Thursday, January 22, 2009

Electrical Madness, Part 2

Well, I couldn't wait till the weekend to look into this problem, it was just too frightening.

I went out Thursday morning and checked some of the fuses. The car has three fuse centers: two in the engine compartment and one inside the car on the driver's side.

The covers for the fuse boxes have a little info on what each fuse is for, but the user manual had complete explanations (though half the things there were fuses for I don't understand. What's HVAC and what's the Body Control Module??).

I pulled a few fuses that I thought might be related to my problem. I checked the fuse for the window motors first, since that's where I've been working. Then I checked some of the ones for the instrument panel, and also the starter since the car wasn't starting. I didn't check every single fuse, but all of the ones I looked at were fine.

Feeling rather discouraged, I got in the car and put the keys in to see what would happen. I turned the key to on, and the dashboard lit up but was acting a little funny. Some of the lights on it were flickering. Trying to start the car again produced absolutely nothing.

Then I noticed that If I stepped on the break, the lights on the dash would dim. That made me suspect that maybe my battery was just dead. I got my trusty voltmeter out and measured the battery, and it read 6.7V! A healthy batter is over 12V, so that's pretty dead.

Later at lunch I was talking to a friend about my troubles, and he was getting me worried. A battery that's discharged that much may not be rechargeable... I may have damaged it and need a new one. That would suck, I could definitely think of better ways to spend $80 than buying a new battery.

He suggested that I could try taking it to a Sears auto center and that they would recharge and test it for me. So that evening I went out to remove the battery, and tested it one more time. I must have been putting the probes in the wrong spot, because now it was reading 11.4V! That still qualifies as dead according to this chart:

Battery Voltage and State of Charge:

12.68v . . . . . . . . . . 100%
12.45v . . . . . . . . . . 75%
12.24v . . . . . . . . . . 50%
12.06v . . . . . . . . . . 25%
11.89v . . . . . . . . . . 0%

But at least it's not six and a half volts dead!

I read a good article on checking for parasitic drains on the battery. Since the batteries dead, it's possible that something I did damaged a circuit or relay in such a way that it's using electricity even when the car is off.

To check for drains like this, I learned you disconnect the negative (black) terminal of the battery, and connect your multimeter in between the cable and the negative terminal of your battery, then set it measure current. Unfortunately, my little pocket multimeter has a 200mA limit, and it just played a tone as soon as I connected it, meaning the current is greater than 200mA.

I also have a test light that I used. It lit up when I connected it, but I don't know how much current it takes to light it, so I'm not sure how much help that is.

Anyway, an acceptable drain is about 50mA, so if it's more than 200mA I've definitely got a problem.

So how do I investigate it? The article says you just pull fuses one by one and watch the current reading to see which fuse causes the current to drop.

Of course, I have a problem here now in that my multimeter can't handle the current...

1 comment:

  1. HVAC: Heating, Ventilation and Air Conditioning. That's what they call 'em on buildings too.