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Old 03-18-2019, 10:29 PM   #1
Tim Warren
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Default Small circuit breaker heats and opens

When the engine is NOT running, if I turn my headlights on, they light, and the running lights are on. After a few minutes all go out and the circuit breaker on the small fuse block on the starter is hot. Wait a few minutes, lights on w/ engine off and the whole process occurs again. Does this indicate a short somewhere? Thank you in advance.
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Old 03-18-2019, 10:48 PM   #2
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Default Re: Small circuit breaker heats and opens

At the min. you have a faulty breaker.


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Old 03-19-2019, 12:01 AM   #3
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Default Re: Small circuit breaker heats and opens

Do you have a volt meter that measures amps?


If the lights light up then you know that the path from the battery to the lights and back to the battery. So that circuit works. If you had a short the electricity would take a shortcut back to the battery and the lights would not light.


Did you know that most of the circuit breakers (CB) that are used in place of the glass fuse have a right way and wrong way to be installed? You will see a copper and chrome rivet. On a negative ground vehicle (modern) the copper prong goes to the battery side and the chrome goes to the equipment. On our Model A positive ground, the chrome prong goes toward the battery and the copper goes to the cars equipment. Hope this is clear. The stock A is backwards from modern cars and the marking are also backwards for our car.


I would replace the CB with a fuse and see what happens.
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Old 03-19-2019, 12:03 AM   #4
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Default Re: Small circuit breaker heats and opens

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When the engine is NOT running, if I turn my headlights on, they light, and the running lights are on. After a few minutes all go out and the circuit breaker on the small fuse block on the starter is hot. Wait a few minutes, lights on w/ engine off and the whole process occurs again. Does this indicate a short somewhere? Thank you in advance.
It could be a load that is not a dead short, but enough to trip the breaker. Does the wire get hot or warm too, or is it cool and only the breaker getting warm? How many amps does the ammeter show when the lights are on before the breaker trips? If the wires get hot and/or you have higher current on the ammeter than the breaker is rated for, then you have an electrical problem. I would imagine your breaker is rated at 20 amps or over?

If your ammeter is showing lower current than the ammeter is rated for, then you would have a bad breaker. How long has the breaker been installed?
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Old 03-19-2019, 08:47 AM   #5
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Default Re: Small circuit breaker heats and opens

You said small size breaker. What size is the breaker and is it the proper size for the circuit ? Remember resistance causes heat. Are all your connections clean and tight.
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Old 03-19-2019, 10:54 AM   #6
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Default Re: Small circuit breaker heats and opens

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Do you have a volt meter that measures amps?
I always understood that a volt meter measures volts, not amps. And an ammeter measures amps.
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Old 03-20-2019, 07:41 AM   #7
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Default Re: Small circuit breaker heats and opens

They are called thermal breakers, and if overcurrent condition exists, they heat up and then open. When it cools it will close again. Sometimes they get weak and need to be replaced. Use an inline ammeter to check the amount of current going through it before you replace it.
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Old 03-20-2019, 08:11 AM   #8
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Default Re: Small circuit breaker heats and opens

Use G.M. Bayonet type circuit breakers, with supplied Pig-Tail plug in.
Use a 30 AMP one for the main feed. Use 2 20 AMP ones for the lights & horn.
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Old 03-20-2019, 11:42 AM   #9
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Default Re: Small circuit breaker heats and opens

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Originally Posted by Tim Warren View Post
When the engine is NOT running, if I turn my headlights on, they light, and the running lights are on. After a few minutes all go out and the circuit breaker on the small fuse block on the starter is hot. Wait a few minutes, lights on w/ engine off and the whole process occurs again. Does this indicate a short somewhere? Thank you in advance.
Read your ammeter on the dash for how much current is passing through the breaker. It should be rated at least 20 amps. Breakers are thermally operated and should be warm when activated. I've never met up with one that is polarized.

Check battery voltage. Incandescent bulbs draw more current with lower voltage
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Old 03-21-2019, 10:33 AM   #10
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Default Re: Small circuit breaker heats and opens

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They are called thermal breakers, and if overcurrent condition exists, they heat up and then open. When it cools it will close again.
Automatic reset are OK in some applications, and some applications, like the main feed, should have a manual reset.
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Old 03-21-2019, 03:27 PM   #11
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Default Re: Small circuit breaker heats and opens

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Automatic reset are OK in some applications, and some applications, like the main feed, should have a manual reset.
For the A, I do not like the the automatic reset. It will sit there and cycle forever if left unattended,
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Old 03-21-2019, 09:36 PM   #12
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Default Re: Small circuit breaker heats and opens

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For the A, I do not like the the automatic reset. It will sit there and cycle forever if left unattended,


I might get you home.
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Old 03-21-2019, 09:39 PM   #13
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Default Re: Small circuit breaker heats and opens

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I always understood that a volt meter measures volts, not amps. And an ammeter measures amps.

Most people call it a volt meter, Multi meter would be the better term. If you google volt meter just about everything that show up is a multi meter.
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Old 03-21-2019, 10:14 PM   #14
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Default Re: Small circuit breaker heats and opens

Wonder what happened to the OP?
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Old 03-21-2019, 10:17 PM   #15
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Default Re: Small circuit breaker heats and opens

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Originally Posted by Mike V. Florida View Post
Most people call it a volt meter, Multi meter would be the better term. If you google volt meter just about everything that show up is a multi meter.

Have to be careful, most multimeters have max current they can measure before they blow an internal meter fuse. A dedicated clamp probe meter is easier to use, and usually measure larger currents.
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Old 03-22-2019, 09:25 AM   #16
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Default Re: Small circuit breaker heats and opens

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Have to be careful, most multimeters have max current they can measure before they blow an internal meter fuse. A dedicated clamp probe meter is easier to use, and usually measure larger currents.
"Have to be careful",

You're so right. Around 25 years ago I had a Fluke 73 multi-meter, I was using it on the ohms scale, a co-worker borrowed it, didn't check the setting and used it on 208 volts. He was working on the rear of a different panel, I heard a loud bang and saw something go bouncing across the floor. The meter had blown up and what I had seen was the knob.
That meter didn't have a fuse in it. The replacement meter the company bought me was a newer version that had fuses in it.
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Old 03-22-2019, 09:55 AM   #17
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Default Re: Small circuit breaker heats and opens

Poor connections and wire that is too small a gauge for the amperage requirements can be problematic. This adds up the resistance in the circuit. Resistance creates heat just like a resistor does. A lot of the aftermarket reproduction replacement electrical parts don't hold a candle to the original equipment. Headlamp connectors and light switches may not be up to the task that the originals performed and same for any connectors in the system.

It's a good idea to have circuit protection of any kind. A lot of cars burned down back in the day. The automatic reset feature can get you home sometimes but it has to be the correct amp rating for the size of the wire gauge in the system. The wire gauge also has to be correct for the application. If the auto reset breakers give trouble, try a new one but be certain it is correct for the application.
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Old 03-22-2019, 02:53 PM   #18
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Default Re: Small circuit breaker heats and opens

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Originally Posted by rotorwrench View Post
Poor connections and wire that is too small a gauge for the amperage requirements can be problematic. This adds up the resistance in the circuit. Resistance creates heat just like a resistor does. A lot of the aftermarket reproduction replacement electrical parts don't hold a candle to the original equipment. Headlamp connectors and light switches may not be up to the task that the originals performed and same for any connectors in the system.

It's a good idea to have circuit protection of any kind. A lot of cars burned down back in the day. The automatic reset feature can get you home sometimes but it has to be the correct amp rating for the size of the wire gauge in the system. The wire gauge also has to be correct for the application. If the auto reset breakers give trouble, try a new one but be certain it is correct for the application.

I am a worrywart - I would be afraid that what ever is blowing the breaker would fry or damage something else - just me. Try to find find the issue/fix it , then If happens more than twice, time to disconnect the battery and call for a tow truck. I am paranoid.
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Old 03-22-2019, 05:35 PM   #19
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Default Re: Small circuit breaker heats and opens

If it has a direct short to ground then it will pop the circuit breaker instantly. If it takes a while then it is heating up slowly due to resistance. This can also indicate a breaker with a bi-metallic strip getting too old and fried to do the job. These things are still available in quite a few amperage ratings. Don't worry about the voltage rating. They are all rated for 12-volts but will work just the same at 6 to 7 volts.

If it still pops after you replace it then I'd say you have too much resistance in there somewhere.
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Old 04-10-2019, 10:10 PM   #20
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Default Re: Small circuit breaker heats and opens

Got it figured out. I had replaced the rear bulbs w/ halogens. The breaker was a 20 amp OLD guy (just like me!). I replaced the breaker w/ a 30 amp fuse, and problem solved.

Thanks to everyone who weighed it!
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