A possible short in my 1976 280z(stop/hazards)

[SeKcGamer]

UPDATE: So I took it to my teacher and after the first two days we found out there was corrosion happening on the fuse box that holds the fuse bussus because there wasn't any power going to them, After I cleaned them up with a circular file most if not all other electrical accessories such as the rear dome light started to function, the emergency light in the engine bay worked and the guages started to work again. But now I have a new problem with the electrical and that is when I put a 10A fuse in the "Fuel Gauge" section the gauges and turn signals start to work on idle, but when you rev it up to 3K+ RPM the fuse blows out and the fuel, temp,oil, gauge goes out along with the turn signals. The only gauge that works it he VOLTS. My teacher was tracing the wires on a wiring diagram to see if there was a short somewhere and testing many wires to see what they were reading and everything seemed fine. What ended up happening is he was testing it with the recommended "10A" fuse but he couldn't get a good read on the fuse box and it would blow before he got something, so what he did was he put a 20A to what kind of reading he would get and when that happened my gauges stayed on but he blew the voltage regulator which I replaced today but the 10A fuse keeps blowing over 3K+ RPM. He told me he didn't know what else to check and hope replacing the voltage regulator would solve the blowing fuse problem. Guys got any suggestions as to why I'm still blowing that fuse?


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[SteveJ]

Yeah, once you said your fuse blew when engine speed increased, I was thinking that you have a bad voltage regulator OR you might have an internally regulated alternator with a voltage regulator. Either case can cause an overvoltage. I hope you didn't blow out any components when testing when using the higher rated fuse.

Life can be a mean teacher. You get the test first and lesson afterward. Here's the lesson. NEVER put in a higher rated fuse when you blow the specified fuse. The wire is rated for only so much current. Putting in a fuse with a higher rating to find a short may cause the wiring to burn up. (Okay, so I dodged that bullet in the past, but I'm not doing that again.)

So why would revving the engine cause a fuse to blow? To understand, you need to know Ohm's Law (https://en.wikipedia.org/wiki/Ohm's_law) and read through the testing of the voltage regulator in section EE of the factory service manual. Go ahead. I'll wait.

Done? Good. Now say you have a circuit with a 10A fuse. The effective resistance in the circuit is about 1.45 Ohms. You can calculate the current assuming you have 12 volts going through the system. (Note: Automotive circuits would be designed around 14 volts with some safety margin, but I'm taking some license with this example.)  Look what happens if you have a bad voltage regulator that is allowing the voltage to go too high.

So you stuff a 20A fuse in the circuit and a component that is rated for 14 volts is now seeing 18 volts for a prolonged period of time. That component might fail in a way that lowers the effective resistance of the circuit, causing it to blow with the 20A fuse, or the insulation on a wire could fail, allowing a short circuit to develop. I hope it wasn't either of those.

If you really want to look for a short, use an ohmmeter. For the circuit you're interested in, find it in the fuse box. Remove the fuse. There is an upstream side and downstream side of the fuse. The upstream side has 12 volts when the car is on (or 12 volts constant for brake lights and some other circuits).The downstream side goes to the load. Measure resistance from the fuseholder on the downstream side to ground. If it's less than 1 ohm, there is good likelihood of a short somewhere. Things to watch for include bad corrosion in the exterior light sockets. The corrosion can bridge the gap between the positive and negative side of the bulb socket. This will lower resistance a fair amount and lead to issues like wires and the fuse box overheating.

I forgot to add this. Here's an example of a wire ampacity chart for DC circuits: https://www.altestore.com/howto/wire-sizing-tool-for-12-24-and-48-volt-dc-systems-a106/

There are plenty more for marine applications.

 

Edited April 30, 2017 by SteveJ

[SeKcGamer]

Yeah, once you said your fuse blew when engine speed increased, I was thinking that you have a bad voltage regulator OR you might have an internally regulated alternator with a voltage regulator. Either case can cause an overvoltage. I hope you didn't blow out any components when testing when using the higher rated fuse.
Life can be a mean teacher. You get the test first and lesson afterward. Here's the lesson. NEVER put in a higher rated fuse when you blow the specified fuse. The wire is rated for only so much current. Putting in a fuse with a higher rating to find a short may cause the wiring to burn up. (Okay, so I dodged that bullet in the past, but I'm not doing that again.)
So why would revving the engine cause a fuse to blow? To understand, you need to know Ohm's Law (https://en.wikipedia.org/wiki/Ohm's_law) and read through the testing of the voltage regulator in section EE of the factory service manual. Go ahead. I'll wait.
Done? Good. Now say you have a circuit with a 10A fuse. The effective resistance in the circuit is about 1.45 Ohms. You can calculate the current assuming you have 12 volts going through the system. (Note: Automotive circuits would be designed around 14 volts with some safety margin, but I'm taking some license with this example.)  Look what happens if you have a bad voltage regulator that is allowing the voltage to go too high.

So you stuff a 20A fuse in the circuit and a component that is rated for 14 volts is now seeing 18 volts for a prolonged period of time. That component might fail in a way that lowers the effective resistance of the circuit, causing it to blow with the 20A fuse, or the insulation on a wire could fail, allowing a short circuit to develop. I hope it wasn't either of those.
If you really want to look for a short, use an ohmmeter. For the circuit you're interested in, find it in the fuse box. Remove the fuse. There is an upstream side and downstream side of the fuse. The upstream side has 12 volts when the car is on (or 12 volts constant for brake lights and some other circuits).The downstream side goes to the load. Measure resistance from the fuseholder on the downstream side to ground. If it's less than 1 ohm, there is good likelihood of a short somewhere. Things to watch for include bad corrosion in the exterior light sockets. The corrosion can bridge the gap between the positive and negative side of the bulb socket. This will lower resistance a fair amount and lead to issues like wires and the fuse box overheating.
 

Thanks I'll go tell him that on Tuesday when I have class with him, see what we can find.


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[SteveJ]

4 minutes ago, SeKcGamer said:

Thanks I'll go tell him that on Tuesday when I have class with him, see what we can find.


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Remember that your homework before class on Tuesday is to read the EE section of the FSM and read about Ohm's Law. That way you can ask questions on the things you don't understand before you get to class.

[SeKcGamer]

Remember that your homework before class on Tuesday is to read the EE section of the FSM and read about Ohm's Law. That way you can ask questions on the things you don't understand before you get to class.

Alright Thanks [emoji6]

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[SeKcGamer]

So my teacher this Tuesday asked me if the fuse was still blowing after I changed the voltage regulator and my response was yes, so he asked me to check my alternator but I completely forgot the rest of what he told me[emoji28], but today when I went to start my car the battery was drained again. What I did was I bought a new alternator and jump started my car but that really didn't work since it wasn't charging the battery, so when I drove off to work in a different car I dropped off the battery and they told me it's completely charged. So I put the battery in and the car is starting to run rough again?.Any ideas?


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[SteveJ]

You're going to have to learn how to use the multimeter to give us more information. It's difficult to diagnose with some qualitative observations.

[SeKcGamer]

You're going to have to learn how to use the multimeter to give us more information. It's difficult to diagnose with some qualitative observations.

[emoji28]I don't have one with me at my house. The only way I can use one is at school tomorrow.


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[sweatybetty]

you can get a el-cheapo multimeter at a harbor freight store for el-cheapo.