Hey so I got this old mazda 626, the 1986 2.0 SL hatchback, and it refuses to start, it works on jump start but as soon as you stop and idle it switches off. I suspect the battery to be dead, as it had been sitting a while (I did manage to start it and drive it a long way before parking it off 2 days ago, but yesterday and this morning it seems to be refusing to even crank) Any advice would be appreciated.
1 Answer
It does sound pretty likely to be the battery, if the alternator wasn't working it wouldn't run once you removed the jumper cables. Firstly, before you start troubleshooting make sure you haven't left your lights on, even a map light left on overnight will discharge your battery.
If you have discounted a mistake the two biggest possibilities are that you have a bad battery or a parasitic battery drain.
First test the battery with a multimeter just after you drive. The voltage should be at least 12.2 volts to start your car, anything lower than that means you probably won't have enough to turn the car over, but really it should be closer to 13.3 just after you drive. If it's much lower than that the battery is definitely dead and you'll need to replace it. Test it just after you drive and then the next day, if there's a big drop in voltage that's a definite sign your battery is dead, or that it's a parasitic drain.
If your battery charge is fine after driving but lowers quickly try disconnecting the negative battery terminal after you drive it and the battery is up to charge. Let it sit overnight and then check the charge again. If there's a big drop then the battery is bad, if the battery has kept it's charge then you have a parasitic drain.
A parasitic drain can be caused by a short or malfunctioning electronic device, you figure out the cause by disconnecting the negative battery terminal, putting your multimeter in between the post and terminal and then measuring the amps. Do NOT try to start the engine unless you want your multimeter to do it's best firework impression. Then pull the fuses one by one and see when the amps drop most, the parasitic drain would generally be a large drop.
Note that if the battery has been drained a lot, and often then it could have lost capacity and may need to be replaced anyway. But, do the check for a parasitic drain even if you think the battery is toast, you don't want to put a new battery in and have it ruined within a couple of weeks!
