308 alternator "gearing"....

As I understand it, 308 alternators are "geared" to handle high revs, so idle speed revs tend to "under-spin" it.

I installed a VDO voltmeter where my old clock was was, so I can really monitor the volts as I operate the car under differing conditions. It indicates 13+ volts at moderate to high revs, but at idle, the volts drop down to around 10. This results in slow windows, wipers, etc.....

Is there any cure for this? I guess a bigger pulley would help, but would "over-spin" it at high revs?

My idle is ~900rpm after warm.

BTW, my alternator is newly rebuilt, so I'm pretty sure it's functioning "normally", for a Ferrari anyway!


Greg
1977 308 GTB USA version

 

Anybody? Bueller? Bueller?

 

Just what are you trying to cure? With a properly functioning alternator and a good battery I have never heard of problems arising from the 308 system.

The only time I have seen an upgrade required was about 1982 or so a 308 competed in the Canonball with so many aux lights mounted on the front the stock system would not keep up.

If it is the windows you are trying to fix go to the other end of the system. The window motors and regulator are a poor design. At 13 volts they are too slow and at 10 they are slower still. That is not the fault of the charging system.
Redesign where the problem lies, in the doors.

 

Actually, you'd want to put on a smaller pulley on the alternator to make it spin faster.

Not sure I'd recommend this though.

 

Sounds like you may have a weak alternator or your idle is set too low. You should still have 13.1 at idle and then about 13.5 volts at above idle.

Weaker older alternators tend to have this problem until you rev up the engine and can sometimes even loose field power lighting up the charging light on the dash.

 

Greg, he is right. I read the original post without really processing the numbers. 10V is too low at idle. Also you can't really test a charging system without being able to put a big load on it but 10V is too low regardless of that. At idle it is not capable of producing many amps (which your windows need also) but the voltage should be better.

 

Have you checked the battery? The battery should normally put out over 12v, and with the car running the fuel pump and ignition will put enough load on it to bring it down into the 11 - 11.5 v range without the alternator functioning. If you have 10 v at idle, the battery may be dropping too much under load. Just a thought.

 

For example- my 308 QV:

voltmeter on the battery-

Not running: 11.9 V

At idle: 13.8 V

this is a fairly old battery and an original alternator (no record of a rebuild).
Honestly- I don't know if it has been rebuilt or replaced and no receipt made it into the file.

 

My battery is a new Optima Red Top, and the alternator was rebuilt a few months ago.

I know what some of you will say--bad alt rebuild. Well, honestly, I don't think so--the thing puts out a fine charge, except at idle, so I think it's a question of idle speed.

I'll monitor my WARM idle volts more closely. I need to make sure that 900rpm my volts are actually that low.

Greg

 

A decent 12V battery, properly charged, will read more than 12V with the engine off, typically ~12.9V. That is the first check you should make. 11.9V would indicate a problem.

Next, it is possible to do a "load test" on your battery yourself. Disconnect the coil wire (and ground it as per mfr recommendation ) so the engine won't start and read the battery voltage while the car is cranking. Helps to have a friend here, or a remote starter button. (I prefer friends, they can buy you a beer sometimes.) What you are looking for is the voltage drop while cranking. Over 10V is OK, and as batteries tend to die a cell at a time, you'll se a definite drop from nominal of 2V for each dead cell, so you're looking at say 8.xV being a definite bad cell.

If you are seeing 10V system voltage at idle, I would be looking at the battery.

And one last thing, MANY charging system problems are due to poor connections. The first thing to do is clean all the terminals and posts. If I had a dollar for every battery and alternator replaced because of bad connections, I'd be driving an Enzo.

 

Thanks Scot. Good info--I'll check it out..


Greg

 

Hey, hope it helps, that's what we are all here for.

Just wanted to add:

The reason you do a load test on a battery is because a dead cell will still take a "surface charge" so that the battery voltage will read 12+ volts without a load, but this is no guarantee that the battery is good. The load test will expose the little imposter cell that's trying to fool you.

 

Can you use an ammeter on the battery to get anything useful?

Ken

 

Is it possible my brand new Optima red top is faulty?
Anybody here ever bought a bad Optima?

Greg

 

Defective batteries aren't uncommon, though I don't have any experience with the Optima. If you turn the key on (not started) and turn the lights on, the battery should still read above 11 v if it is in good shape. That's why I questioned the whole thing about being below 11 v with the engine running.

 

I'd bet the alternator before the battery if the battery is new. You rebuilt the alternator, but did they install new diodes. It still sounds like a weak alternator and you have to rev it to excite the diodes on the field side.

Seen it a jillion times. I'm assuming you do not have a VAT (volt amp tester). You can drop by any Interstate Battery dealer and have them hook up a VAT and it will completely diagnose the battery, starter, alternator charging and diodes.

We have a little Midtronics handheld VAT unit that has an IR printer to do a receipt for the customer. It tests the battery CCA and general condion, starting voltage, charging system at idle and fast idle, with and without load (headlamps), checks the diodes, etc. I think even the dreaded Autozone has this piece of equipment and will do it for free....but for heavens sake, keep your Optima. It also has a 2 year complete replacement warrantee and prorated up to 60 months after the first 24 months.

I'd go to Interstate Batteries first, it will save you time and money.

 

We are an Interstate Battery dealer and Optima is now owned by Interstate. For Ferrari, it's our no. 1 selling battery. I've sold probably 30 Optimas in the past 18 months. Not one has been faulty, most arrive with higher CCA than specified as well.

 

Thanks Speedmoore. I'm going to have the complete electrical system checked out in the next few days..


Greg

 

Apparently 12 volt lead acid batteries have the following characteristics
charge --- volts
100% --- 12.7
75% --- 12.4
50% --- 12.2
25% --- 12.0
discharged - 11.9

So if your battery reads 10 volts at tickover it is sort of dead. Alternatively it could be supplying a terrific current at the time, but it would have to be a lot more than just the fuel pump and the ignition.

good luck

 

So, if I'm getting 11.9 v on a static battery, it needs replacing?
I think the battery is about a 4 year old Interstate. I've had the car for 2.5 years and it has only needed a charge once after sitting for three months.

Thanks for the info- good stuff.

jwise

 

11.9 volts means its currently discharged. It wouldn't necessarily mean it needs replacing, maybe just re-charging. Old batteries seem to need charging more often. I usually work on the premise that if the engine starting gets a bit sluggish in the Autumn, you may as well replace it (the battery!) as there is no way it's going to get through the winter without causing problems.

 

I think if the battery were actually reading 10 V you wouldnt be able to even start the car. Maybe the first thing is to verify you have an accurate guage?

 

i agree 100%. i dont see how it would run at 10. i would also suspect the rebuilder only did the brushes and windings and did dot replace or check the diodes. the diodes are the # 1 reason alternators go bad.

any reading under 12.5 from a lead acid or gel battery means its discharged. 13.1 is the the bare bones minimum for a running car at idle. older cars are more forgiving that the new computer crap cars . lots of little "bad connections" due to corrosion here and there in a 25 year old will cause all sorts of issues. i would start by tracing the big 12v draws and all the ones that have constant 12v, horn relay, brake lights, then go to the ignition and headlights. also check for a parasitic draw. disable any fancy stereo and double check to see if its wired AND GROUNDED properly.

 

Good advice guys, thanks..

Greg

PS--It ran at 14 volts all day yesterday....(except at idle, where it's always a bit lower)...Weird.....