Installed battery, now horn blaring and all electrical accessories are on!
 

I installed a new battery and when I connect the second (negative) battery cable the horn blares constantly and all electrical accessories are on i.e. parking lights, windshield wipers, interior fan etc. and keys and remote fobs have no effect; cannot turn off accessories/horn, cannot lock/unlock or start car. I've pulled the horn fuse (to work in quiet :-) but cannot attach the negative battery cable without all the electrical things turning on.
Advice/recommendations wanted on why "electricity is flowing" when I attach the second battery cable and how to stop it and get control of the ignition/locks again with the keys/remotes.

Not a F'n Clue. Bad computer?
Your car is Possessed!

what was the reason to replace the battery? lets see if there is something deeper.... because this sounds to me like a big short in the fuse wiring harness. any funny smells? and i dont mean funny as hahah....this is serious. when you replaced the battery, did you replaced the terminals? just making sure you put the cables on their correct position.

You were right.
Battery cables were reversed.
Now all electrical items work and fobs synced.
Only problem is no sound at all when I try to start the engine. Possibly starter relay was damaged when battery connection was reversed??
Or is it simply something more I have to do with the anti-theft system for it to recognize the key?

i dont think so.... do this test: turn on the headlights, try to start the engine. if the headlights dim or shut off, recharge the battery. if the headlights stay on and nothing happens, check that starter. or fuses. the headlights are on a different circuit than the starter. they should dim a little bit while the starter is working. that is one nice trick to determine if you have battery/alternator problems or starter problems. there is a thin wire going to the starter to engage the solenoid. check that it did not fall somewhere and you did not connect it. first make sure the battery is fully charged or swap it with a known good battery and try again. and the car computer has nothing to do with the starter motor.


pd. what is fobs?

Seriuosly! Do yourself a favor and never touch your car again! I have no idea how anybody could screw that up
Battery terminals are usually clearly marked, and positive wire is usually capped red and negative wire goes to the body!
I'm sure you screwed something up! Pray the computer is OK!
Take it to a qualified shop

i will share a secret... i have done that. there are circuit protectors that will prevent catastrophic damage. if the car starts then we will know if the computer is ok. but the car does not needs the computer to engage the starter motor circuit. go ahead frank, as far as i know, you are ok.

Let's pray all he did was ruin a main fuse/fusible link/relay.
This still requires a visit to a pro, take it to an authorized dealer (via tow!).

"fob" is the remote control for locking/unlocking doors and activating the emergency alarm.
Thinking about whether to trace to the solenoid or have it towed to the dealer - probably will have it towed to dealer.

before you tow it to the dealer, do the headlight test and let us know. a dc motor wont get damaged because the polarity is reversed but some starters have a diode to protect against inverse polarity and it could blow. just do the test and make sure the starter trigger cable is connected. if all your fuses are ok, you can remove the starter and get it checked. or, go to the dealer and dont tell them that you inverted the battery cables.

and also, try to reset the car fob. it could be that. reset it or bypass it if you can.

I've got my car back now and it's running fine. The cause of the problem: I installed the new battery backwards (i.e. positive cable to negative post, negative cable to positive post. After re-installing the battery the correct way:
- I replaced the blown 15A fuse for the interior overhead lights;
- Confirmed that all electrical accessories worked;
- Re-programmed the keys and remotes so they communicated with the car's computer;
- Attempted to start the car. All instrument panel lights indicated that the anti-theft system was accepting the keys;
- There was no sound from the starter solenoid/relay therefore no engine cranking - no start.
- Towed the car to the dealer.

What the dealer did:
- Determined that the main fuse inline with the positive cable had blown; Installed a new main fuse; started the engine; determined that the power steering was not working; mechanic tried but was not able to configure (program) and activate the power steering electronic control module; price of a new power steering control module and motor unit: $1,400 (!); I suggested looking for a used one; they found one for $500. The mechanic decided to swap the used control module and install it on my (original) power steering pump. I was charged 2½ hours labor plus the price of the parts.
I was fortunate that none of the other control modules (i.e. electronic modules) were damaged.

nice! good thing it was not that bad!

Ran into this issue after installing a new battery backwards (never install a battery in the middle of the night without light lol). Push start did not work, along with advanced key fob, power windows, power locks, lights, radio, bose, etc..

I blew out the main fuse (and a couple others) but the ECU/PCM was safe. This happened in a 2009 Mazda6, but I'd bet the electricals are similar. Also, the alternator probably received some kind of damage/wear because it's not protected by the 125A Main Fuse.

Edit: All dashboard lights were on and headlights worked