fuses when installing aftermarket devices. If
>you
>> are protecting the device with a fuse, you will put the fuse as close to
>the
>> device as you can. If you are protecting the wiring, you will put the
fuse
>as
>> close to the power source as you can, which is as far away from the
device
>as
>> you can get. Installing a radio with the fuse located right at the radio
>is
>> almost as bad as not having a fuse at all. The power feed to the radio is
>> completely unprotected. It's far, far more likely that you will have a
>> problem with the wire insulation chaffing and exposing the bare wire to
>the
>> car body than it is that you will have a problem inside the radio.
>Especially
>> if the person doing the installation is not experienced in wiring
>techniques,
>> as most of us are not.
>>
>> Regards,
>>
>> Dan Masters,
>> Alcoa, TN
>>
>> '71 TR6---------3000mile/year driver, fully restored
>> '71 TR6---------undergoing full restoration and Ford 5.0 V8 insertion -
>see:
>> http://members.aol.com/danmas/
>> '74 MGBGT---3000mile/year driver, original condition - slated for a V8
>soon
>> '68 MGBGT---organ donor for the '74
>>
>
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