Blew the 80 AMP main fuse tonight. *** FIXED ***
 

I blew the 80 amp in the V8 Miata. Was just driving around the block and it blew. Tried to put another one in and it blew instantly. Where do I start checking?

What you need to know -

1) Just pulled the battery out while the hurricane was approaching. Used it for generator testing. Battery is back in and I do not see any problems with the wiring on that.

2) Got some rain in the car a week ago. Could have corroded something.

3) I'm tired and drunk and my mind is not working right now.

So ONLY the 80A fuse is dead?

I don't know at this time. I was letting a friend drive it around the block. We got about 1/4 down the road and the car shut off. I know it has headlights but I do not know about anything else. Of course, putting the key in the ignition does nothing. Too dark and too late to play with it. I'll know more in the morning.

Well the headlights and factory fans are connected on the battery side of that fuse so they would still get power. The rest of the harness is on the protected side of the fuse, but most (all?) of those circuits should be protected by lower amp fuses.

My first guess would be the battery cable to the alternator thats the only direct connection to that fuse. Next would be to pull up the fuse box and check for any shorts underneath it.

Good guess. I'll check it in the morning. OOPS, I guess this is the morning. Still some left in the Southern Comfort bottle. So I guess I'll check it in the afternoon...

Yea next step I would take is check to see if any other fuses are blown. If so then at least you have a circuit to work with. If it's just the 80A that should be easy. Not much touches that with out another fuse.

The starter solenoid is about the only thing I can think of that goes through the main fuse without also going through another, smaller fuse. Not to say that a short downstream of a branch fuse couldn't also take out the main fuse, but it would most likely have blown its branch fuse as well, and then the second main fuse shouldn't have blown.

Oh, the alternator. It feeds the battery through the main fuse, but a short in the alternator would pull power from the battery through the main fuse. Disconnect the alternator and see if that fixes it.

(Also, to avoid blowing a bunch of main fuses, take it out and install a 12v incandescent lamp in its place. A headlight would work. The idea is that the lamp should not light up when everything is "off.")

The blower motor has a resettable breaker on it, so a short on that circuit could blow multiple main fuses. You could try pulling out that breaker.

This all presupposes that there's not just a chafed wire somewhere, of course.

OK, not sure what all is blown but this morning when the sun came up (it was night when we broke down) it was very easy to see what happened. The harness that goes to the alternator was tie wrapped to the frame. The tie wrap snapped and let the harness fall onto the exhaust where it melted the insulation and shorted out. I'm pretty happy this all happened in my neighborhood and not out on the road somewhere. If you are going to break down, do it on the street you live on! Anyway, the car is up for sale and I would have hated for this to happen to whomever bought it. An easy fix - just have to find some wire to splice in and tie wrap it back up with heavy tie wraps.

Thanks for the help guys!!!

That looks like it came from the emergency bag I keep in all my cars.

Nope, still broken. I cleaned up and wrapped the harness that had melted and shorted across the exhaust. I was happy for a few minutes until I pulled my meter out and found there was still 12V across the 80 AMP fuse connector. Ugh!

What? 12v across the fuse... what should there be?

I was thinking 12V across it means that there is an closed circuit somewhere down the line (meaning a short somewhere). Am I wrong? When I work on stuff like this I get frustrated to the point that I no longer think clearly. It is a horrible condition and that is why it takes me a week to fix simple stuff.

So far I have pulled every fuse/relay out of the main fuse block and pulled the ignition switch harness. Still 12V across the main fuse. Will look at it again later when my head clears.

In case anyone wants to look. The 1990 Miata wiring diagrams (main fuse block on page #3). It looks like the ALT is my next point of attack.

http://www.lightweightmiata.com/docu...ing/90diag.pdf

n/m

Why am I thinking that your reading of 12v is correct...

I mean, if you check the voltage of the battery it will read 12v... (assuming it's good). I think you want to measure amps, to see if anything is drawing power? If you put your positive lead on the 80A connector, and the other to ground... it is correct if it's reading 12v.

Or am I going crazy.

I think it only gets power if one of the other systems is active (like you turn the key in the ignition and that switch gets closed). When I pulled all the other smaller fuses for the other systems I was probably just wasting my time. If the problem caused an 80 AMP fuse to burn it would have surely killed one of the smaller fuses. BUT, I think disconnecting the ALT will hopefully show that it is indeed the problem. I was hoping it wasn't the ignition switch because that would have been a pain to deal with. A crapped up ALT might be a pain too. Sucks because I had a Corvette alternator sitting around here for 20 years - threw it away a year ago. I did manage to find a Miata wiring harness that I thought was long gone. Cool because it has an 80 amp fuse on it (meaning I can save $5-10 on replacing a third one).

Edit - What I am saying is that there should be 12V to the 80amp but NOT ground. It only gets ground if another system provides a connection to ground.

Yep, I couldn't stand it any longer so I ran to the garage and disconnected the main hot wire going to (coming from) the alternator. My 12V across the fuse is gone. I don't know crap about alternators but I am assuming that means it is bad. Anyone an ALT expert here?

Maybe I'll try google <G>...

Nope, I'll do one better. Calling my dad....

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...

Dad says the diodes are burnt up. He knew what the problem was before I told him any details. I should have called him first <G>.

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).

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.

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.

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.

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.

n/m

misread again.

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...

With the fuse removed and the battery disconnected, measure the resistance between the load-side of the fuse and a good ground.

Found a good tutorial on replacing the rectifier:

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.



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.

could that be my issue?

What's your issue?

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.

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.

Wrong is such a non politically-correct word. Think of it as being factually challenged.

https://www.miataturbo.net/attachmen...ine=1314998110

The starter main winding is the only unfused circuit in the car.

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.

I see.

(Posting from my phone.)

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

Well fuck me. Good thing my WHOLE HARNESS BUILD IS BASED OF THE MITCHELL DIAGRAMS..

:vash::vash::vash::vash:

Although I AM using a bit of common sense, and the old harness as a reference.

It's ok. On average, the Mitchell diagrams typically only have 3 or 4 critical mistakes per page. :D

Finally got some days off work and pulled the alternator last night. Ran it up to Advance Auto and they put it on the machine. Guess what - the rectifier tested good. I've never been more disappointed that I had something not broken. Not sure where to go from here. It just HAS to be the rectifier. I don't get it.

What if you switch on without the alternator in place?

Hmmm, had not done that but would be interesting to see what would happen.

What happened:
- car dies, 80a blows (wire is laying on the exhaust)
- put in another 80a and it blows instantly
- tow car home and fix wire
- test across open fuse and see 12V
- test across wife's MSM open fuse and see 0V
- pull wire from alt and see 0V
- determine that my alt is bad (bad rectifier probably)
- get alt tested and find out all is fine with it
- scratch head and rethink

I have never blown a fuse after the wire was fixed (haven't tried). I just put the fuse in, pulled the NEG battery cable and measured for amps across battery cable and NEG battery post (reads 0.1 amps - dark current). This seems fine but the alternator is still out. I'm wondering if I have 12V but only 0.0000000001 amps. Could it be that I was just inventing a second problem (blown alternator)? I'm going to have to put the alt back in and see if the fuse blows. I hate doing it (I don't have another fuse - 3 was my limit). I'm hungry and tired (night shift) so I think I am going to grab a bite and take a nap.

First off, stop trying to measure voltage across the fuse. The only thing that's going to tell you is whether the fuse is blown or not. If it's blown, you will see 12v across it. If it's not blown, you will see 0v across it, regardless of how much current is flowing through it.

Try hooking everything up (except the alternator) and start the car. If it runs and does not blow the fuse, then attach the alternator. If that blows the fuse, then the alternator is bad- end of story.

I still don't understand what you mean by testing voltage across the fuse, do you mean you are putting one lead on the incoming side of the fuse and the other on the outgoing side (one on battery side and one on load side)? I agree with Joe, that's not a good way to test for your problem.

As much as I hate to say it, Joe's suggestion above probably should have been one of your first steps. Let us know what happens though.

OK, the car is fixed and back on the road. There was nothing wrong with the alternator after all. I did take it to three places to get it tested (they were across the road from each other). Two said it was fine and one place said the rectifier was bad. I've since learned (well, learned before and that was the reason I wanted to see what three different places said) that those testers are not to be trusted. Anyway, I was seeing 12V across the fuse socket (with the fuse removed). No, that 12V was not supposed to be there and did not exist on my other Miata. What I did not think to test for until I was told the alternator was good was AMPS. I was getting 12V but only 0.03 amps (what I thought in post #46).

In hind sight, testing for amps should have been my first test. Believe it or not, that is the first time in my life I have ever switched my meter to amps (never had a reason until now). So, in the end I learned something and fixed a problem (hanging wires) on the car.

rmcelwee, i just experienced the same thing after getting the car for 3 days. my 80 amp fues blew and i had the car towed to my shop. I had a suspicion that the PO had jumped the car a few times and that may have weakened the 80amp fuse over time. My mechanic says that fuses go from time to time and made the same comment as you, you're luck it blew locally and not happen while you were a hundred miles away.

Glad to see your car is up and running again. g/l on the car sale.

I've never had a fuse "just go". Nope, "millions" of miles under my belt and have not seen it. I guess anything is possible though.

Did it happen to you a hundred miles away? I had TWO spark plugs back out of my pickup truck about 300 miles from home once. Like I said, anything is possible.