is this A127 alternator dead?

[Tessy]

I have just taken off my 120A alternator put back my older, weaker A127. For some reason, I'm getting no warning light turning on when I connect to the smaller spade connector (and so no charging upon start). However I have noticed that I DO get warning light turing on when I touch the wire to the case / earth. I haven't used this alternator in a long while, is it buggered? No problems when I reconnect the newer one with the warning light cable going to D+. 

 

In case you are wondering, I need to put on the old alterator to make a long trip as I still haven't put on the double engine pulley that I need for the more powerful alternator. The 10mm belt holds up when idle charging but snaps once on the move as too powerful! 

[BEngo]

 Take the regulator out and check the brushes are OK and moving freely. They should still be long enough that the hole half way is still present.   Regulator is the ribbed curved  thing held in with 3 screws  and a (usually yellow)  wire from the insides of the alternator.

Whilst the reg is out have a look at the slip rings.  Check they are clean (polish with a very little brasso if needed).  If you can check the resistance between the slip rings.  Should be about 3 or 4 ohms IQIRC.

 

With yellow wire and warning light  wire connected, the regulator case connected to negative and ignition ON check the voltage between the brushes.  Should be close to battery voltage.  If not the reg is banjaxed. If OK reassemble and try.  If still no light/charging I think the rotor is open circuit.

 

N

 

Edited March 9, 2021 by BEngo

[Tony Brooks]

Just to be sure - you have connected it to the warning lamp blade and not the one for a radio suppressor?

[Sir Nibble]

2 hours ago, BEngo said:

 Take the regulator out and check the brushes are OK and moving freely. They should still be long enough that the hole half way is still present.   Regulator is the ribbed curved  thing held in with 3 screws  and a (usually yellow)  wire from the insides of the alternator.

Whilst the reg is out have a look at the slip rings.  Check they are clean (polish with a very little brasso if needed).  If you can check the resistance between the slip rings.  Should be about 3 or 4 ohms IQIRC.

 

With yellow wire and warning light  wire connected, the regulator case connected to negative and ignition ON check the voltage between the brushes.  Should be close to battery voltage.  If not the reg is banjaxed. If OK reassemble and try.  If still no light/charging I think the rotor is open circuit.

 

N

 

Before getting tools out. With engine off is there a voltage at the D+ terminal. If so, the rectifier has failed, common fault. The regulator test described above, forget it. It won't work unless the regulator is earthed and the terminal for that is tucked away at the bottom of a bolt hole. With the regulator fitted there is a rectangular aperture on the side of the alternator, half in the black plastic of the regulator and half in the alternator casing. If you introduce a thin screwdriver or straightened paper clip or somesuch into this aperture, it will contact the -ve brush and you can earth it on the alternator casing to bypass the regulator. Do this with ignition on and if the regulator is at fault the light will light. It's worth popping the regulator out to look at the slip ring, they do wear badly and more pertinently have a habit of fouling from corrosion if left idle.