Oldiebut goodie wrote:Ideally yes, but with the petrol setup no, it is because of these losses otherwise the sense would be connected directly to the output at the alternator. That's the way it works best! That's why you need to check exactly what alternator you have as there are differing methods with differing results.
I don't see the purpose of the separate 'sense' wire though. If it's there to measure battery voltage so that the alternator knows what charge to put out, it can surely never do that when the alternator is running. If it's not seeing the output voltage because of cable losses, then it's not seeing the battery voltage either. In your diesel setup, with two short fat wires, it doesn't matter whether they are connected together at the battery terminal or at the alternator end, the sense wire will just see the alternator output. Won't it? Am I misunderstanding the purpose of the sense wire?
"I'm a man of means, by no means....King of the Road!"
Oldiebut goodie wrote:Not all vehicles are the same! My diesel setup with its very short cabling is not affected but we are talking here about a petrol van not mine.
OK, but in any vehicle, how can the sense wire measure the raw battery voltage when the alternator is running?
"I'm a man of means, by no means....King of the Road!"
Oldiebut goodie wrote:It is simple to work out really - are there 3 wires or just a single wire from the alternator. Only the single wire ones are self exciting. If there is a single wire self exciting alternator there shouldn't be a charge light on the dash.
three wires or three connections?
My Bosch has only two wires but three connections as the body is the ground? so is it self exciting or does the blue wire exite it?
There should be ideally a sense wire that detects the battery voltage, an exciter wire (in our case the blue one) to start the alternator and the heavy output wire. Some alternators use the output as a sense wire which defeats the object and doesn't always sense the battery voltage correctly. It will charge but not necessarily at the correct voltage as it thinks that the output, at say 14.2v, is the battery voltage whereas the battery may be sitting at - say 13v. Earthing is carried out by the body. I don't know how the single wire ones manage to sense the battery voltage correctly though! With the long cable runs on your petrol setups via the starter solenoid there is scope for maybe 0.5 - 1v drop over the length. The diesel ones are so short that it really has no effect. (appx one foot cable)
The problem is that most of our vans have had alternators replaced over 20+ years so what was true at the time they were assembled is all up in the air and each alternator needs research as to exactly what type is currently fitted.
Mine has a Lucas one with the blue exciter and two heavy wires direct to the battery being a diesel, one being the sense and the other the output.
Yep, read that when it was posted. Still don't get it though. Even if the battery is sitting at 13V, there's just no way to measure that when the alternator is running, since the battery's apparent voltage will be pulled up by the alternator charge. So I don't see the purpose in trying to measure the voltage of the battery while the alternator is running.
"I'm a man of means, by no means....King of the Road!"
Well if you cannot see why you cannot! Maybe you are missing the point that the alternator changes its output according to what it senses at the battery.
Maybe you will never get it. Just accept it.
Try..... 13v output at the alternator, 12.5v received at the battery, sense sends signal that only 12.5v received so alternator increases output to compensate. It will continue to adjust the output until the same voltage is received as sent. (or its max which will be 14.5v if you are using a 14.5v regulator)
Even on a short run on my alternator there was a drop in voltage due to crimped terminals creating a resistance. Remaking the terminals and soldering them removed that drop so now my sense cable isn't doing much work. Of course you need your sensing wire to be in tip top condition with nigh on zero voltage drop to get an accurate voltage.
Last edited by Oldiebut goodie on 25 Jun 2014, 17:12, edited 1 time in total.
itchyfeet wrote:Interesting didnt know the alternator sensed voltage, i assumed it just pushed out the same voltage regardless of battery state
getting back to the regulator/brush pack, anybody know was was std and what brickwerkrs are selling, i have a 14v one and a 14.5v one?
The 14v one will likely as not give you 13.8v max output and the 14.5v one will get you a whole 14.2v which apparently is better suited to modern batteries. (they all seem to run at .2v below the stated voltage) Most people are now fitting the 14.5v one in preference to the older lower voltage ones.
Yes - maybe that is what is confusing CJH.
(Although it does briefly before the alternator is excited )
We mustn't lose sight of the fact that the condition of the battery also has an effect on that voltage.
Ooooh .... are we all getting excited now?
Time for sirloin steak and new potatoes now.
Oldiebut goodie wrote:It is simple to work out really - are there 3 wires or just a single wire from the alternator. Only the single wire ones are self exciting. If there is a single wire self exciting alternator there shouldn't be a charge light on the dash.
We haven't had battery sensed alternators on light vehicles since the early seventies and VW have never fitted them. It doesn't matter if it has got two cables three, four or five they are machine sensed alternators and they will self excite.
1982 Camper 1970 1500 Beetle Various Skoda's, Ariel Arrow
How does that explain that if the excitation wire is missing the alternators don't charge then? How often do you hear on the forum that this has happened - quite often.
Most of us have alternators that have the excitation wire.
Practical experience says different to what you say.
You appear to have run together the voltage sensing and the self exciting into being the same subject.
itchyfeet wrote:the thin wire blue? If so thats the charge light wire and needs to be connected, im unsure if its needed on this alternator to generate power( i read it is on some), i dont think it is, its just the light but somebody will confirm im sure
Next time I have the lid off my engine I am going to test mine by disconnecting that blue wire and firing it up, just so I know for sure
.
Im going to do just that at the weekend when i swap my newly rebuilt bosch 90A back on
1988 DG WBX LPG Tin Top
1989 DJ digijet WBX Holdsworth Villa 3 Pop Top itchylinks
Oldiebut goodie wrote:How does that explain that if the excitation wire is missing the alternators don't charge then? How often do you hear on the forum that this has happened - quite often.
Most of us have alternators that have the excitation wire.
Practical experience says different to what you say.
You appear to have run together the voltage sensing and the self exciting into being the same subject.
If it doesn't then the alternator is faulty, practical experience says it does.
One alternator with the exciter wire connected.
Click to play.
With the wire disconnected.
Not the alternator needs higher revs before it will self excite.
1982 Camper 1970 1500 Beetle Various Skoda's, Ariel Arrow
Thanks bigherb so we are getting to the bottom of the confusion by the sounds of it
blue wire needed for exitation at low revs, will self exite at high revs with blue wire off
so always worth reving the engine when testing a faulty alternator, this will indicate wiring if it srarts to generate voltage or brush pack if it dosent.
1988 DG WBX LPG Tin Top
1989 DJ digijet WBX Holdsworth Villa 3 Pop Top itchylinks
Just accept it.
