sgtskid

The factory alternator went bad and I had it tested at autozone to verify. I put a new (rebuilt) alternator in it and the car still wouldn't charge. Drove to autozone and had the battery and alternator tested and they told me the battery was dead and the test wouldn't be accurate unless I charged it. This after being on a charger all night.

So, I went home removed the battery and drove back to autozone. Instead of charging the guy just gave me a new one (under warranty anyway). I put the new battery in and drove the vette back to autozone again. .. he hooked the tester up and for the first ten seconds of the car running the output was 12.2 volts. Then it suddenly jumped to 14.8 and stayed. He said the output should remain constant and gave me another alternator.

I put the new alternator in and went for a drive. Volt Guage (on dash) reads fine initially but after a few minutes of driving it falls close to 8 volts. I verified this morning with a volt meter.
Car off and cold, battery measured 12.8 volts and resistance between the alternator and positive terminal was 1.8 ohms.
I started the car and measured the output on the back of the alternator and it measured 14.87volts and the Gauge in the car looked good. Went for a drive and Guage slowly dropped by the time I got home. 15 minutes time frame. Again measured the output and it was 11.73volts. I turned the car off measured the battery at 12.1 volts and the resistance rose to 2.1 ohms. I statted the car again and voltage was back up to 14.31 volts at the alternator and 13.8 volts at the battery.

I have checked and cleaned battery connections and alternator connections. Is it possible I have another bad alternator? Or am I missing something?

Pics in order of description.

sgtskid

I apologize ahead of time for the typos. I'm typing on a phone.

whalepirot

It is possible.It appears the voltage regulator is not exactly doing its job with some amount of run time.

Good news: it's easy to r/R.
Bad news: rebuilt parts are different from manufactured. The former just get what tests bad; the latter, mostly all new inside.

At 2.2 volts/cell, a fully charged battery will test at 13.2vdc. The regulator will typically produce show more than that, as shown on both your gauges, dependent upon electrical load from the car, including recharge requirements.

I'd check the battery terminals for lack of corrosion; similar with ground and (+) wire attachment integrity, then have that rebuilt ALT tested and probably replaced by a new or reman unit.

sgtskid

Yippy.

aDigitalPhantom

Yes it is possible to get two bad reman alternators in a row.
I once got 3 bad remans in a row for a 71 GMC....

jtc44

A battery at rest should have 12.6 volts 2.1 per cell.
A resistance check is not good enough you must do a voltage drop test IMHO you have a hi resistance connection or a poor ground both can be found with a voltage drop test.
Google realfixesrealfast.com It will open up new doors for you.

sgtskid

I ordered a new alternator. Should be here Tuesday. Lets see what it does.

sgtskid

I understand the concept of a voltage drop test. But I fail to see how it applies here. If I tested the back of the alternator, the source, and it reads 14 volts one minute and the next it says 11 what would be in between consuming the volts?

Cliff Harris

I have gotten a bad alternator right out of the box. Since then I have them test it in the store before taking it home.

My alternator went wacko a few years ago. Sometimes it would charge and sometimes it didn't. I took it apart and cleaned everything and put it back together and since then it has been fine. Your alternator might have an intermittent connection internally or a bad voltage regulator.

davewhtt

Cliff is the voltage regulator built into the alternator? I read somewhere in an old post that it could cause the battery to drain when car is off? Back in the old days the regulator was on the firewall or inner fender. Dave

whalepirot

Yes, and stop calling me Cliff!

There is a rectifier circuit inside the rear of it that may have a dying diode or whatever; not your problem. The return is.

sgtskid

What do you mean by the return? I bought a new alternator, not rebuilt, and installed it. When i started the car the lights were dim. All of the sudden the alternator made a whistle and the lights became bright. Voltage output was good. Took the car for a drive and 30 minutes into it alternator stopped and voltage gauge dropped. Now I'm stumped.

whalepirot

Return of the part that will probably test faulty, new or not.

GQ-ROD

OP, do not get any alternators from autozone.

Their return policy has changed, and they are now testing them in house, not all associates know how to do this properly and they will show that the alternator you bought is still good in order not to honor the warranty, because WITHOUT the test showing it's bad they will not always return it.

Had to take it to another autozone to have it tested again, then the manager authorized the return even though their machine showed that it was good, it was not since it was over charging.

Had this problem twice in six months with their alternators.

Got and advance auto parts one and it's lasted 2.5 years.

Get a napa one or advance auto parts one they seem to be better.

volkswagens-for-life

Did not read thread, but a bad battery will zap the diodes in our alternators, thus frying them. Maybe your model year ditched the diode-based alternators, but on my '87 this happened over winter. Put the car away w/ a poor battery (too lazy to disconnect it), summer came, installed new battery.. alternator was fried due to the bad battery zapping it while sitting.

aDigitalPhantom

I would be hesitant to buy an alternator from o'reilly auto also. The average o'reilly auto employee believes that if you clock and alternator 180 degrees you reverse the output polarity. I asked that to a few of them just to see what they would say it was extremely pathetic.

sgtskid

Three bad alternators?

desertmike1

Think of an Diode as a check valve! The Alternator's winding has three + and three - Diodes for a total of 6, if any one of these fail you loose 33% of the total amount of the Alternator's potential output..

These diodes prevent the Battery from discharging in reverse flow through the Alternator's winding, that's all they do, and if they go bad, they stay bad!

the OP has what sounds like an intermittent failure that might be heat related. It seems odd that he has three Alternator's in a row that exhibit the exact same symptoms, but then again maybe he should play the Lotto!

just saying..

GQ-ROD

Yes it's highly possible if all the alternators are coming from the same place, and that lot was rebuilt at the same location.

However 3 does seem odd statisticly.

Double check that your battery is actually good and was not fried by the alternator by doing a load test on the battery again.Should read no less than 9 volts while cranking at the battery using your dvom.

Double check the alternator exciter wiring to ensure that it has current and not shorted to ground.

Double check charging voltage at the battery not the alternator to ensure you have 13.5 to 14.5 volts this at least rules out bad wiring from the alternator to the battery.

The wiring in these cars is old, and gets brittle, it's difficult to see any real damage unless you test and follow each wire from beginning to end testing along the way with a test light/dvom to see if there's continuity.

It can be frustrating but methodical testing is the only way to ensure you have ruled out the obvious things.

Worse comes to worse get you money back and get an alternator from napa. They offer 10 % discount if you have a AAA card.

Lastly check your battery main ground, this can be notorious for causing issues.

Hope this helps.

volkswagens-for-life

Very interesting.. thanks for explaining. Why 6 diodes to prevent reverse draining though?