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Re: Update - please help Posted by Ari [Email]  (#2847) [Profile/Gallery]
(more from Ari) on Fri, 19 Mar 2004 05:32:27
In Reply to: Update - please help, Karthik, Thu, 18 Mar 2004 22:46:26 Members do not see ads below this line. - Help Keep This Site Online - Signup |
What was the voltage on the battery (read with a DMM) when the engine was running? Has the battery light been on during any of this activity?
A few items-
Car batteries hate being fully discharged. Every time it gets fully discharged, it loses life. This is due to lead sulfate building up on the plates. So even a 'new' battery can lose its capability to hold a charge. What's evil about this is that you'll charge the battery, and it'll appear to be fully charged - nice voltage, the whole bit. Turn your back for an hour, and it'll be as dead as a stump.
If you've had sticky brake lights, and the battery has been fully discharged, and left fully discharged, a few times, it could be sulfated up. The only solution is to replace it.
With the engine running at idle, and most electrical loads off (headlights, fans, etc.) you should see a voltage at the battery of around 13.5 or more volts. That's the alternator at work. Nominal voltage of a battery just sitting around is from 12.5 up to around 13.0 for a fully charged one. In order to charge a battery, the alternator has to output a voltage HIGHER than the nominal battery voltage, so current flows into the battery.
So - with the battery connected and the engine running at idle, measure the voltage at the battery. Don't watch the EDU - it's not a precision device and it gets wonky at low voltages. If you're seeing less than 13 volts (give the engine a rev or two, see if it jumps up), then it's a charging system problem. That's not to say you don't also have a battery weak from repeated discharges - it's just that replacing the battery without addressing the charging system means you'll be setting the new battery on a fast path to an early grave.
A battery light is a good indication that the charging system isn't working right. However, there is one caveat - make sure the battery light is working! Turn the car to ON, but don't start it (with a charged battery). The battery light should come on, along with the oil light, and a bunch of others. If the Battery light doesn't come on, then there's a problem in that circuit. Why care? The current through the battery light is used to energize the alternator at start up. If the circuit is open, the alternator will never start working, and you'll be running on the battery. It could be a burned out bulb, but that's rare. The bulb gets maybe an hour's use in the life of a car. Most likely is the thin yellow or green wire that runs from the bulb to the back of the alternator. It hangs out in the breeze, and it tends to corrode away.
Another thing to check is your battery cables. Pull the clamps off the battery, and polish the posts and inside of the clamps with some fine sandpaper or Scotchbrite. Look into the insulation at the wire - is the wire clean, or is it gunked up with white or green powder? As an owner of an '88, I know you'll see some pretty ugly wire. If you suspect the cables, replace them. Also check the fuseable link mounted on the side of the battery box - look for corrosion.
If you're not getting a good charging voltage, also check alternator belt tightness (serpentine belt), alternator to chassis ground connection, and voltage regulator. The brushes on the voltage regulator wear down, and you can get poor contact and poor charging. If your VR is more than three years old, it's something to check. The solution is a new VR, as they come with fresh brushes.
If you're seeing voltages at least in the mid 13 volt range with the engine running, measured with a DMM at the battery, the charging system is probably fine. Charge the battery, and then shut the car off. Now, turn the car to ON, but don't start it. Turn on the headlights. Measure the voltage at the battery posts - Not the clamps, right at the posts. 11 volts or so isn't unreasonable with a headlight load and a not great battery. Now, measure from the negative post to the chassis. You should see a voltage less than 50 millivolts. This is checking the clamp connection, negative battery cable, and cable connection to chassis. More than 100 mv is a sign the connection is bad.
As to starting voltage as measured on the EDU - It's useful for trending, but not a great absolute number. The EDU grabs the voltage during starting as a way to roughly check the health of the battery. A lower voltage is 'worse' than a higher voltage. But it all depends on your car. If you normally see starting voltages at 12, but see it dropping down to 11 or 10 over time, then chances are the battery is getting tired. But you've got to take a lot into account - how cold is it? How warm is the engine? How quickly did it catch? All of these things can make the starting voltage higher or lower with no indication of battery health. And this voltage varies from car to car. I've gone through (or still have) 3 9000's. One shows a starting voltage of 12, and 11.x means I've got a week left on the battery. I've got another car that regularly shows 9.1, and will happily start showing 8.5 volts. Is that really true? With the first car, I know the battery voltage is dropping below 12 - it's just when the EDU circuitry in that car picks the voltage. So you've got to know the car.
posted by 192.249....
, Fri, 19 Mar 2004 07:47:11, Fri, 19 Mar 2004 05:32:27 <-- Viewing This Message
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