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Technical The Main Fuse

Discussion in 'The Hokey Ass Message Board' started by squirrel, Jun 13, 2021.

  1. squirrel
    Joined: Sep 23, 2004
    Posts: 52,803


    I had an adventure tonight.

    I took my Chevy II to the drags in Tucson, which is an hour or so away. I made a few runs, but it was pretty slow, since I left the street tires on it. I left after making 4 passes, at about 9:30.

    I took the lonely highway home, which goes through the low mountains north of Sonoita. I just got into the hills, and the car went black--the engine shut off the lights went out, instantly. No smells or noises, it just quit. There was a new moon a couple nights ago, and it was really dark out--so dark, I could not see the road at all. I managed to get stopped without running into anything, and rolled partly onto the narrow shoulder. This scared the crap out of me.

    I opened the hood, and didn't see anything weird. I found my volt meter in the trunk, and checked for power at the horn relay, under the hood (it's the early 60s Chevy type that has the main power wire attached to a couple screw terminals). There was 12v there. Then I looked at the fuses, which are on the driver side kick panel. The top one, which is the main power feed to the electrical system, was blown. After searching the glove box for a while, I looked in my parts box in the trunk, and found my new fuse supply. I replaced it with a new 20 amp, and everything worked.

    Driving home, I was pretty careful to keep my speed down around the curves, unless there was a car ahead or behind me. And I kept the high beams off. There isn't much electrical load on the car when driving, just the coil, lights, and the overdrive solenoid on the Gear Vendors.

    I made it home fine. The whole time, wondering if it would die again if I hit a bump...I don't know why the fuse blew.

    And I had a while to think about how I might change the electrical system. In the 50s, Chevy didn't have fuse panels, they had circuit breakers built into the headlight switch, as well as a few in line fuses on things such as the heater and radio. With a breaker, you get blinking lights when there's a short, which is better than no lights at all. I had experienced that once in my 59 truck, many years ago, when the tail light wiring was flaky and shorted.

    What do you think? Did this fuse just get tired, or did it get blown by a wire shorting momentarily?

    I'm going to look for a circuit breaker to replace it, just in case.

    flatoz, Deuces, Stogy and 14 others like this.
  2. I think it was designed to work well (probably better than the older system). Drag racing, hot weather(?), and it sounds like an unknown condition (age/miles) fuse. Carry spares and don't worry about it unless it blows through a couple in a short period of time.
    Last edited: Jun 13, 2021
    Stogy, Boryca, bchctybob and 5 others like this.
  3. I love old cars and the stories that go with them.:)
    Stogy, 1Nimrod and flatheadpete like this.
  4. Lil32
    Joined: Apr 4, 2012
    Posts: 2,534


    lucky man
    Stogy likes this.

  5. blowby
    Joined: Dec 27, 2012
    Posts: 8,656

    from Nicasio Ca

    What do you figure the total wattage was at the time? High beams?

    Had the same thing happen, on a motorcycle!. Scary experience coasting to a stop in pitch black wondering where the road ends.
    Stogy likes this.
  6. oldiron 440
    Joined: Dec 12, 2018
    Posts: 3,092

    oldiron 440

    No power is much better than a flaming inferno...
    Glad you made it home...
    Stogy, 1Nimrod, bchctybob and 5 others like this.
  7. Budget36
    Joined: Nov 29, 2014
    Posts: 10,419


    I should have paid better attention, or maybe I just better retention, but in my Industrial Electronics classes one of the books I had show what fuses looked like when shorted, excessive current draw and one or two other things.
    Before someone says it, excessive load and a short are not the same thing.
    Google might yield some results.
    I seem to recall a short would leave a splatter on the glass, but I’m doing my 6th 12 hour night shift right now for the week, so take it with a grain of salt;)
    Stogy, bchctybob, 41 GMC K-18 and 2 others like this.
  8. deathrowdave
    Joined: May 27, 2014
    Posts: 3,048

    from NKy

    Damn , I’m thankful you and the tin are , ok . Scary stuff to happen . They do make a breaker that will snap into the same location as the glass type fuse . Maybe replace the many with an auto resetting breaker . The breaker , still does not define why the fuse gave up the chase on you . It looks to me as an overload popped the conductor. There had to be some heat from current flow . 20amp main fuse with headlights on high, seems a touch on the light side to me . The headlight circuit on my 32 has a auto reset breaker in it , it is not wired into the fuse box with the other circuits . Power is fed from the main buss of the fuse panel , just no fuse for lights .
    None of this suggests why , the fuse popped and why installing a new one , operating on low beams never popped the new fuse , other than 20 amps will not carry the load .
    Stogy, 1Nimrod and 41 GMC K-18 like this.
  9. Petejoe
    Joined: Nov 27, 2002
    Posts: 11,617

    from Zoar, Ohio

    That’s as scary as having a hood pop open. Glad your safe.
    Since the headlights were the only thing different during your trip. I’d pull them and the tail lights and the switch and look for heat traces.
    Did you run your headlights on the drive there?
    Stogy, 1Nimrod and 41 GMC K-18 like this.
  10. G-son
    Joined: Dec 19, 2012
    Posts: 1,100

    from Sweden

    Well, the upside of separate fuses for separate functions is probably rather obvious now, you want to keep the ignition circuit separate from anything else (so a minor problem in a "unimportant" system doesn't force you to call a tow truck), and you want the front lights separate to not lose them suddenly because of a problem somewhere else. You can even argue for daytime running lights and regular low/high beams on separate fuses so everything doesn't go completely dark at once, or right and left light on separate fuses.

    I think I only would need that experience once to do some electrical work on the car to make sure it won't happen again.
    Stogy and 1Nimrod like this.
  11. Ziggster
    Joined: Aug 27, 2018
    Posts: 1,172


    Like @sgtlethargic said, I’m guessing heat/age/vibration. My 97 Lexus LX450 quit on me a few days ago while doing some off-roading/back road driving. Parked it at an old church to take a few pics and would not start. Voltage was down to 12.7V, and my aux battery wouldn’t start it. No spark on either of the two battery terminals when I removed the cables. Called youngest daughter to come get me and her sis (over an hour away). Just before she showed up, I tried it again, and it started right up. I’m thinking the extreme heat may have done something to my battery isolator. Was 29C out and coolant temps were creeping up to 100C. Been fine since.
    Stogy likes this.
  12. Replace the fuse, and inspect your electrical system, possibly a chafed wire touching ground or something like that ?

    ive seen it many times where a fuse pops, you replace it a never to pop again.

    hard one to diagnose , put a circuit breaker in and monitor it, perfect excuse to drive the car more !!
    Stogy, 1Nimrod, rusty valley and 2 others like this.
  13. mcsfabrication
    Joined: Nov 26, 2006
    Posts: 1,048


    A 20 amp fuse sounds kind of light duty for the main fuse. The failed fuse appears to have a bit of light surface rust on the contacts. I'm sure you're checking if all the contacts are clean and making good positive contact.
    Using a breaker, is normally a good decision.
    Can't imagine this stumping the HAMB guru.
    I'm thinking you're testing the rest of us!
    Stogy, 1Nimrod, bchctybob and 8 others like this.
  14. Jalopy Joker
    Joined: Sep 3, 2006
    Posts: 29,482

    Jalopy Joker

    waiting for others to shed some "light" on this subject that all of us have dealt with on one level or another
    Stogy and 1Nimrod like this.
  15. I'm thinking 20 amps is a little light also, you have headlights, tail lights, ignition, dash lights, gauges, brake lights, that overdrive solenoid and I'm sure I missed something all on that one 20 amp fuse. But I'm sure this is not the first time you drove at night with this car. As said above, sometimes you can tell why the fuse blows from looking at it, short or overload. What I have heard is that if the fuse splattered metal on the inside of the glass it is from a short, if the small part of the wire inside the fuse is distorted, melted or even the solder is melted at the ends, it is from overload. I don't know if this helps.
    Stogy, 1Nimrod, 41 GMC K-18 and 2 others like this.
  16. spanners
    Joined: Feb 24, 2009
    Posts: 1,726


    My theory is that glass fuses are just a fancy type of light globe. After years of having electricity shoved through it, the filament says "enough's enough. I'm out of here".
    Stogy, 1Nimrod, Chicster and 4 others like this.
  17. BrerHair
    Joined: Jan 30, 2007
    Posts: 4,853


    Haha! That’s what I thought. Where’s squirrel when you need him!
    1Nimrod, bchctybob, clem and 2 others like this.
  18. Jim, if memory serves me right, I believe that Mack Truck used a self resetting circuit breaker of that style years ago. They were square shaped but had the round contacts like the glass fuses.
    Stogy, 41 GMC K-18 and VANDENPLAS like this.
  19. Paul
    Joined: Aug 29, 2002
    Posts: 15,963


    What is the combined total amp draw for all systems running at time of fail?
    Stogy, bchctybob, 41 GMC K-18 and 2 others like this.
  20. squirrel
    Joined: Sep 23, 2004
    Posts: 52,803


    I took a closer look at the fuse, it has that "old" look to it. I expect it's one I took out of an old car, so it's probably 40-50 years old.

    And the part that "popped", looks like it was slightly molten for a while, as though it was not a sudden short circuit that caused the failure--just a load at or near capacity, for a long time.

    I have a couple of those little square breakers with the round terminals, to replace a glass fuse. They're old ones, so old they say USA on them. But they're 40 amp, and I think 30 would be more appropriate. Or 25. They're available new, although a different style, they have a plastic housing now, of course.

    The wire sizes are not that big on old cars, I used pretty much OEM size wires for a 62 chevy.

    I'll have to dig into the wiring to see how I connected everything together under the dash. It's been 7 years since I put the car together.
    Stogy, quick85, dana barlow and 10 others like this.
  21. deathrowdave
    Joined: May 27, 2014
    Posts: 3,048

    from NKy

    A dirty ground or insufficient one will cause the same issue as high load . It’s all about current flow. High resistance causes a heavy load to complete the circuit . Just something else to think about .
    theboss20, Stogy, 1Nimrod and 4 others like this.
  22. As an old rule of thumb so to speak, a fuse that blows dark with the appearance of an arc was caused by a direct short to ground. And a fuse that just melts clear was caused by high amperage draw.
    Stogy, 1Nimrod, clem and 5 others like this.
  23. Soo glad your okay! But I know how you felt. Many moons ago. I had the same experience. I had one of the old headlight switches with a fuse in it. I dimmed the lights with the floor dimmer for a car coming toward me. Then to high beam. I had the Quartz halogen bulbs, and like you it went DARK fast!:eek: I did get off the road safely. But I didn't have a fuse with me. So I twisted a piece of foil just to get me home. Replaced the fuse with a heavier 30 amp. fuse. It never happen it again. But made for a moment that I will never forget.
    Last edited: Jun 13, 2021
    Stogy, quick85, 1Nimrod and 6 others like this.
  24. HotRodWorks
    Joined: Dec 6, 2005
    Posts: 369

    Alliance Vendor

    No. That’s not correct. The current goes down as the resistance goes up. Ohms law is V=iR. The voltage is the same because it’s regulated at 14 ish volts. If the resistance increases, the current has to decrease.
  25. I had that happen to me in an OT VW Beetle, but it had a self-setting circuit breaker. It did it 4 more times on the 100 mile trip home, always at the worst time. Dealer could not find a fault, and said it was a bad breaker. Glad your situation worked out and worked out well. A 30 amp breaker?
    41 GMC K-18 and squirrel like this.
  26. junkyardjeff
    Joined: Jul 23, 2005
    Posts: 8,394


    The main fuse in my 37 Chevy p/u would blow every couple years,it would quit at a light and the stereo would go dead but it would start right up but quit at every light and then when I got home replaced the fuse and it would be good for a couple years. Last month it did it again so I dug deeper and found a loose wire at the starter,when I started making the harness I did not have a good soldering gun and they were just crimped but now are soldered. I think the only reason it ran was from the feedback from the alternator going through the ignition switch and since the alternator put out more then the 30 amp fuse it blew it.
  27. That car was designed for a 37 amp charging system. The 20 amp fuse was fine for that time period. They are glass and can only take so many cycles before they finally break down.
  28. My 37 has Halogen headlamps and going to a car shoe early in the AM the lights went out pulled over and reached up under dash to touch headlamp switch , it burned my fingers . so waited for the circuit breaker to click on at the switch . Then drove with lamps on low beam . Switch got warn but lamps stayed on . Next day installed relays on both headlamps , that cured the problem switch stays cool and headlamps are much brighter .
  29. 20 amp fuse has been fine until now, should be adequate going forward.

    I have read that one of the reasons glass fuses were phased out is the fuse holder clips corrode/ get dirty causing a bad connection. Maybe clean with some alcohol and use some dielectric grease.

    Glad you made it home safely.
  30. deathrowdave
    Joined: May 27, 2014
    Posts: 3,048

    from NKy

    Essentially, a "ground" is an electrical connection to a common return in circuit. In automotive terms a ground is a connection to the body or other metal surface of the vehicle. The body is connected to the negative terminal of the battery. Thus any connection to the body of the car is a connection to the negative terminal of the battery. Electricity flows from the positive of the battery, through the circuit to do some work, then back through the body to the battery . This is a "negative ground" system which is by far the most popular automotive ground scheme used in the automotive world. The main reason this is done is to greatly simplify the wiring of a vehicle. Since most car bodies are made of metal (at the very least almost all frames are) they provide a perfect ground plane, eliminating the need to run a separate ground wire to each circuit which almost chops the amount of wire necessary in half. Good grounds are important because poor grounds cause voltage drop. The consequence of this voltage drop is that the systems in that circuit can malfunction. How do bad grounds cause voltage drop? Well, the answer is Ohms Law. Ohm's Law states that V=IR. This means that the voltage drop (V) is proportional to the current through the circuit (I) and the resistance (R) of the resistor (or bad ground in our case). An easy example is attempting to draw 10 amps through a bad ground with a resistance of 1 Ohm. So V=10(1) thus making the voltage drop a whopping 10V. This leaves only 2 volts available (in a standard automotive 12V system) to power the circuit and means that all that extra power must be dissipated as heat at the resistor (bad ground). Generally this heat makes the condition even worse, causing more voltage drop and more heat. In the most severe cases the ground will function under light loads yet fail under high current draws causing what appears to be an intermittent failure. Many people have seen this behavior caused by a loose (high resistance) battery terminal. Bad grounds can cause poor performance in almost any circuit. Voltage drop can manifest itself as dim headlights, one circuit malfunctioning when another is switched on (a common symptom is that the heater blow slows and dash lights dim when the brake pedal is pressed) and all kinds of other random weird issues.

    I give up , install a new .20 fuse and drive it like you stole it ! Just try to be safe and have a good time doing it .

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