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Date:         Tue, 29 Sep 2009 22:16:16 -0700
Reply-To:     Rocket J Squirrel <camping.elliott@GMAIL.COM>
Sender:       Vanagon Mailing List <vanagon@gerry.vanagon.com>
From:         Rocket J Squirrel <camping.elliott@GMAIL.COM>
Subject:      Re: No High Beams
Comments: To: Mike <mbucchino@charter.net>
In-Reply-To:  <4E39705A74E048AF8264AE00A5EF612F@mike2d93581d7f>
Content-Type: text/plain; charset=ISO-8859-1; format=flowed

> ... corroded contacts in the bulb sockets, poor > grounding (both common complaints on our older Vanagons), and corrosion > that 'wicks' up into the wire strands on the ends (invisible under the > insulation), and it all adds up to more total, continuous current draw.

Nuh-uh. Nothing you just described increases circuit current! It all increases series resistance, which reduces current.

(If someone wants to chime in here with a description of how electrical bits in something simple like the brake light circuit can age in such a way that the on current is greater in an older van than in a new one, it would be helpful.)

Moving on, there's no way -- no way -- a short to ground on a circuit fused with a 16A fuse would cause 18 gauge wire to glow before the fuse pops on account of the fusing current for 18A gauge wire is more than 80 amps.

Yes, the current-handling charts for wire assume open air -- so let's derate the wire to 75% of its free air rating when bundled. This would reduce the ampacity of 18 gauge wire from 13A free air to about 10A bundled.

If we were talking 15A, just below the fuse's rating, then I think we'd be talking trouble. But 7A? Ptui! I don't see fireworks happening on account of A) the brakes are not on continuously, and 2) the normal load is half that -- it's only when I've got the trailer hooked up that we see 7A.

For those reasons I still predict that the one 18 gauge wire going from the brake switches to the brake lights will outlast the engine, unless someone sneaks up in the middle of the night and slashes it.

All that aside, the bottom line is that your deeper point is right-on: this is all back of envelope calculations and guessed egg-headery. I should measure the current.

(We're taking bets that when the brakes lights are on the current is pretty close* to the values I calculated.)

============= * "Pretty close" is the fudge factor required to allow for current tolerances on 21W FLAPS brake light bulbs. -- Mike "Rocket J Squirrel" Elliott 84 Westfalia: Mellow Yellow ("The Electrical Banana") 74 Westrailia: (Ladybug Trailer company, San Juan Capistrano, Calif.) Bend, OR KG6RCR

On 9/29/2009 6:37 PM Mike wrote:

> It's not as much about in-rush, start-up current or even continuous > current draw in a normal circuit. You could install a slow-blow fuse of > the original rating to counter start-up in-rush currents. > But, now factor in corroded contacts in the bulb sockets, poor > grounding (both common complaints on our older Vanagons), and corrosion > that 'wicks' up into the wire strands on the ends (invisible under the > insulation), and it all adds up to more total, continuous current draw. > This gets the entire circuit warm (even hot!) and will break down > insulation and plastic parts over time (remember how our headlight > switches melt down even with an all-stock circuit?). > You need to do an actual ammeter measurement on the van, not a > theoretical mathematical calculation based on book and diagrams. Your > circuit is far from new and is now modified from stock. > When a conductor shorts to ground, accidentally, anywhere along it's > entire length, you now have a wire that'll blow (and WILL glow!) before > the higher-rated fuse will blow. As I said, it's not a good scenario, > no matter how you like to ratonalize it to yourself. You MUST upgrade > the conductor to a larger cross-section to prevent this possibility. > Alternatively, you could also add another conductor, in parallel, > alongside the existing wire, but I don't like that idea much for a > couple of reasons (what if it come apart somewhere?). > Also, your wires are not running in "free air", but are cabled in a > harness alongside conductors, and thusly, the circuit needs to be > 'de-rated' accordingly. This means that your calculations are wrong, > so, an engineer, you're not! (neither am I, but I use my > 'book-knowledge' and real-world experience to guide me in making > decisions to 're-engineer' things sometimes....) Hope I've explained it > properly. > > Mike B. > > > ----- Original Message ----- > *From:* Rocket J Squirrel <mailto:camping.elliott@GMAIL.COM> > *To:* vanagon@GERRY.VANAGON.COM <mailto:vanagon@GERRY.VANAGON.COM> > *Sent:* Tuesday, September 29, 2009 8:58 PM > *Subject:* Re: No High Beams > > I don't dispute your knowledge and experience. > > Let's see how closely VW cut things. > > 97.58 (Bentleys) shows the brake light switches fed by a single 18 gauge > wire through a 8A fuse (S8). 18 gauge wire is rated to carry 13 amps, > continuous, in free air, forever, if it has insulation rated for 60C. > There are insulations rated for higher-temperature operation, and > the wire > can be run up to 18A or more with better insulation. But I don't > know what > kind of insulation is used in the Vanagon, so I'll use this rating, the > most conservative I could find. > > The two brake lights on the Vanagon are 21W, so they each draw 1.8A, > steady-state, although the turn-on surge of a tungsten filament can > exceed > the steady-state value by 5 to 10 times because a cold filament has > lower > DC resistance than a hot one. > > So the initial turn-on current of the two bulbs together can be anywhere > between 18 to 36 amperes, but that surge will be brief - somewhere > in the > tens to hundreds of milliseconds. Less than a second, anyway. > > Clearly the steady-state current draw of the two bulbs (3.6A) is well > below the 13A current rating of the wire and the 8A fuse. The fuse can > handle the brief turn-on surge. > > When I doubled the load by adding a pair of 21W trailer lights, the fuse > blew the moment I stepped on the brakes for the light test. Maybe > because > the steady-state current (7.2W) exceeded the 8A rating of the fuse -- > that's darn close (anyone know the tolerances on these cheap > fuses?). More > likely, doubling the inrush current to somewhere in the range of 36 > to 72 > amperes popped the fuse. > > But doubling the brake light loading doesn't exceed the current carrying > capacity of the wire. "Noxious smoke and fumes" are not likely to result > from passing 7.2A through wiring rated to 13A @ 60C insulation. The > inrush > current of the bulbs is too brief to heat a length of wire 12 or > more feet > long by more than a few degrees. It certainly won't "glow like a > lightbulb > filament" even if I was passing 16A through it steady-state due to a > wiring fault. It would get darn warm and eventually the insulation would > fail somewhere. > > Going from 3.6A to 7.2A in wiring very conservatively rated for 13A is > safe enough for me, especially since I only drive with the trailer a few > times a year. I reckon the engine will give out before the wiring will. > > BUT -- Mike B. is right: DO NOT try this at home. Just because I > jump off > a bridge doesn't mean you should. I do not need a phone call from your > widow's lawyer. Never modify any wiring or electrical accessories on > your > van unless you know what you are doing and are willing to accept the > risks > of a miscalculation. > > -- > Mike "Rocket J Squirrel" Elliott > 84 Westfalia: Mellow Yellow ("The Electrical Banana") > 74 Westrailia: (Ladybug Trailer company, San Juan Capistrano, Calif.) > Bend, OR > KG6RCR > > > > On 9/29/2009 5:16 PM Mike wrote: > > > You should NEVER swap up to the next size fuse, when any type of > circuit > > fault causes the correct size fuse to keep blowing. > > The fuse rating is engineered to blow at nearly the maximum > > current-carrying capacity of the wire. If you add a greater load > to an > > existing circuit, and that causes the original size fuse to blow, > that > > means that the wiring is pulling more amps than the wire size is > able to > > safely handle. If you proceed to install the next-size-larger > fuse, you > > now have created a fire hazard that could cause a total loss of your > > treasured vehicle. When a wire draws too much current, it gets hot, > > melts insulation, the copper can even glow like lightbulb > filament along > > its entire length! Not good! At this point the wire becomes the > > weakest link and 'blows' instead of the fuse! Ii's also capable of > > filling the vehicle with noxious smoke and fumes. Do you want to > keep > > your family in this vehicle? I can't recommend it. > > > > Mike B. (licensed FAA aircraft mechanic, and licensed electrician > in 2 > > states) > > > > ----- Original Message ----- > > *From:* Al Knoll <mailto:anasasi@GMAIL.COM> > > *To:* vanagon@GERRY.VANAGON.COM > <mailto:vanagon@GERRY.VANAGON.COM> <mailto:vanagon@GERRY.VANAGON.COM> > > *Sent:* Tuesday, September 29, 2009 7:09 PM > > *Subject:* Re: No High Beams > > > > Actually, Rocket J. you can discern the wire size to the > brake lights by > > gazing at the current track in the bently. With that in mind > you can > > estimate the current carrying capacity, then see if the total > draw > > exceeds > > the reasonable capacity of the wire. If it does't then just > fuse to the > > next higher value. If it does exceed the RCOFTW by some you > might > > have to > > run a second circuit. Works for all accessories of course. > > > > Pensionerd. > > > > On Tue, Sep 29, 2009 at 7:20 AM, Rocket J Squirrel < > > camping.elliott@gmail.com <mailto:camping.elliott@gmail.com> > <mailto:camping.elliott@gmail.com>> wrote: > > > > > Along those lines, I instantly blew the fuse for the brake > light > > circuit > > > when I connected up my little trailer and tested the > lighting. Not a > > > miswiring, just the additional load of the trailer's two > little brake > > > lamps was sufficient to pop the fuse. VW must rate the fuses > > pretty close > > > to the expected load current so even a non-shorting fault is > > sufficient to > > > open the fuse. Had to swap up to the next-sized fuse there. > > > > > > -- > > > Mike "Rocket J Squirrel" Elliott > > > 84 Westfalia: Mellow Yellow ("The Electrical Banana") > > > 74 Westrailia: (Ladybug Trailer company, San Juan > Capistrano, Calif.) > > > Bend, OR > > > KG6RCR > > > > > > > > > > > > On 9/29/2009 4:15 AM Frank Condelli wrote: > > > > > > In a message dated 28/09/2009 3:14:30 P.M. Eastern > Daylight Time, > > >> LISTSERV@GERRY.VANAGON.COM > <mailto:LISTSERV@GERRY.VANAGON.COM> <mailto:LISTSERV@GERRY.VANAGON.COM> > > writes: > > >> > > >> If you were to read my _Headlamp Relay Upgrade_ > > >> (http://frankcondelli.com/hdltrela.htm) webpage. It > > specifically says > > >> that it is necessary > > >> to change the 10 amp fuses to 15 amp when installing the > higher > > wattage > > >> bulbs. > > >> > > >> > > >> > > >> Mark Drillock got it, right on the button. Both fuses > 9 and > > 10 were > > >>>>>>> > > >>>>>> blown. > > >> I guess those little 10 amp fuses don't like the bigger > lamps I > > have in > > >> both > > >> units. And since I drive at night very seldom they > could have > > been out > > >> for > > >> some time.<<<<<<<< > > >> > > >> > > >> > > >> > > >> > > >> Cheers, > > >> > > >> Frank Condelli > > >> Almonte, Ontario, Canada > > >> '87 Westy & Lionel Trains (_Collection for sale_ > > >> (http://frankcondelli.com/trainsal.htm) ) > > >> _Frank Condelli & Associates_ > > (http://frankcondelli.com/busindex.html) > > >> - > > >> Vanagon/Vanagon Westfalia Service in the Ottawa Valley > > >> _Vanagon Stainless Steel Exhaust Systems_ > > >> (http://frankcondelli.com/exhaust.htm) > > >> _BusFusion_ (http://www.busfusion.com/) a VW Camper > camping event, > > >> Almonte, ON, June 11 ~ 14, 2009 > > >> > > >>


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