I'm No Pro

I need to test the alternator and battery for voltage. My volt meter in the IC is reading lower than it should, and I recently had a no start situation due to dead battery. I'm thinking possible cable-splice problem.

I have not used a DMM before and in the instructions it states that it(DMM) should not be used to test voltage on car batteries. How do you test the battery for voltage if you aren't supposed to use these for car batteries? Can anyone tell me where to touch the two leads when testing the battery and alternator?

TIA!

Katman

>the instructions it states that it(DMM) should not be used to test voltage on car batteries.

That's kind of odd, without any reasoning..?

Anyway, on to testing with the meter.
Set teh meter to ~20V range, as you are trying to measure in the 10-15 volts range.

First, measure the battery voltage with nothing on, and motor not running. Remember thae value. Probably hight 11's, or low 12 volts.

Then, start the motor, and measure the voltage (with no big loads on) Should be higher than when everything was off. (Makes sense, as it wasn't charging before, right?)

Voltage at the battery when charging probably will be in high 13's to low 14's. without changing anything, also measure the voltage at the 'jumper post' thing under the hood. Should be a 'simailar' voltage. If not, further investigation may be required later.

Turn on some high currrent loads (high beam headlights, rear window defrost, heater fan) and voltage should go down a bit at the battery.(But not as low as when motor was off) . Turning off big loads, and voltage should go back up again.

Now to test for a bad splice:

With motor not running, turn on some high current loads. Hook one lead of the meter to the +ve of the battery, and run the other lead all the way to the "jumper post" under the hood.

(You'll need to extend at least one of the leads to reach the two places at teh same time, using some wire. Quick and dirty shold work, and the wire doesn't have to be able to carry any large curent)

As you are measuring, connected to both ends of what is essentially the 'same wire', in theory, you should get a reading of zero volts. In reality, you will get some voltage (but lower is better.) Set the meter to a lower voltage range, or you won't see anything. Say 2.0 volts, as you are looking (in the perfect world), to measure 0 volts.

If you get more that say, 0.5 volts then you may have problems at the splice.

(I'm not sure exactly what is acceptable. Let us know what voltage you measure?)

Also, can't hurt to clean all of the battery post terminals, and check the conditionj of the ground strap connection to the car body. All should be clean, tight and shiny.


Rereading your original reason for testing (no start)

Think about this: If the splice IS the problem, then the splice is limiting current flow to the starter (Read: Not loading up the battery as much). But could be the battery failing too, right?

A good test of the battery is to measure it's voltage WHILE CRANKING. (Assuming the starter even cranks? If not, you aren't even ready to try this yet)

To do the this properly, you have to put the meter tips on the battery posts themselves. Not even on the battery clamps, but on the top of the posts themselves (the connection to the clamps can reduce the voltage a bit, while under high current loads). Poke the leads in good to the battery posts, and crank the starter.

With the amount of current that the starter draws, you'll see the voltage go down. Maybe even down to 10 or less volts.

However, if it goes really low, then a good chance it's your battery at fault, asit can't supply enough current. If the voltage hardly goes down at all (and yet, still no start) , then not that likely that the battery is at fault, but rather probably a bad connection further down the line.

Even a crusty connection at the starter itself can cause problems. Replacing the starter can fix that (Even if teh existing starter is good), because you have to fix the crusty coneection to install the NEW starter.

GOod luck!

chmnyboy

I'm not sure how new you are to DMM's, but make sure you switch it to 20 volt direct current (DC is symbolized by the dotted line with a solid bar on top; This symbol ~ is AC)

**It is also important to note that, in my experience, a no start situation can be tricky to diagnose, despite the simplicity of the system. For instance, I've had times when both the battery and the starter tested good at Napa, but didn't work in my car. As it turns out, the starter and the battery were just weak enough (probably wiring related, too) to work at the store, but the additive effects of their weaknesses caused problems. Wow, this is getting really wordy...My point: Just because your battery shows 12v, don't immediately go waste money on a new starter...still might not be the problem.

I'm No Pro

What I'm concerned with is the voltage reading I'm getting on the IC gauge. It's registering just to the right of 12. I'm not sure if its the gauge or this is an accurate reading but with the battery being weak I'm thinking the gauge is probably accurate and the battery is not being charged. If the problem was at the splice would I see a low reading on the gauge? Or would an alternator problem be more likely to cause the low reading on the gauge?

The no-start was from low output at the battery, not the starter.

I'm No Pro

Do I touch the battery post or the cable with the red DMM probe, and ground the black probe?

Thanks again

mofologic

my dash gauge reads around 12 or a bit over, but 13.6-14 volts at the battery and jumper post.

I'm No Pro

My 100 gauge shows around 13-14, this car's reading around 12 has always made me wonder what was up.

If the gauge is the problem would it be the gauge or maybe some sender?

Katman

Nice writeup? Only fair, as you answered my autobox questions!!

Anyway..

When testing while cranking, put the red probe right on the battery +ve post, and the black lead right ont the -ve battery post. That is because you want to read acutuall battery voltage while cranking. Not battery voltage minus any losses through connections along the way! :-)


When measuring voltage for charging, you can do the same, or if easier, put the black lead to the body or whatever is convinient. No large currents to disrupt things when just charging.

Katman

It's not so much to know if that gauge is accurate, as to know if it's reading has changed lately.

In other words, if everything was always ok and it read 12v, and now all of a sudden it reads 10V, then something is probably wrong.

But if 12v on your gauge is really 13.5 volts on the car, sholdn't be a problem, as long as you KNOW that's how it works.

Now if the car is new to you, and you don't know where the gauge is 'supposed to be' under normal conditions, then you should figure out what's up.

Is the gauge reading low, because voltage IS low, or does the gauge have a bit of offset in it's readings, and that's how it normally works?

All an Audi adventure... :-)

boxerfan

when you turn the ignition key to the 'ON' position the charge indicator on the dash must light. If it does not then you need to check the bulb in the dash. IIRC the bulb is in the path of the exciter wire that goes to the alternator. If that circuit is open due to a blown bulb you will not have any charging because the alternator needs to have 12V on the exciter wire to initiate charging.

Good write up Frank.