Blew the 80 AMP main fuse tonight. *** STILL BROKEN *** - Page 2 - Miata Turbo Forum - Boost cars, acquire cats.

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Old 09-02-2011, 12:06 AM   #21
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Oh I see... the harness was still attached to the alt. when this happened... makes sense you may have a burnt up alt.

For some reason I was thinking that the cable fell off the alt. (no longer connected).
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Old 09-02-2011, 12:10 AM   #22
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Quote:
Originally Posted by Joe Perez View Post
For this to be true, the fuse must be blown (or missing.) Is the fuse blown / missing? If not, your test procedure is faulty.

If it's blown / missing, and it's because you haven't replaced it since fixing the alternator wire, then this is to be expected. There are circuits downstream of it that draw power even when the car is "off" such as the radio and the ECU.

If it blew again after you fixed the alternator wire, well...
As a recap (probably my final one until I pull the Alt and see what is going on with it) -

I fixed my burnt up wires and there are still 12V across the fuse leads (fuse was pulled).

I pulled all fuses/relays in the main box and the IGN switch harness. Still have 12V across the leads.

I pull the big wire off the alt and the 12V goes away. I'm assuming that you (Joe) agree that this means it is the Alt.

I'll be pulling the Alt, opening it up and buying a rebuild kit. I've never cracked an alt apart before so this should be fun. Dad says it is the diodes so that will be the first thing I check.
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Old 09-02-2011, 12:12 AM   #23
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Originally Posted by FRT_Fun View Post
Oh I see... the harness was still attached to the alt. when this happened... makes sense you may have a burnt up alt.

For some reason I was thinking that the cable fell off the alt. (no longer connected).
Hey, does this mean I toasted the Alt when the cable burnt through and shorted on the exhaust? Just trying to sort out what happened. This sounds like a plausible explanation.
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Old 09-02-2011, 05:00 AM   #24
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I don't see what the alternator has to do with reading 12 volts across a fuse.

Plain and simple, if you're reading 12 volts ACROSS a fuse (which is essentially a jumper wire), you have an open circuit aka your "jumper" isn't "jumping".

With the fuse removed, you will still have 12 volts ACROSS the terminals. Because your jumper wire is gone.

With a melted fuse, you will have 12 volts across the terminals.

With a good fuse, you should have 0 across the terminals. A fuse =/= resistance. No resistance = no voltage drop.
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Old 09-02-2011, 05:08 AM   #25
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The ground side of the 80amp fuse should not be grounded. The only way to ground it is if a circuit is closed (i.e. a burnt up alternator). If I pull the main fuse and measure across the leads I get 12V. That means I am getting +12v from the battery (that is normal) and (-) from the other side (abnormal).

FWIW, my MSM Miata has a 100amp fuse. It reads 0V across the leads with the fuse removed.

One thing that might be confusing you is that the car is off. If you read 12V across all fuses in the car at all times your battery would only last a few hours and be dead every time you wanted to start it. There should not be any movement of electrons across the main fuse.
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Old 09-02-2011, 08:35 AM   #26
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n/m

misread again.
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Old 09-02-2011, 08:37 AM   #27
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Quote:
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there is no ground side of a fuse...
I do not know the correct terminology, but ok, the side of the fuse that is closer to a ground than the battery. Not sure what else to call it...
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Old 09-02-2011, 08:39 AM   #28
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With the fuse removed and the battery disconnected, measure the resistance between the load-side of the fuse and a good ground.
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Old 09-02-2011, 08:40 AM   #29
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Found a good tutorial on replacing the rectifier:

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Old 09-02-2011, 01:40 PM   #30
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Quote:
Originally Posted by rmcelwee View Post
Hey, does this mean I toasted the Alt when the cable burnt through and shorted on the exhaust? Just trying to sort out what happened. This sounds like a plausible explanation.
Ah, I see what's going on.

Yeah, I think your dad is probably correct. If the rectifier stack in the alternator has failed short (which is a little weird, as diodes normally fail open, but it's possible I guess) then current is flowing backwards from the battery, through the main fuse, through the alternator and to ground.



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I don't see what the alternator has to do with reading 12 volts across a fuse.
With the fuse removed, he's reading 12v across the fuse socket.

This isn't a very reliable test, as you normally expect to see voltage across a fuse socket with the fuse removed. The low side isn't "grounded" per se, but there is a path to ground through whatever load the fuse would normally be protecting.

The more reliable test is to wire a lamp into the fuse socket (the lamp acts as a current-limiter) and then measure the current through the circuit.


But I think Rob is on target here. The insulation on the alternator wire failed, grounding the alternator wire. This short-circuited both the main fuse and the alternator's rectifier to ground, destroying both. So even after the wire was repaired, the now-destroyed alternator was still causing a short-circuit.
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Old 09-02-2011, 01:56 PM   #31
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could that be my issue?
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Old 09-02-2011, 01:57 PM   #32
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What's your issue?
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Old 09-02-2011, 01:57 PM   #33
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shitty car syndrome.

i have continuity between 12v and ground dpending on the direciton of the probes, if I touch the main fuse to the engine gorund.
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Old 09-02-2011, 02:34 PM   #34
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Quote:
Originally Posted by Joe Perez View Post

This isn't a very reliable test, as you normally expect to see voltage across a fuse socket with the fuse removed.
Exactly...I am not understanding how he diagnosed a bad alternator by seeing something that is normal.
Wish I had a diagram in front of me to understand this better.
From what I see, the only way to diagnose this would be to look at amps through the terminals, not volts.
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Old 09-02-2011, 03:13 PM   #35
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Quote:
Originally Posted by MartinezA92 View Post
Exactly...I am not understanding how he diagnosed a bad alternator by seeing something that is normal.
Wish I had a diagram in front of me to understand this better.
From what I see, the only way to diagnose this would be to look at amps through the terminals, not volts.
Correct me if I'm wrong, but the alternator is on the unprotected side of the fuse.... A short there would kill the alternator most likely, and could blow the fuse.

So with alt. cable attached, you are basically giving the battery a ground, so it never makes it past the fuse, so technically you have a 12v drop across it. With alt cable removed, no ground pre fuse, current goes in and out of fuse 0v drop, so meter reads 0 with it removed.
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Old 09-02-2011, 06:15 PM   #36
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Quote:
Originally Posted by FRT_Fun View Post
Correct me if I'm wrong, but the alternator is on the unprotected side of the fuse....
Wrong is such a non politically-correct word. Think of it as being factually challenged.



The starter main winding is the only unfused circuit in the car.
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Blew the 80 AMP main fuse tonight. *** STILL BROKEN ***-9zhqy.gif  
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Old 09-02-2011, 07:31 PM   #37
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According to the wiring diagram posted ealier, it is unfused...
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Old 09-02-2011, 10:06 PM   #38
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According to the wiring diagram posted ealier, it is unfused...
That diagram came from the Mitchell manual. The one I posted came from the Mazda service manual.

One of them is incorrect.
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Old 09-02-2011, 10:31 PM   #39
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That diagram came from the Mitchell manual. The one I posted came from the Mazda service manual.

One of them is incorrect.
I see.
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Old 09-03-2011, 01:20 PM   #40
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(Posting from my phone.)

Fsm is correct when I've traced white 12v wire off alternator
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