Alternator or regulator

[John L]

Can somebody help with diagnosing a no charge problem please.

I have a TR4 4 terminal regulator, there is power on the B+ on the regulator, there is nothing on the next + with the key on, and nothing on the brown and yellow on the alternator, but the alternator light is on although a bit dim.

Does this mean the regulator is duff or the alternator?

Are there other tests I can do to prove where the problem could be.

With the engine running there is no charge showing on the ammeter, and the charge light doesn't go off.

John

[RobH]

If you have a four-terminal regulator it is battery-sensing and the B terminal connects direct to the battery. The alternator warning light is connected from the ignition switch to the other + terminal on the regulator to supply it , and hence the field winding, with power. With the engine not running but ignition switched on you should see some small voltage at that terminal. Without some voltage on the field winding the alternator cannot start up.

If there really is nothing there then either you have the incorrect ignition bulb fitted or the regulator is short-circuit or there is something wrong with the field winding connections. The wiring from the bulb must be OK otherwise the bulb would not light but its worth checking the two links from the alternator to the + and F terminals.

(You cannot use an LED bulb as the ignition indicator unless you have a fixed resistor in parallel with it- an LED cannot supply enough current).

 

One possibility is that the slip-ring brushes in the alternator are worn and not making a connection. If you pull the lead off the F terminal on the regulator you should be able to read the resistance of the field winding between that wire and the second + terminal on the regulator. If that is open-circuit then the fault is in the alternator field connection.

 

There is a diode inside the regulator which is directly across the field winding, intended to protect against back-emf. If that diode goes shorted then the field cannot be energised. If you pull both the + and F connections off the regulator and do a resistance check between the two terminals in both directions by swapping the meter leads over, there should be a low resistance one way and open circuit the other. If it is low both ways then the regulator is faulty.

Edited April 9, 2016 by RobH

[John L]

I friend has lent me a 3 terminal TR4.

Can I fit and run this, what does the live wire on the B+ do? Is it just to show a negative on the ammeter when the key switch is off and you turn on the lights, or does it do something else?

John

[RobH]

The B terminal on the 4-terminal regulator is the sensing wire which tells the regulator what the output voltage is. A three-terminal regulator has an internal connection which does the same job so you can just leave the B wire disconnected while you test things (but make sure you insulate it so it can't short out, as it is connected to the battery).

[John L]

Well fitted the 3 terminal regulator, and the light still doesn't go out, and there is no increase in voltage on the battery, so I presume now it must be the alternator at fault?

Are there some simple tests I can do to prove it before I order a lot of parts unnecessarily?

John

[RobH]

As I said above - you can check the field continuity but that's about all that is easy to do without dismantling. You haven't said which alternator you have - do you know the type number?

[John L]

Its a Lucas 15AC.

J

[RobH]

There is an exploded diagram of your alternator here:

http://www.collection.archivist.info/showresult.php?prog=1&srcprog=searchv13.php&srcdata=title&Type=PS&Accn_no=2920&subject=8057

 

and the circuit diagram for the whole system is here:

http://www.collection.archivist.info/showresult.php?prog=1&srcprog=searchv13.php&srcdata=title&Type=PS&Accn_no=2909&subject=8046

 

By removing the plastic end cover you can check the slip-ring brushes easily. If you know what you are doing you should also be able to check the diodes and the stator winding without dismantling further.

[John L]

Just to finish up this article, had the alternator off, and looked at the brushes, the centre one was fine, but the other was well worn, swapped them both around and all works fine, now need to order some brushes now and perhaps some bearings.

Thanks RobH for your guidance.

John