Weird Charging (or not) Problem

[RichardH]

Calling all yo automotive electricians.

I have a basic understanding of how an alternator works...... Voltage supplied via Warning lamp which energises the system. The alternator induces an AC voltage which is rectified and compared to the supplied voltage, but at an increased current. This is supplied back to the battery to charge it.
I have a weird problem with my '40.
I have had the Alternator tested and it is "fine".....

However........
On initial turning of ignition key, the "Alt" warning lamp is lit.
As soon as the engine fires up, the warning lamp goes out, but the Alternator does not seem to charge.
When the engine is turned off, the "Alt" light does not come on - repeated turning of the ignition key does not induce the light to come back on.
If the car is left several days, the "Alt" light comes back on until the engine is started.

I have checked the electrics today....
There is power to the warning lamp at all times.
The warning lamp bulb is OK.
Earthing of the energising wire to the alternator, turns the warning lamp on.
The engine and the Alternator body are both grounded to "earth".

This seems to me like the energiser part of the alternator is faulty and is leaving a back EMF on the energiser wire, thereby putting out the warning lamp, but not energising the Alternator to produce any current.

How could the Alternator have tested "OK"?
Any ideas as to what could be causing this?

Any tests I can carry out in-situ, bearing in mind where the Alternator is mounted on a '40 (OS front of engine)?
The Alternator is similar to a Delco LRA177 or a DRA3457, but to be honest, I am not exactly sure what it is.:huh:

 

[Robin Little]

How many wires are there to the alternator ?

 

[RichardH]

sorry - two
well to be precise - three - it has one exciter wire and two to the output terminal (just doubling up on capacity).

 

[Robin Little]

Ok. Sounds like the standard LRA 101 type wiring

Basically the exciter wire as you call it, should have the same voltage on it as the battery eg 12 to 14v + What this does is excites the rotating part of the alternator which in turn creates a rotating magnetic field ( just like a permanent magnet) in the circuit is a Zener diode which will only conduct below a certain voltage. 13.8V approx. which in turn regulates the charging.

So to test disconnect this wire from the alternator and measure its voltage to earth ( wire should be +) alternator will only charge if this is below around 14v . ( usually about 13.8v) with ignition on

If this test is ok switch the ignition off and test from the heavy cables to earth again this should have the battery voltage on it. 12-14v +

If these to tests prove correct then chances are the fault lies with the alternator.

Do not have the excitation wire connected and the main charging circuit disconnected at the same time with engine running as this will destroy alternator.

What is the battery voltage with the engine stopped then with it running?

 

[Robin Little]

on another though . Sticking brushes!! if there is no circuit through the brushes then charging light may not come on depending on the circuit.

 

[RichardH]

So to test disconnect this wire from the alternator and measure its voltage to earth ( wire should be +) alternator will only charge if this is below around 14v . ( usually about 13.8v) with ignition on

If this test is ok switch the ignition off and test from the heavy cables to earth again this should have the battery voltage on it. 12-14v +

Battery volts is low (running motor at night from West Meon to home without charging). - about 12V.
voltage at end of small wire = batt volts. Will check heavy cables, but think its the same with ign on, but engine not running.

 

[Seymour Snerd]

Just a dumb generic thought about how the alternator can test "good" but the symptoms come and go: the problem could be intermittent and triggered by heat. However that does not explain why the warning lamp behaves properly while at the same time there is no charging current. So to double check: is it true that if you fire it up cold the light comes on with the ignition switch, goes out when the engine is running, but does not charge the battery?

Just remembered something: if there is high resistance between alternator and battery (like a corroded connection to battery + post) the alternator can "think" it's charging the battery but actually isn't, while ohmmeter and volt meter checks won't show this because those all happen at low current. So check the integrity of all your "high current" connections. For example, with the engine running and the headlights on march your voltmeter from alternator down the charging wiring to the battery post itself. If they're not all exactly the same voltage there's a bad connection. Of course if they're all 11 V then my theory is wrong.

And, BTW, corroded connections are temperature and humidity sensitive.

 

[RichardH]

So to double check: is it true that if you fire it up cold the light comes on with the ignition switch, goes out when the engine is running, but does not charge the battery?

It is true that when it hasnt been used for a day or so, the warning light comes on, and goes out when the engine is running. The Voltmeter reads just above halfway (no scale), If, however, I turn on a load, the voltmeter drops fairly quickly as if the Alternator is not charging. If I then turn off the load, turn off the engine and then turn the ignition back on, the warning light does not come on. Checking the energising wire shows that the alternator end is not earthing the wire, and therefore the lamp is not coming on. Earthing the wire, off the alternator, immediately brings the lamp on.
I have not checked the voltage at the Alternator energiser position or the output terminals with the engine running.

 

[Seymour Snerd]

.
I have not checked the voltage at the Alternator energiser position or the output terminals with the engine running.

That's probably the next step.

 

[RichardH]

reet folks...

injun off
Batt - 12.42V
Energiser contact - 0.00V
Power output - 12.42V

injun off/ Ign on (No warning light on )
Batt - 12.42V
Energiser contact - 11.81V
Power output - 12.38V

Engine on
Batt - 12.23V
Energiser contact - 11.78V
Power output 12.18V

Grounding Engine - No difference
Grounding Alternator - No difference
Grounding Energiser contact - Warning lamp lights.

I think this proves that the Energiser circuit does not go to ground. Maybe a broken wire/ poor joint inside the alternator?

 

[Robin Little]

Looks to me that you have an alternator problem!!

 

[RichardH]

i think so too. Am gonna remove it (again) and get it checked (again) .

 

[Maurice Butler]

While you have it out replace the regulator - have had them play silly games once they get warm. Also check all the small diodes that feed the regulator as well as the big ones that feed the battery.

 

[Dave Wood]

I didn't get that it was an internally regulated alternator. From the terminal descriptions it sounds like an externally regulated one. Except the one reply regarding having it replaced( I am supposing that was in the context of an internally regulated one), there is no mention of the regulator at all. Could it be that it needs a regulator and that it is wired as if it was internally regulated? Just a thought.

 

[RichardH]

I have replaced both the regulator and the diode pack about 4 months ago. This time I will leave it to the "expert". Mite even opt for a more modern higher current Alt if it needs replacing. I think this one is 45A.

 

[Jack Houpe]

100 amp GM 1 wire, I love them.

 

[Maurice Butler]

picture here on how it all works Automotive Charging Systems -  A Short Course on How They Work on some regulators the IG & S terminals are connected internally to the B terminal.
the charging voltage should be 13.8-14.5, fully charged the battery should be near the 13.8v. The warning light should come on every time when the engine is not running, no if buts maybes. If it doesn't the wiring to/from the warning light, regulator, brushes and rotor is suspect. If the warning light fails it no longer passes power and will cause the alternator to not charge as it needs the small amount of current from the light to kick start the charging.

 

[Howard Jones]

The battery is 6 cells. Each cell generates 2 volts and the battery totals 12volts DC. In order for the battery to take a charge, the DC voltage coming from the charging system must be greater than 12V. Typically it's about 13.5 -14Vs DC.

If it is a internal type then the regulator should maintain the desired 13-14V output throughout the current capability of the alternator at the big lead output cable going back to the battery. Or at least that's the way it should work. Engine revs and current load will effect this. However if you have a external voltage regulator then the alternator voltage will vary with engine revs and then the external voltage reg. will regulate the alternator output to 13-14 volts.

If you have a volt meter in the car it should read the engine off / no load battery voltage of 12V. When the engine is on and running the voltmeter should read the alt output voltage of about 13 -14 Volts. Any load such as lights will pull the voltage down somewhat. When I have my high beams on, engine running, mine reads just about 11.5-12V
from a lights off/engine running reading of about 13 VDC.

Usually a good alt, good battery problem is bad connections at the battery. Take then OFF the terminals and clean both the cable connections and the battery posts. Steel wool and a wire brush will do a good job. While you are at it make sure the battery is full of water if its that kind. One dry dead cell will get you 10volts (remember 2 volts per cell) and create issues including damaging the alternator. The charging system isn't meant to "see" 10 volts and it will run at very high current draw until it over heats and blows a diode. This is why OEM systems use a fusible link. The link will open and protect the alternator.

You don't have a fusible link do you?

 

[RichardH]

Apparently it was the regulator playing up. Bit of a bum, coz it was only replaced about 6 months ago - lets hope this one lasts longer.