My Alternator Test: opinions requested.

[Turtle]

Hello all,

I noticed that my alternator was doing some strange things (I have an analog voltmeter and the needle was doing a lot of dipping during load, signal and wipers especially) so I installed another (not new, but fresher) one. The voltmeter needle was steadier after that, so off I went. Soon after that (about 3 weeks), my battery died. Kaput. Damaged beyond repair. Couldn't even jump start it. At the time, I didn't know what the true culprit was: The alternator I just installed, the alternator that I removed, or the battery itself.

I bought a digital voltmeter, ammeter combo and installed it with the new battery along with upgrading the "big 3" as a precautionary measure. With the new alternator, the voltage remained steady at 14.6~14.8. However, the amps would read below 10 at first, which is normal, but after some driving, the amps would drop to as low as 1.3, but was consistently high 2s, low 3s. Even when I turned on the blower, the high, beams, etc., the amps would not jump up or down greatly, just maybe up to 9 amps, then go back down to 2~3. According to the FSM, the amps should read more in the 30 range and above. If it does not, the FSM states to replace the alternator (no internal fixes can be made, i guess). Here is a pic of the meter at the end of my drive:



Today I went to O'reillys with the old alternator and the car and asked for some tests. The old alternator was plugged into the machine and surprise, surprise, it failed all 3 tests (i saw FAIL 3 times, dunno if he retested it 3 times, or if there are 3 parts to the test). Then we went to the car and ran some diagnostics with an EXP800 on the new one. The test was normal, and he gave me a printout:



The printout seems to be based on the voltage. I asked about AMPs and he dismissed them, said they didn't matter, so I left.

Here are some questions:

Assuming that the ammeter is correctly wired, should I worry about the low amp readings?

Seeing that the EXP800 test says I'm good to go, am I truly good to go? I just don't want to do another song and dance to warranty my stupid battery.

It would appear that the old alternator started a chain reaction in the battery, so I would deduce that the problem all along was the old alternator. I just want to make sure that the new alternator will last me so I can move on reasonably worry free (because all corolla owners should never drive completely worry-free), I just can't seem to find any definitive answer about the amps. The FSM doesn't even explain why lower than 30 amps under load is a problem, nor does it explain its cause.

Thanks, your opinions are appreciated.

R

[Barry]

replace alternator and inspect all grounds. i personally ran a same generation supra alternator which has a higher amperage rating than the AE86 alternator. the wire terminal is on the opposite side, but still works with any factory wiring.

[Turtle]

Yesterday, in addition to the "big 3", I added a 4 gauge wire from the alternator bracket bolt (top bolt) to the passenger inner fender. During my drive in this morning, the amps seem to have decreased. I saw it dip as low as .9 amps.

This is a brand new optima yellow top battery I installed, BTW. The FSM states that if the battery is full, the amp test under load can be less than 30amps, but it doesn't say how low.

Could my newly installed grounding system just be very efficient? My volts have not dipped at all. It's a very consistent 14.7~14.9.

Could it be the yellow top? I keep reading up that the yellow top requires low amperage to get charged, but I don't know if the alternator adjusts for this kind of thing.

another thing to note, the shunt that came with the ammeter is connected across the ground between the battery negative and the ground point on the driver's inner fender area.

A typical start up would make the ammeter read close to 30amps (because of the starter), then seconds after it will wind down to around 10amps, then settle at around 2~3amps.

R

[assassin10000]

Alternator grounds directly through the engine. Any power going through it will not register on the ammeter, given where you describe the shunt being.

The only thing you will see is anything drawn from the battery.


Andrew

[gotzoom?]

Maybe this will help answer your question. Think of electricty like water. Voltage is the pressure at which the electricity is "pushed" down the wire. Current (amps) are pulled down the wire more like vacuum pulling water through a pipe. The amount of amps you see are caused by various things in the car consuming electricty. If you drive at max rpm with every light in the car on, the wipers on fast, the rear defroster on, the radio at max volume, etc, you should be able to find the theoretical maximum current draw on the vehicle. The alternator is rated to deliver some number of amps for a given amount of time. As long as your max current draw is significantly less than the max rated current delivery capability of your alternator, you're safe. Typically engineers design circuits to operate at 50% of max capacity, so if the car came with an 80A alternator, the car should never draw more than 40A at any given time.

I would expect the apms on your meter to vary as you do different things. The injectors pull a different amount of current when the car idles than when it's near the rev limiter. The ECU pulls more current typically as RPMs rise. 3A seems like a reasonable normal operating current to me.

And yeah, what Andrfew said. In order to correctly measure the current being drawn from the alternator, you would need to wire the + post of the alternator to one side of the ammeter and the wiring to the car to the other side of the ammeter.