Tell me what happened and how I fix it

OK, I'm going to need a little help here today, fellas. I'll do my best to make a long story short:

'95 Miata M with OEM AGM battery. Car was sitting for a while and had enough juice to run the door buzzer and the dome light, but not to start the car. Buddy of mine decides to just jump start it and hooks up the cables to another car, but he got the cables wrong on the Miata end (the negative terminal is painted bright red for some reason on my Miata). There were no theatrics, no blown up cars or batteries, and the other car is perfectly fine. However, the Miata does nothing now; even the dome light won't light up when cables are connected correctly.

I put a 2 amp trickle charger on it last evening and it seemed to show that it was charging, but when I went to check on it this morning, it showed that it wasn't charging at all. I figure at the very least, I screwed up my battery and need a new one, and at the very most, I royally fvcked up my car and I need a new electrical system.

Can someone tell me what I need to check or do? Thanks in advance.

Reply to
tooloud
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There are several things that could have happened. If your battery had not been so low it would have acted as an electrical shock absorber an provided a bit of protection to your own car's electrical system. One of the most common things that happens during the scenario you explained is that the reverse current damages the rectifier diodes in the alternator. The rectifier "bridge" is made up of six diodes. When a diode burns out it usually fails in one of two ways, by either going open circuit or by getting a short carrying current on both directions. If it gets a short any charge you put in the battery will go straight to ground through the alternator as a short since the battery + and alternator output terminal are a parallel connection.

If your ignition was turned on at this time you could also have blown your engines computer module which controls you ignition and fuel injection systems since they are largely made up of transistors, diodes and IC Chips all of which can be damaged by reverse polarity.

The first thing I would do to test this system is to check all of your fuses inside the car and in the engine compartment. After they have all checked out OK disconnect the main output wire market + or BAT from your alternator, install a decent fully charged battery and see if it starts and runs. If it runs test the rectifier in the alternator by touching the wire you disconnected from the alternator to the post it was attached to. If you get a spark (even a faint one) you definitely have one or more damaged diodes. If that is all you have replace the alternator and consider yourself lucky.

Only electronics are damaged by reversing polarity everything else should be unaffected.

Reply to
Lug Nut
Reply to
Leon van Dommelen

Sounds like you've popped the main fuse or fusible link; I'm not familiar with the '95 fuse box arrangement or location of a fusible link, but I'm pretty sure there's a main fuse there of some kind that almost certainly blew during the attempted backwards jump start.

Perhaps one of the other regulars can help out here.

Good luck - Dana

Reply to
Dana Myers

Actually, you've all been a big help--the 80 amp main fuse I've read so much about today is blown. I've taken the battery off the car and am trying to trickle charge it now--anyone know if it stands a chance? I'm going to pick up a new fuse tomorrow and figure out how to remove the blown one--looks like it's bolted in.

Anyway, thanks for the help so far, guys--I'll let you know what happens. ;)

Reply to
tooloud

My battery was killed by the door being ajar all winter, and after a while on the trickle charger to bring the charge up high enough to put my battery minder on...all is well! You would never know it went dead. I figured it was the 80 amp fuse.

Reply to
Mike

I just hope you get it fixed and get back on the road so we can resume sparring about less important matters, like that damn fine motor in my S.... never mind. ;-)

My water-injected 2002 SE (it has a JR S/C kit + BRP 60mm pulled, good for a little over 7psi boost) just about gave me a heart attack the other night; it started idling terribly and lit the check engine light, throwing a P0301 - Cylinder 1 misfire. It was partially warmed-up at the time. I pulled the plug and found that it had liquid in it. Oh sh*t - blown head gasket?

Liquid proved to be clear water; no oil/petrol in radiator coolant and no white smoke in exhaust.

It was not a blown head gasket - my flakey 1 bar check-valve in the water injection system had fully failed, and water was being sucked into cylinder 1 at idle.

Pulling the check valve and plugging the line to the nozzle, and zip-tying the S/C bypass, the car runs great, no more P0301. I cleaned my shorts out and I'll replace the check valve as soon as I can.

Whew.

I hope your experience is at as pleasant, Jake.

Dana

Reply to
Dana Myers

It is. IIRC you need to remove the fuse box.

Reply to
Per K. Nielsen

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