Alternator 101 or I'm apparently STUPID

It is normal for alt light to stay on after starting until motor is reved. In the absence of other problems, should be left alone.

Dave

 

No, it doesn't work like that. It sounds like you still have volt drop. Did you test for voltage drop in the manner I suggested earlier in this thread?

 

I agree with gbsti, you may have excessive voltage drop in the cable.

Take the idle & 2500RPM voltage meas'ts at the alternator or starter, & also measure at the battery. That way the battery's at the same state of charge for both meas'ts.

BTW, the drop will be higher for 10-15 min after starting the engine while the alternator is recharging the power drawn during cranking.

 

You checked it with an Ohmeter? This still won't measure your voltage drop.

 

Correct!

Steve - -> Just to give you a little background (I may have mentioned this before on FChat), I'm the starter motor & alternator application engineering manager for DENSO UK, so I can give you some advice on the best way to measure what's going on! I'm not familiar with the 308 power supply system (well before my time!), so I will comment generically. Ideally, you need several hands or at least some help with this, but try to get the following readings in one hit for the main power cables;

1) <Vbatt> Battery Voltage
2) <Vsta> Voltage at the starter motor B+ terminal (assuming this is in series between the battery B+ and the alternator B+)
3) <Ista> Current flowing between the above 2 points (using a current clamp)
4) <Valt> Voltage at the alternator B+ terminal
5) <Ialt> Current flowing from the alternator (using a current clamp)

Using R=V/I, will give you the resistance down each wire, which should be less than 10mOhms (typically around 3mOhms) i.e.;

(Vsta-Vbatt) / Ista = XX mOhms
(Valt-Vsta) / Ialt = YY mOhms

XX + YY should be less than 10mOhms.



Similarly, for the D+ line (which is the most likely cause of your problem) between alternator and warning light bulb, check the resistance by measuring the voltage at each end and the current flowing (again, use R=V/I). The resistance will be higher than the main B+ cables, but should be less than 50 mOhms.

Hope this helps!

Dem.

PS:
Regarding maximum speeds, latest alternators are capable of 20,000rpm continuous, and the output voltage setting is controlled by the engine management system. How times have changed!

 

I'm not going to get into qualifications, experience and noting in this thread, but taking measurements with an ohmeter and calculating potential voltage drop is NOT the way to diagnose or fault find a problem of this nature. If my previous explanation on how to measure was not clear, let me know and I'll break it down further. Cheers!

 

Lets summarise a little:

1. Unless someone has changed the alternator pulley on your car from stock to a larger one for racing purposes or some other reason, don't change it. The standard one works fine.

2. Your warning light operates normally. It takes about a good rev initially to excite the alternator and start charging. If this was your only concern and you aren't having any battery problems, you could probably turn a blind eye to the voltages and carry on enjoying life.

But,
3. Your voltages do seems a little low @ 11.98V (Do you have any lights or Ign on when checking static voltage) and 13.2 running. This could be because the car has sat and not run/charged for a while, but I'd also check to see if the alternator drive belt is tight enough, incase the pulley is slipping.

4. Your regulator is internal sensing - That is it senses the battery voltage from it's output or "B+" terminal. The regulator switches the field winding (DF or the rotor) on and off at a high frequency to control the upper voltage limit. (Normally 14.2~V). It may help to put the higher setting regulator in, but I would still like to see some direct voltage drop readings first.

Even though you measured the voltage at the battery and then at the alternator, it will float depending on how much current is flowing. So, if you are still interested in checking the real time voltage drop, grab your meter and try this:
Connect one lead of the meter to the ground terminal of the battery and the other to the case of the alternator. You will need a long piece of jumper wire (small gauge is fine) to connect to one of leads of the meter extending it to check this as your taking a reading the length of the car.
Check the readings at idle and 2500+ rpm.
Then perfrom the same between the Battery positive terminal and the B+ terminal on the alternator.

As mentioned by Verell if the car has sat for a while and just been started and stopped without much driving the voltage will be a little lower and the charge current higher as the Alternator tops up the battery after the starter draw. This is good because the higher the charging current the more obvious any volt drop will be.

 

Steve,

That's a reasonable assumption, but additionally, if we assume that the reg and pulley are correct specification and the alternator is functioning correctly, the low voltage at idle could be due to excessive current being drawn (as mentioned in the last post, by a very low battery SOC, or possibly some other kind of short circuit / high current draw from somewhere in the car - this latter being much more difficult to trace, but a current clamp on the battery negative cable with the engine on and off will advise you of this current drain).

 

Reminder,
This all started after he had the alternator rebuilt. (Of course that may be when he started making meas'ts as well).

I've been assuming that you doesn't have ready access to a high current clamp-on ammeter & am trying to advise accordingly. If you're near an AutoZone, they do loan tools & you may be able to borrow one there. Having one would definitely speed analysis.

My $0.02 on the meas'ts:

Even given time between meas'ts the voltage drops look reasonable for a 308.
I was suspecting we'd see 0.5 to 1V drop somewhere indicating a pretty poor connection.

The ~13+ V at 2500 RPM is still low tho, s/b closer to 14V. I now suspect that the rebuild used a 13.3V or 13.5V 'universal regulator' rathar than the exact one which should be 14.3V for a 308. A severe load hasn't been eliminated & could cause this.

Even with a wrong regulator, the voltage while idling seems low, as does the battery voltage when the engine is off. This does seem to point towards the battery or a heavy load somewhere.

A good check of the battery would be to disconnect a lead, charge the battery over-night or longer with a 10A-15A charger, measure the terminal voltage, then let it sit for 4-8 hours, then measure it's terminal voltage again. Both meas'ts s/b up around 13V. If it's still down around 12V or even less, then the battery's got a problem for sure.

If the battery checks about 13V -13.5V when charged, & doesn't drop much when setting for afew hours then it's probably OK & something's pulling it down when installed & connected to the car.

Another check after charging the battery would be to take to Sears, AutoZone, or another place that sells batteries & ask them to do a load test on it.

 

If that's 14V at the battery after running a while to replinish the power used when starting the engine, then that's a good number.

If that's measured at the alternator output terminal or the starter, then that's a bit low, factory regulators are set to 14.3V to allow for cable losses to the battery.

 

My 1981 308 GTSI has behaved like that since I got it 7 months ago. During that time I’ve put 2400 miles on it.

I found this thread a while ago and realized that it’s normal for the battery light to come on (after the engine is started) and stay on until 2000rpm. That all changed yesterday, though. The light went off the first time the tach hit 2000rpm, then a minute or so later it started to come back on again (dimly) and within a mile or so it was on solid again. The car’s 41 years old and anything deserves to fail on it at this point. Where do I start?

 

Clean your battery terminals, all ground points and the connection to the alternator to start.

 

Ok, that’s a good place to start and I’ll certainly do that.

A little more background on the car: it’s got 36K miles and I’ve got no service history other than the timing belt service and spark plugs right before I bought it. I wouldn’t expect an alternator or regulator to fail at this mileage, but they’re very likely 40+ year old parts and anything’s possible (as is a loose or corroded connection or even a broken wire somewhere). I did find a slightly loose connection at one battery terminal, but after tightening it and driving the car again the problem is unchanged.

I checked the battery voltage after securing the cable (12.50V). After starting the car and driving it long enough to get above 2000rpm, it showed just 12.02V at idle, and voltage came back up to 12.34V right after shutting the engine off. I checked about an hour later and the battery had recovered to 12.48V. It would appear that the charging system isn’t working at all.

Knowing that the alternator has to be removed to even access the connections on the back, I want to have a plan in case I don’t find a bad connection or ground.

I’ve read that the alternator is just a “Bosch 65A” and is pretty common. Are there different configurations? (Is there a particular part number that I need to look for?). Would I need to just remove and match what I’ve got, or is there a way to round one up in advance? Are decent-quality remanufactured units available?

I’ve also read that the voltage regulator IS particular to the 308 due to the higher resistance of the long positive cable running to the battery. It doesn’t appear to be particularly expensive even if sourced as Ferrari 107396 ($72), but is an equivalent non-Ferrari part available that has the same output? I suspect it’s just a certain Bosch regulator and I’m hoping it’s available without having to pay the “F” premium.

Having said all that, if I must go through the trouble of draining the coolant and removing the alternator, I’m not going to put it back together without replacing something (even if just the regulator, since it’s the least-expensive part and is the easiest to access). I want to KNOW that what’s going back together is good.

Has anyone run into a similar failure?

 

Update: I guess I got lucky in that replacing the alternator/regulator fixed my problem. It was an original Bosch that came out and aside from being a little dirty I was relieved to find that fluid leaking onto it isn't what killed it (photos attached). By the way: are there any good remanufacturing facilities here in the states? I'd like to have my core refurbished.

Draining/refilling/bleeding the cooling system wasn't as bad as I thought it was going to be. I disconnected the heater hose under the front of the car and about 3 gallons drained out. That was enough to be able to disconnect the coolant tube at the thermostat housing and rotate it downward (to access the alternator) without any leakage.

It appears that the 'right' way to do this job would have been to drain some of the engine oil, remove the dipstick tube, and remove the alternator pivot bolt. Instead I used an s-shaped 'obstruction wrench' and removed the two 17mm nuts holding the alternator bracket to the block (as detailed elsewhere in this forum). With the coolant tube disconnected at the top and rotated downward (horizontal) access to the alternator wasn't absolutely awful (thanks, Verrel!) and the alternator/bracket came out without having to remove the fuel tank, a/c compressor, or even the a/c belt.

I used Birdman's bleed procedure and it worked just like he said it would. During the post-bleed drives the coolant/oil temps never got above normal. In all honesty, had I not known any differently, the car acted completely normal during the first drive.

This forum has proven to be absolutely invaluable both in terms of information about the car in general and especially regarding any type of repair.


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