Alternator 101 or I'm apparently STUPID

Gentlemen:
I've been fighting this problem for 6 months now and I guess I just don't understand (electricity has never been one of my high points). When starting the car the Alternator Warning light stays out until approximately 1,500 RPM then goes out and never comes back on. Voltage at idle is 11.0 to 11.5 while at 2,000 RPM or above runs at 13.5 or so. I had the alternator rebuilt and upgraded.
The man at the alternator shop thinks the problem is a weak ignition wire to the alternator. He says that if the wire will not light a test light it will not excite the alternator. When I put a voltmeter on the ignition wire it will read battery voltage, 12.0 to 12.5 volts (a new battery which I have had tested - it was fine). However, it will not light a test light going from the ignition wire to ground nor from the ignition wire to the alternator. This I do not understand - how can there to be voltage yet not light a test light? This problem is driving me crazy and I am determined to correct it. Please suggests, ideas, explanations.

Thanks in advance
Steve Schroeder

 

Sounds normal. At least on the 3X8 series

 

My 308 has exactly the same conditions as yours--I just run it as-is and it has never been a problem.

Specific to your question on voltage--any high resistance circuit (wire with resistors) will show the same voltage at any point in the circuit just like a low resistance circuit (i.e. a plain wire) so long as current isn't flowing (i.e. there is no connection to ground). When you connect the test light to complete the circuit to ground higher resistance somwhere in the circuit is preventling sufficient current from flowing to light your test light. A perfect example is a a headlight with a poor ground. Plenty of voltage going into and out of the headlight but because the ground connection is poor (maybe due to corrosion) the resistance is too high and restricts current flow out from the headlight, therefore the current is blocked from flowing and the headlight won't light. Hope this helps!

 

That the way it works . So it is not broken , just drive it

 

Is this at the battery or the Alternator? Which terminal are you probing on the alternator?

What was upgraded?

The alternator is getting excited by the alt lamp. If it wasn't the alternator would not start charging. Also depends on the wattage of your test probe bulb V. your dash bulb.

The alternator warning light feeds a very small amount of current to the field winding (in an alternators case the rotor). When the rotor spins fast enought to induce current into the stator it starts switching the 6 main diodes and 3 warning light diodes. The three warning light diodes applies a +12V potential to the warning light wire floating it above ground.ie applying +12V to the warning light on both sides of it, "switching" the light off.
There probably isn't enough current supply to power a probe light but it will give you a reading of 12V when measured with a meter. You will probably find the alt light will glow dull when you probe it with a test lamp.

Is the problem you have been having for 6 months just high revs to turn off the warning light? This is normal.
Because these cars rev higher than your avearge V8 the pulley ratios is a little higher. If it's a major concern you could put a smaller pully on the alternator which will spin it faster at lower rpm and switch the warning light off at lower rpm, but also spins it a lot faster at higher rpm and wears out the bearing faster.

 

Perfectly normal, my 78 308 GTS and 87 328 GTS did this....there are MANY threads on this in both this site and old site archives.

 

Be careful about increasing the pulley ratio! I'm not sure exactly what type of alternator you have, especially as it's been 'upgraded', but if you reduce the pulley diameter, then it will have a greater max speed. This means, a greater chance of it flying apart!

As for the reason it 'wakes up' after giving the engine some revs, this is because an alternator will normally 'self-excite' after 3,000 (alternator) rpm, due to residual magnetism (effectively by-passing the ignition feed).

Still not sure about why you were able to measure a voltage, but the test lamp didn't light, but I'll talk to my colleagues about that one. If you can confirm the manufacturer of the alternator, that will help me.

 

Right,

Had a chat with a colleague of mine and this is what he commented, which seems reasonable to me;

I reckon it's high resistance down the ignition wire. If measuring the open circuit voltage, it will always read battery voltage (regardless of the resistance). Only when current is flowing will there be a voltagedrop due to high resistance. As you'vetried connecting the lamp down to ground, it doesn't sound like an alternator problem. I would suggest taking a separate wire from the battery, through the bulb down to the D+ (ignition feed to the alternator), and note the behaviour. If the bulb works OK, replace the ignition wire, or trace the source of high resistance (loose connection?).

 

This behavior is normal for the BOSCH alternators. gdbsti's descriptoion of the details is correct. In other words, the alternator has to be spun fast enough generate a minimum amount of voltage before the voltage regulator brings itself on line. This behavior is documented in some of the workshop manuals, and in BOSCH technical literature.

Can you be more specific about which 'ignition wire' you're talking about?

 

i disagree. if you have less than 12.5 volts out of the alternator at idle then something is wrong with the battery, the alternator and / or the wiring to and from both. its called a 12 volt system for a reason. yes the diodes open at 2000rpm in most applications to provide a extra charging boost to keep the battery charged. you should not have a reading of under 12.0 -12.5 volts on a running car. something is wrong.

 

On my alternator there are two wires. The larger runs to the battery by way of the starter. The small one is used, I am told, to excite the alternator. On my car it is a yellow wire with a green strip. This small wire is the one I am talking about. There is no resistence from the fuse block to the alternator measured on the large wire.
I thank everyone for the input.

 

The small green/black wire is the wire from the battery thru the alternator warning light (X) to the D+ (see figure below) terminal for the regulator inside the alternator. While lit, the light is providing minimum energizing current from the battery for the alternator's rotating field coil (G) until the alternator produces enough voltage from it's 3 coils (U,V,W) to run the field coil.

If your test light is from ground to the D+ terminal, the alt warning llight won't pass enough current to light the test light, but I"ll bet the alt warning light glows dimly (assuming the car is off), or gets brighter assuming the car is running. Actually, assuming the test light is a fairly low wattage bulb, I'd espect the 3 diodes going from the stator winding to D+ to be able to supply enough current to light it, at least dimly. However, never tried to see.

If the test light is from the battery's + to the D+ terminal, then it's in parallel with the alt warning light, but the field may have enough resistance so that the test light won't seem to light.

re:

Are you measufing this voltage at the D+ terminal, or at the battery?
If at the D+ terminal, don't measure there, as that's the field coil actuation voltage & will be being varied by the regulator. Instead measure across the battery.

If at the battery, they're too low, should be more like 12.5 at idle. Try measuring the large terminal on the back of the alternator, or where that cable connects to the starter.

If that voltage is significantly higher at the alternator, then something's wrong with the heavy cable between the battery & alternator. If the voltage is low at the alternator's low terminal, then something's wrong with the alternator. Possibly a regulator with a lower than spec voltage was substituted, one or more of the alternator's main diodes is bad, or one of the stator coils is opened up.

Here's an off-the wall one, is the alt. warning light an exact replacement? A lower wattage bulb would light up pretty bright, but wouldn't pass as much current to the field coil.
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Mine has been doing the same thing for 8 years with no problems otherwise.

 

the glowing light is a common old ferrari thing.



however as stated in my orignal post, you should never have less than 12.0 volts at idle on a running car unless something is wrong. most computer cars will not even run on less than 12.5 / 12.65 volts, depending on year and systems. my light flickered on my 78 when cold. i dont think my 77 has ever done it, and i know the light works as i just did the bulbs and rheostat bypass. i will check though

 

If an alternator drops a phase it will growl or have a low whine. One of the purposes of using the 3 warning light diodes is to indicate when there is an open or short circuit issue as it changes the voltage potential on the warning lamp, therefore making it glow.

This is very true. An easy check for Voltage drop is to use a multimeter (on DC volts) testing the voltage directly across the 12V+ and ground circuits. Connect one probe to the Battery + term and the other to the B+ term of the alternator while running/charging at 2000rpm. Loading up the system with headlights etc is helpful too. Also check the ground circuit between the alternator case/engine and Battery negative terminal. Anything over .25V (Volt drop) is excessive and needs remedying.

Most automotive electronic systems will run down to 9 Volts at least. That way when the battery voltage drops down to 9.5V~ during cranking the ECU will still operate allowing the car to start.

 

I've been taking the voltage readings at the battery, which matches the reading on an installed voltmeter. I'll try at the connection at the starter but I suspect it will be the same. Given the kind responses to this post I am inclinded to accept the fact that this is a 23 year old car and that the way it operates.

In any event, I once again thank everyone for their input.

Steve Schroeder

 

We're NOT trying to say that below 12V at the alternator output (or starter high current connection which s/b essentially the same) during idle once the alt warning light has been turned off by up-reving the engine is normal. It isn't. Should be at least 12.5V - 13V. If it isn't, then I'd suspect the voltage regulator.

Was the regulator replaced as part of the alternator rebuild?
Was it an exact replacement?
The 308 regulators are set to a higher output than most cars to compensate for voltage drop in the long cable to the battery. There are physically identical regulators that will fit right in, but are set lower. There are also aftermarket adjustable regulators that can be used.

What is normal is for the warning light to stay on until the engine's been rev'd up to 1800-2000RPM once.

 

Perhaps the answer is an after market adjustable regulator. Yes, the regulate was changed at the time of rebuild but I don't have the part number. Where might I obtain and aftermarket adjustable regulator?

Thanks

Steve Schroeder

 

The set voltage on the regulator will not effect the cut in or lamp turn off point. Keep the origional regulator if it is controlling the voltage properly. As previously stated you may decrese the cut in speed by installing a higher wattage (4w) bulb. This will improve it slightly, but if the problem is a poor connection or some other wiring problem, it will not help much. Do not change the pulley ratio, because as previously stated it WILL damage the bearings, especially at high RPM, and may cause the rotor to explode into the stator - causing much damage to the alternator and othe engine components.

 

Damian hit on a point that is more serious than most think. If you measure the pulley od's on some models, and calculate the RPM @ a 7500 shift point, it is not uncommon to have the alternator running well over 14000 RPM!
Alternators do not need the rpm to work like the old generators did, so all MFG's went to them long ago.
I've put a 4" pulley on the alternator, with the stock 2 valve dampner pulley @4.25" and ran it just fine. Even at an idle, with lights on and opening both windows at once didn't get a glimmer out of the light. Food for thought.
Kermit

 

It's available from TRANSPO Electronics. They have both a fixed & variable regulator that can replace the BOSCH regulator that Ferrari used.

http://www.transpo.de/catalog/US_cat.html

Select [Manuf. to Transpo]
Select Ferrari, then click on the p/n: IB354
IB354 is their fixed 14.1 V regulator. The data sheet lists the Bosch regulators it replaces.

Once at the IB354 page, the IB354 data sheet also lists the IB372 which can be clicked on to bring up it's data sheet

IB372A is the variable regulator, comes set at 14.4V which is a bit high, so will need adj.

I"m doing this away from my Ferrari records, but am pretty sure these are the right regulators for the 308. They are definitely a physical match.

TRANSPO only sells thru distributors. Many auto shops have accounts with a dealer that handles Transpo. I think I still have one, but haven't ordered in a couple of years, so they may have purged it.

The distributor may have the part listed under the Ace-Electric brand/pn which is shown in the Transpo data sheet. I remember something involving Ace last time I ordered something made by Transpo. Not sure what the relationship between Transpo & Ace is, possibly Transpo makes them for Ace who sells to the distributors under their brand.

BTW, Transpo also makes 28mm slip rings that can be used to rebuild the 308 alternator if oil gets on the brushes & destroys the slip rings.

Make the meas'ts at the large cable at the alternator, or the starter before you go replacing voltage regulators. If you don't have at least 12.5V there , there, then there's definitely something wrong. Even with a wrong regulator you should have at least that much voltage.

 

Every 308 & 328 does this. My advice" LEAVE IT ALONE!

Dave

 

I thank everyone for their knowlegdeable input. I believe I am going to take the conservative route here and just leave the bloody thing alone.

 

Are you saying that this is normal: