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>.