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Username Post: Power at headlight switch vs. fusebox        (Topic#342280)
LGK 
Contributor
Posts: 195
LGK
Loc: Cincinnati, OH
Reg: 04-26-14
01-29-17 07:21 AM - Post#2674078    

As I understand it from the detailed shop manual schematics, the stock '54 3100 only had that simple firewall mounted in-line fuse box for the Tail and Stop lights. the truck I recently bought is far from stock with a 350 chevy/auto trans/1 wire alternator, etc. Everything works well.
The previous owner did install a 12 position ATO type fuse panel under the dash which is great but did not label any functions.
I already made up a replacement fuse panel (not installed yet) and a relay (horn, led flasher, spare) panel. The fuse panel I built has both a Switched and an Unswitched section.

Questions/comments:
1) I want to get rid of that firewall mounted in line stock fuse box and fuse the Tails and Stops to my own panel.
2) Here's my real question and just having a hard time understanding it. If the 12 position fuse panel that the previous owner installed (or the one I built for that matter) has battery power going to every terminal then why would I need power going to the headlight switch to energize the circuits that it does?

As an example one of the terminals of the headlight switch is for the parking lights. Well if you are providing 12v to the lights at a fuse panel then you should not have to at the switch I would think.

Admittedly I am having a brain block on fusing an item in-line, vs fusing it thru the fuse panel as well. Thanks



 




Shepherd 
Frequent Contributor
Posts: 1039

Loc: Lake George, NY
Reg: 11-11-15
01-29-17 11:01 AM - Post#2674127    
    In response to LGK

Internally the h lite switch may have seperate circuits for the pk lites, headlites , so 2 power inputs, aftermarket switches are sometimes set up this way.Also some switches have their own internal cicuit breaker so there is a direct feed, no fuse.



Edited by Shepherd on 01-29-17 11:04 AM. Reason for edit: No reason given.

 
65_Impala 
Very Senior Member
Posts: 3928

Reg: 12-29-02
01-30-17 07:01 AM - Post#2674295    
    In response to LGK

Power typically goes directly to the headlight switch and then the headlight switch has a separate tail-light output. The tail light output then goes to the fuse block where it powers 2 fuses, one for the tail lights (typically around 10A) and one for the instrument cluster (typically around 2A).

A similar thing is done with any ignition switched circuit like the windshield wipers or heater. The ignition wire out of the ignition switch goes to the fuse block and powers these fuses.

So, no the fuses for circuits that are switched on and off by the headlight switch should not have power all the time.



 
LGK 
Contributor
Posts: 195
LGK
Loc: Cincinnati, OH
Reg: 04-26-14
01-30-17 08:34 AM - Post#2674318    
    In response to 65_Impala

Very helpful. Explains a lot. So....if I have the fuses on the ATO panel that exists, can I not remove the stock fuse box (the one with the tail and stop lamp in line fuses) that is on the firewall?



 
65_Impala 
Very Senior Member
Posts: 3928

Reg: 12-29-02
01-30-17 09:42 AM - Post#2674330    
    In response to LGK

Probably. Is there a new fuse duplicating the old fuse? They might already be replaced and only not removed.



 
LGK 
Contributor
Posts: 195
LGK
Loc: Cincinnati, OH
Reg: 04-26-14
01-30-17 11:38 AM - Post#2674355    
    In response to 65_Impala

Here is what I saw in identifying each wire/circuit to the existing ATO fuse box:
A) parking/tail and dome were all on the same fuse. Usually you separate dome which I will do.
B) Brake lights at this point are not working. I gotta figure that out whether I have a bad stop switch, etc. I did not test the brake lights before I replaced the bulbs with 1157 LED equivalents from SuperBrightLED's. Should not matter though.

Both fuses are good in that firewall little fusebox.



 
65_Impala 
Very Senior Member
Posts: 3928

Reg: 12-29-02
01-30-17 01:19 PM - Post#2674375    
    In response to LGK

The more I think about it the more I thought I was wrong so I double checked the wiring. There will be a fuse for the tail lights (running lights) that has power all the time and provides power to the light switch. There will also be another unfused power source for the headlights. Then, there is a wire from the light switch for the dash lights and it gets fused again at something like 2A. So, the tail light fuse would be powered all the time but the dash light fuse is only powered with the lights on.

So, park/tail lights and dome lights could exist on the same fuse since they both get power all the time. But yes typically they are separate fuses so a dime light wiring issue doesn't take out the park/tail lights. Same as the dash lights having another fuse - it keeps a short in the dash from taking out the tail lights.




 
LGK 
Contributor
Posts: 195
LGK
Loc: Cincinnati, OH
Reg: 04-26-14
01-30-17 02:40 PM - Post#2674396    
    In response to 65_Impala

I follow you. But...for sure I am going to fuse the headlights. Even if there is one of those small fuses in the headlight switch itself (it is still mounted to the dash so I cannot see it ) that protects the headlights, I will run the 12v wire to the fusebox with a 30 amp fuse. May have mentioned it but I just installed a new heavy duty headlight wiring harness with the dual relays because I install H4 HID headlights. They tested great!
Going back to my original question though...I am still having a brain block about why a switch has to be powered at the switch itself, UNLESS one or more of its' outputs is not fused and goes directly to the load device. I am guessing it is because back in 1954 there was no fuse box (except for that in line box on the firewall for the tail and stop lights) so they ran power to the switch. But then again, I looked at modern day headlight switches and I think I saw power connections to those as well.



 
CowboyTrukr 
"6th Year" Silver Supporting Member
Posts: 3821
CowboyTrukr
Loc: Salt Lake City
Reg: 06-20-09
01-31-17 11:16 AM - Post#2674510    
    In response to LGK

Your assumption is partially correct. There must be hot 12V either to power the loads directly or through a relay. It is all fused. It needs to be there at the switch to complete the circuit.

Something that can sometimes trip a person up is when you're measuring voltage across a switch that is fed from a relay. You connect the hot source through the relay coil then over to the switch. The other side of the switch gets connected to ground like the door jamb switch. Intuitively one might think "I'm going to blow the fuse if I close this", but the coil resistance prevents that.

Greg

'95 K1500 Z71 EC Short Step 5.7L+0.040/NV3500
'00 Explorer XLT 4.0 V6 Auto
'94 K2500 5.7 NV4500 ECLB - SOLD

"The only thing necessary for evil to triumph is for good men to do nothing" Sir Edmund Burke


 
65_Impala 
Very Senior Member
Posts: 3928

Reg: 12-29-02
01-31-17 02:58 PM - Post#2674532    
    In response to LGK

The headlight switch (most of the push-pull ones anyways) has a self resetting circuit breaker for the headlights since by law the protection device for the headlights has to be self resetting. So, that is why the headlight power goes directly to the headlight switch.

The dash has some main power wires that go to the light switch, ignition switch and lighter that are not fuses with fuses. But, there was often a fusible link put on the main wire coming off the battery feeding all those circuits which protected all that main wiring, especially on vehicles later then your truck. The fusible link was less likely to blow or cause issues then a fuse under the hood.

I would suspect fuses weren't put on the main wires under the dash for a couple of possible reasons. One being that having a single fuse that could kill the ignition power and stop the car isn't a great idea. Another being that protecting the wire 3/4 of the way to it's destination didn't make much sense.



 
LGK 
Contributor
Posts: 195
LGK
Loc: Cincinnati, OH
Reg: 04-26-14
02-01-17 04:51 AM - Post#2674629    
    In response to 65_Impala

  • 65_Impala Said:
The headlight switch (most of the push-pull ones anyways) has a self resetting circuit breaker for the headlights since by law the protection device for the headlights has to be self resetting. So, that is why the headlight power goes directly to the headlight switch.

The dash has some main power wires that go to the light switch, ignition switch and lighter that are not fuses with fuses. But, there was often a fusible link put on the main wire coming off the battery feeding all those circuits which protected all that main wiring, especially on vehicles later then your truck. The fusible link was less likely to blow or cause issues then a fuse under the hood.

I would suspect fuses weren't put on the main wires under the dash for a couple of possible reasons. One being that having a single fuse that could kill the ignition power and stop the car isn't a great idea. Another being that protecting the wire 3/4 of the way to it's destination didn't make much sense.



IMHO...while all the responses are very helpful, most direct answer yet. Thanks. Finally making sense why the designed the H'Iite switch the way they did. I do have a megafuse (125amp) coming directly off the battery.
I've ID'd all the wires that previous owner fed to a fusebox. Remember this is a non stock truck in many ways: Htr, gas gauge, tails/park lights, headlites, carb electric choke, radio, cig lighter, voltmeter, signal and spares. What I did not see was a separate brake light fuse, although as mentioned the stock in-line fuses are still active on the firewall which includes Stop and Tail fuses. Is it common to also fuse the brake lights at the fusebox. I think so, right?




 




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