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Accessories draw (electrical)
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Author:  slafolle [ Thu Dec 21, 2006 1:34 pm ]
Post subject:  Accessories draw (electrical)
I'm upgrading the electrical system on a 1966 Dart - going to solid-state regulator, headlamp relays, replacing old wiring, etc. I don't know much about electricity, just the basics. A couple questions:

Is there some way to determine the current draw of accessories such as headlights, heater blower (on high), cigar lighter, etc.? The FSM? Maybe a chart online somewhere? This old quote from Dan, regarding installing a main fuse for pre-'63 cars got me thinking:
Quote:
. . . The proper rating can be estimated by totalling up the wattage of all of the car's electrically-operated items, dividing by 13, adding 8 to this result, and rounding down to the closest available slow-blow/time-delay maxi-fuse. Such a fuse would get installed inline in circuit A-1, the main feed
I'm going to keep my fusible link in place, but I'd like to double-check that the correct accessory fuses are installed (I don't trust anything the previous owners did). The amp ratings printed on the fusebox are illegible.

Also, the bulkhead connector is melted where the alternator Bat wire connects. I'm thinking of drilling through this junction and running a new, thicker-gauge wire straight thru. Is there a better way of doing this? I plan to eventually run a shunt circuit from the alt to the battery, but for now I'd just like to clean up the mess.

Finally, can you recommend a website or publication that will help educate me on all this? I've read through Dan's tech page and a few other electrical theory websites, but I suspect there's more out there that I'm not seeing. Thanks!

Steve
Author:  slafolle [ Tue Dec 26, 2006 12:18 pm ]
Post subject: 
Re: good source of auto electrics info, I picked up a copy of:

Diagnosis and Troubleshooting of Automotive Electrical, Electronic, and Computer Systems (3rd Edition) by Halderman and Mitchell.

Used copy about $10. including shipping from Amazon.

I have a couple other auto tech books which I don't like by Halderman and Mitchell (aren't those the Watergate guys?), but I'm finding their electronics book very useful.

Cheers,

Steve
Author:  dakight [ Tue Dec 26, 2006 3:15 pm ]
Post subject: 
The Factory Service Manual should give you the fuse types and ratings for your car.
Author:  SlantSixDan [ Tue Dec 26, 2006 4:48 pm ]
Post subject:  Re: Accessories draw (electrical)
Quote:
I'm upgrading the electrical system on a 1966 Dart - going to solid-state regulator, headlamp relays, replacing old wiring, etc.
Sounds like a good project.
Quote:
Is there some way to determine the current draw of accessories such as headlights, heater blower (on high), cigar lighter, etc.?
Well, anything that you know the wattage of, you can determine the current draw. Headlamps, for example, are stamped on the back with their wattage. Typical for sealed beams is 60/50w (high/low beam). That is at 12.8v, so we divide 60 watts by 12.8 volts and get 4.7 amps, then we multiply it by two (because of two headlamps!) and we get 9.4 amps, then round up to 10 amps for purposes of calculating total amperage.

Likewise, we can do-up all the other exterior lighting. Brake and turn signal lamps are 27w apiece (= 2.1 amps apiece), tail and parking lamps are 8w apiece (= 0.63 amps apiece)

It gets harder when you don't know the wattage. You can kinda reverse-engineer the approximate amperage by looking at how the factory protected the circuit. *generally*, the fuse rating will be the circuit amperage plus a margin of between 30 and 60 percent. So, for example, say you find a circuit protected by an 8A fuse. You're probably looking at a circuit that routinely carries 5 to 6 amps. Lights are generally given a larger margin, because they don't tend to draw "somewhat too much" current. Either they draw correct current or there's a dead short on the line and they draw tons of current, so it doesn't really matter if a lighting circuit routinely carrying 10A is protected by a 15A or a 20A fuse...that dead short is gonna blow either in a hurry!

Motors, on the other hand, can draw too much current—it's not an "all the way right or all the way wrong" situation as with lights, so the margin is smaller to prevent the cumulative damage that occurs when a motor draws somewhat too much current for a prolonged period of time, heating up the wires and switches.

The job of the main circuit protection is to prevent catastrophic destruction of the car's wiring in the event of a dead short in a high-current portion of the wiring. The main fuse doesn't want to know about non-catastrophic faults in individual circuits; that job is handled by the individual-circuit fuses or circuit breakers. You can use the method I previously wrote (which you quoted), and if you want to double-check your maths, you can total up the amps, rounding your 0.5s up and your 0.4s down, add 40 to 45 percent, and see if you come up with a similar number.

Remember also that when you put the headlamps on relays, you will be removing the headlamps from the car's existing harness and providing them with their own circuit protection.
Quote:
I'm going to keep my fusible link in place, but I'd like to double-check that the correct accessory fuses are installed (I don't trust anything the previous owners did). The amp ratings printed on the fusebox are illegible.
Oh, izzat all? Well, the FSM will have a fuse chart, and you can probably also find the values in the wiring diagram you can pull down for free here.
Quote:
Also, the bulkhead connector is melted where the alternator Bat wire connects. I'm thinking of drilling through this junction and running a new, thicker-gauge wire straight thru.
Good idea. I choose to disregard the blather about how you've got to replace the ammeter with a voltmeter.
Quote:
I plan to eventually run a shunt circuit from the alt to the battery
How 'bout doing it in a way that'll let you keep your ammeter? Read the instructions for putting together an "active shunt" in this thread.
Quote:
Finally, can you recommend a website or publication that will help educate me on all this?
I very heartily unrecommend the nonsense you can read at the MadElectrical site.
Author:  slafolle [ Sun Feb 11, 2007 7:33 pm ]
Post subject: 
Thanks Dan! This will be a big help for verifying/determining fuse ratings and wire size.

Strange that I didn't notice your reply 'til now, perhaps due to the holidays.

Cheers,

Steve
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