Charging system mystery.

[RoyBatty]

Made the 100 mile drive to Hayward yesterday to balance a propeller for a guy at the airport there.
The trip was great with the spider running great and no indication of trouble.
That is until I pulled off the 880 and noticed my turn signal were not cycling at all.
Right away I was concerned about a low voltage issue. Note that the low voltage lamp in the bottom of the tach never came on.
Turns out my customer is an Alfa and Fiat lover also. He had owned several spiders and currently has a 74 Alfa GTV and an 85? GTV.
Once I was finished with his propeller, I started into the troubleshoot of my problem. Low voltage at the battery resulted in the typical no start with the starter solenoid clicking at us. Once jump started the alternator showed 7-8 volts on a digital VOM.
FIrst thought was that the 95 amp alternator died. I did notice that the rear bearing dust cover is gone from its opening.
The closest O'reillys did have a stock externally regulated alternator, but it did not come with a pulley and I did not know if the existing pulley would transfer. My prop balance customer had given me a ride to the parts store and I did not want to burn up his time. Not knowing for sure if I could get the replacement alternator to accept the pulley I had, I opted to buy another battery and make the trip home just running the ignition. So I had to race the sun home. Once the battery was installed I started up and just drove straight home with no issues. Still no low volt light on in the tach.
Did the 100 miles home and got out my meter and found I had an alternator that was putting out 13.8 to 14.8 volts depending on how I loaded the system.
So what do you think?
I think I have an intermittent failure of the 95 amp internal regulated alternator. It is one from Allisons I think. And I am NOT complaining about the alternator purchase, its a mechanical part made by people, failure is expected.
Coupled with the alternator, I think I have a bad lamp in the low volt warning light. I have had the guage cluster out recently and I suppose it's possible I have a bad connection to that lamp?
Any input?
P.S. the dual IDF's are running about 22 MPG with me cruising at about 3700 RPM at 70 MPH. That and more power than before!

[aj81spider]

How old was the old battery? I had one fail once that had symptoms like yours.

[RoyBatty]

WalMart battery less than 1 yr .
Edit:
Lamp for low volt warning was bad.
System still shows good alternator output.

[spider2081]

Not sure about the 95 amp alternator circuit, but for my stock 1981 Spider the alternator needs to have the low voltage light working in order for the alternator to work. The alternators internal voltage regulator needs voltage on the light wire at the alternator for the alternator field circuit to operate properly.

Does your alternator have a single a single black/violet wire that pushes onto a spade connector on the alternator?? If so I believe if you ground that wire the low voltage lite in the tach should light. That is a simple test for the light in the tach and its circuit.

[TX82FIAT]

I have the 95 amp alternator in my car and the last battery only lasted a year. Spent some time with voltage readings at the alternator and the battery. Put a USB/voltage meter hook up into the cigarette lighter to measure volts when car is off and on. What I came up with after a while was the battery ground strap was insufficient to complete the circuit. I put new ground strap in for the batter and no issues in over two years. Alternator puts out 14 volts consistently.

[81SPIDERMATT]

I have read also that a non working light will cause problems .... it comes on at startup and i always look to make sure it does as it can be a frustrating thing to track down when problems arise ...

[RoyBatty]

I think we may have hit on something here.
If I am correct, the blown low volt lamp prevented the field voltage from reaching the regulator?
This then prevented the alternator from putting out a charge.
Is this correct?

Edit
Whoa back up the bus.
The above explanations do not work as my charging system was showing proper operation when I got home from my trip. Before I went out this A M and replaced the low volt lamp.

[RoyBatty]

Before I went into any great deal of tearing into the electrical system, I took the battery into the local Wally world to have it load tested. The put it on their machine and after about 1 hour the verdict came back. BAD battery.
Wahooo. I lucked out and bought the correct part out of necessity to get home.

[TX82FIAT]

Good to get confirmation that the battery was bad. Batteries are pretty durable these days so i would lookin into what may have caused the battery to go bad. The good news is that you really don't need to do anything now. However, if this battery goes bad in the next 18 months I would take a look at the overall system. A new well established ground wire may help and they only cost a few bucks at any auto parts store.

[So Cal Mark]

the charge light MUST work to activate the alternator. Our installation instructions explain this. If the light doesn't come on with the key on, engine off the alternator won't work

[btoran]

mark - which charge light are you referring to? i had a few idiot lights in the center section of my 1975 dash that did not contain any bulbs. i had a local servcie station install the 95 amp alternator last year. how would i know if it is working properly? thanks.

[majicwrench]

A bad battery did not cause your failure to charge. Once running, the battery really doesn't matter much.
Most Wally world type places have no idea how to properly test anything. And the test equipment is there to HELP THEM SELL PARTS so what do you think it is gonna tell you??

If alt was designed to be powered from charge light, yes, light needs to work. BUT a lot of alternators will "Self excite" if spun fast enough just from residual magnatism (my theory), which is what likely happened on way home. Used to see this on GM alternators a lot. Once "exited" they will charge fine, even without functioning charge lite.

Light needs to work, belt needs to be tight enough you cannot spin pulley.

[RoyBatty]

Only problem is, it did not self excite with the old battery.
Question, how long will a fully charged battery run an ignition system with the alternator offline?

[majicwrench]

They don't always self excite. That is why you need the current from the light. If your old battery started the car on the beginning of your trip, it was likely OK. It just ran down because charge system did not work, thus the 8v at battery. Very very very dead battery. Then unless Wally widgets charged the battery for hours they did not test a fully charged battery. You cannot accurately test a battery till it is fully charged. And it takes hours to fully charge a battery.
How long would it run on fully charged battery wo charging system working?? Hmmm, gonna guess. If it just running ignition, once in a while brake lights or t/s, 3hrs. Just guess. Might be fun to do the math and figure out closer.

[So Cal Mark]

the charge light is the red light in the bottom of the tach