I have a 1991 MAZDA 626 2.2i sedan auto
I have just had a dead battery.
I will be getting a new one today.

1] How do I reset the ECU - method required?

2] Further, my son was trying to recgarge this battery with the terminals still connected [about 10 mins] - my undersranding is that thats a no no. Could he have caused some damage to the electrical system? Does anybody know exactly why you're supposed to leave the terminals disconnected when charging?

3] We temporarily replaced the dead battery which died at a main set of street lights with an older partially uncharged battery that still just worked. As I was driving the 626 home I noticed the charging light was on [understandable], but also the rear tail light and brake warning lights came on [on the dash panel]. There was to the best of my knowledge nothing wrong with the rear tail light and brakes/fluid. Would these lights have come on because of the flat battery charging heavily whilst driving home?

 

Put a new battery in today - motor starts but volt meter says 12.45 volts when engine off and 12.13 volts when engine running. I take it this means the alternators stuffed?

 

yes bad altinator and this is why the lights were on and it would not have done any damage by charging with the battery connected for i do this and have for years and never damaged one besides the altinator puts 12.5 to 14 volts on the battery anyway so no
ok so to reset the ecu disconect the batt and hold brake peddle for 5 m in and if the battery is dead and has been dead than it has been reset because it will only hold memory for a short while

 

Modern battery chargers in fast recharge mode will put out 17 volts or so in pulses for a short time, long enough to make the car think there's an alternator fault, and shut down the excitation to the alternator. In short order, the battery boost voltage will destroy the voltage regulator and/or the alternator diodes.
What's more, if you try to run your car on a half-dead battery, and it won't produce at least 12V after the engine has started, the battery voltage will not be enough to kick the alternator into life.

When you have electric problems, always follow these rules.
a) Fully recharge the battery out of circuit. b) Ensure the no-load fully charged battery voltage is greater than 12.5 V, or else use booster start cables to get underway.
c) Don't assume the alternator is no good until you've done a and b first, and that your alternator drive and wiring are in good shape.
d) If you've done a through c, and the alternator won't deliver more than 13.5 V to the battery at a fast idle, your alternator is most likely shot.

 

OK - I took it to the auto electrician who was busy, but told me if my dash lights [brake/rear light/charge] comes on its definitely the alternator.

So I went to the first wrecker and got exactly [lucky] the right used alternator [30 day guarantee] and decided to replace the crook one with this. My son and I tried to remove it from the donk, but what a mungrel - no room to take it out anywhere after disconnecting the bolts and alt belt. Definitely didn't have room to go down and no room to go directly up and out. So after some pondering we moved it as far as possible to the passenger side [left] [hard to get it past the tubes and speedo cable] then I removed the casing holding the fuel filter and after some squirming it popped out like a baby out of you know where.

The wrecker mitsubishi alternator went in OK and first time it kicked over OK all dash lights out. Sweet!

Just remember to leave one battery terminal off in all this as you can easily short out the alternator if your tools hit the alternator teminals.

Another thing was getting the alt belt tension right so that it wouldn't squeel on start - took a few retightens.

Thanks guys who replied to help! All done back on the road!

 

Well that sounds like a Mazda; you really have to sweat it to get at anything.
You were lucky though. Half the time you end up stripping the threads or shearing the head of the tension adjustment. Any time I think alternator or drive belt work, 2 days or so ahead of time I saturate the tensioner hardware with a release oil and clean the crud off with a brass bristle suede brush (careful not to contaminate the belt or pulley though). It's still worth doing this, because you may have to readjust the tension after the thing's been running a couple of weeks.

 

Thanks oldeng,

The drive belt on this MAZDA 626 alternator is a little thin and therefore a bit stretchy, so your right, it probably will require another tension. You don't get the feeling that its at the end of the tension - feels like a rubber band. Heh heh! But better to do it bit by bit than to overtension.

 

to manofsteel420204

ur reply to speedy123 about ecu reset, wer u asked him to disconnect the battery n push brakes for 5 min, do i need to disconnect both the terminals or jus negative terminal,

i had a check engine light on my 99 mazda 626 automatic 4 cl, error code was for mass air flow sensor(maf), so changed it, but now can't get the car started with maf connected, n without connectin it my car vobbles n shuts down at stop light, do you think ECU reset is a good option

 

Just the negative cable.

To avoid cramps in you lower extremities I suggest to limit your brake application to 5 seconds and don't push hard you only need to trigger the brake light switch.

Looking out for ya,

Tan

 

to tanprotege,

jus wanted to re-confirm wat im gonna do,

=> take out the negative terminal of the battery
=> without inserting the keys, press brakes for around 5 sec so the brake light turns on
=> let it be tat way till the complete battery discharges n the brake light goes away
=> put back the negative terminal, n jump start the battery
=> hope dis resets my ECU

plzz lemme know if im doin anything wrong

thanks