Possible alternator issue....need assistance

Use the meter to check the voltage of the battery before you start the car. Then check it after it's started and at low idle, check it again at 1000rpms and again at a little over 2000rpms. It may be that the light is just "picky" and there isn't much of a problem. Let it drop back down to low idle for a few minutes and check it again, it should be higher then the first low idle measurement. It may not be a big number, but if it's higher, then you know the battery is getting a charge. This is all just meant to be a simple check and will give you an "idea" as to what's going on.

 

Just to pile on a little bit... You should see just about 11.8-12.4 volts at the battery with nothing on, 11.5ish with the key on (no stereo or anything else on, just the pump going) and 13.2 to 13.8 with the car running. If the voltage stays in the low 12's and starts dropping when running and doesn't go up when you raise the rev's it's probably a weak alternator/regulator. If the voltage keeps climbing as the revs climb (16v-up) it's a bad regulator. A bad diode plate can cause the regulator light to stay on and also cause screwy problems. Fixing that requires alternator removal. And I'll say the same thing....this is just to give you an "idea" of what's going on. Good luck!

 

Most 308 owners report that the alt light is on when they first start the car and stays on until you rev the engine a little. My alt light is always on a tiny bit, especially noticable at night. It's a very crude circuit that runs the light, and even when everything is fine, a little bit of current runs through the warning bulb. If everything checks out with the voltage using a meter, don't worry about it. How old is the battery? Very often, you have a crappy battery and the alternator hasn't got much of a "reservoir" in the battery, so you will see a much more direct correlation between engine speed and electrical function, such as headlight brightness.

Get a trickle charger ($40 at Sears) and install a quick disconnect plug so you can keep the car on a trickle charger when you aren't going to drive it for a couple days. It makes a big difference in battery longevity and cold starting.

Birdman

 

My 78 did the same thing and so does my current 83.

On fire up the light will come on until the rev's go up. You will also notice a faint glow at night when the lights are on and it will be a touch brighter when the turn signal and lights are on.

I would only be concerned if the light was on bright regardless of RPM

 

Aaron,
Sounds pretty normal to me.

Birdman

 

Rev over 2000 RPM, and see if it tops out at 13v or so. If it gets to 14 or beyond, the VR is dying. But this won't necessarily illuminate the red light (I don't think) because the alternator doesn't see an undercharge.

How old is the battery? As they age and they can't take a charge as well, that would likely illuminate the red light. I think 11.75 is actually a touch low at idle but maybe on a Ferrari that's okay? I'd prefer to see 12.5-13v.

But if the battery continues to hold a charge and you can replicate the numbers again after a few weeks, you're probably okay and it's just a quirk of your car.

Ken

 

Sounds like your alternator is going bad, to me......

A local shop could rebuild it, I'd think. They are not special......my red light goes out as soon as the engine climbs above idle.....no need to crack thru 1K RPM....although I guess that's pretty much right above idle!

It really has to be above 12V to charge.....13.5 - 14, IIRC.

 

Don't hold me to this, but I think your car has a "GM" style alt. They don't put out much at idle and really need to be over 2000RPMs before they do much. Get a tender or trickle charger for your car. If you are like most of us then the car sits way to much and you need to keep the battery in good shape.

 

My alternator seized on my car and inturn broke the belt. Does anybody have another paert number for the alternator and belt? I would guess that someone would be so kind to post a model and part number as a cross reference. Thanks-Tom

 

Well as one or two have said previously, at idle (1000rpm) your alternator should output ~13.2-13.8V AND NO MORE! Right to the redline, no load to full load ie all lights blazing! The regulator pack integral to the alternator is preset to 'clip' the output to a max level, varies depending on the type of regulator pack but typically 14V when operating normally.

I've literally just refitted my refurbished alternator and all is OK. The first failed on the way to Le Mans and was outputting 15.8V+ cooked the battery & blew all of the H1 headlamp bulbs. LOL

A few things to check: Disconnect +ve battery terminal first.

a) Chassis grounds wrt the negative battery terminal, run a cable from the
alternator casing & connect either Black or re lead on your New Multimeter.
set onto max Ohmic range, connect remaining lead to Neg battery lead &
the reading should be fractions of Ohms, but more likely to be 2 -5 Ohms.
This is due to the fact that your average Multimeter only outputs a couple
of volts on this Ohmic range, DO NOT use an AVO/Meggar as this could
damge a whole host of electronics if there is an alternate root to ground
other than from the test point (Meggars output 500V or 1KV) Although the
2 -5 Ohms may seem way to high using a Multimeter, this will easily be
reduced/overcome by the current flowing and not cause a problem.
b) If your alternator connects onto the starter motor (like my Mondi) clean-
up the contact stud & apply a smear of Vaseline.


Need more info drop me a pm.

 

Older Ferrari’s have a self exciting or "One wire" alternator as it some times called, has only one wire running to it, that one wire is the main large battery wire. Don't confuse one wire because the plug going to the alternator only has one wire to it, this is a two wire system. The one wire, self exciting alternator uses a special voltage regulator that doesn't need an ignition wire to activate it.

This type of alternator only requires a battery wire to be hooked to it. The voltage regulator contains circuitry that uses the residual magnetism in the alternators fields to determine when to turn the alternator on. The only drawback to this system is the regulator does this by sensing the RPM the alternator is turning. So when the alternator gets to a certain rpm the voltage regulator "turns on". Typically you start your vehicle, rev the motor slightly then the alternator starts charging. This can be overcome by using a smaller pulley or by adding an ignition wire but I only recommend you add another wire if you know what your doing or have it done by someone who rebuilds alternators.

Basically, what does the little red light mean when it’s on and says ALT ? Well, it means that either the alternator output voltage is lower than the battery voltage, or the battery voltage is lower than the alternator output voltage. If the light gets dimmer as you keep revs up at the engine, then you most likely have a problem with the alternator. If it gets brighter, then the battery is most likely bad. You only need to rev it once to get it started, not to keep going unless your seriously putting a draw on your system (headlights, foot on the brakes, heater, audio system, fog lights, playing with the power windows all at the same time as an example)

Voltage @ battery before starting: 12.19 not good……should be over 12.7 to 13+
- Voltage after starting: 11.75 after staring…pretty normal
- Voltage @ 1000RPM: 11.75 after starting…..very bad, you only need to tap the gas to get it started then it kicks in. The misconception that people think how an alternator works is this, they think you have to keep the rev up, wrong, your not inflating a tire, your turning the pump on…
- Voltage @ 2000RPM: 12.50 and rising through 13 when I signaled to the wife to let off………..still not good, too high an RPM to get normal idle power.
- Voltage @ idle again: 12.00 and falling, your regulator or complete alternator needs to be replaced as well as a new battery due to the voltage before start.

I have 25k invested in car audio/video running 2000 watts RMS. Voltage and power is everything when pushing 140 DB. When I turn on my truck my system goes on as well. At the rear of my truck I have 3 digital volt gauges, one is on the main fuse block with power coming from the front and it reads 14.2V then two, foot long cable going to two 1.2 Farad caps and they read 14.1V and this is on idle. When my truck is off the rear fuse block reads 12.7V and the caps read 12.6V and this is after 15ft of 4gauge wire that creates power loss due to cable resistance.

Sorry for the long post…..just want to help.

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