Hi All

Firstly fabulous forum, I've really been baffled by the range and detail of information available!
I hope this thread is of use for others, beg your pardon if this is described already somewhere – then I was not smart enough finding it
Oh, and sorry for my English, I’m no native to this language…

This is how to cure low voltage issues caused by a bad rectifier in the alternator (or generator as some prefer to say).
Mine did well while cold, but getting warm the volts would drop and eventually go below 12V. Although the battery warn light did not come on the volts were low enough to cause the radio to creak while idling, and the battery did not charge very well.
Now low voltage in combination with no battery warning light is usually a sign of one or more diodes burnt in the rectifier. (A bad regulator or worn carbon brushes usually trigger the battery warn light)
Standard cure is to replace the alternator, which can be costly and involves (on an XKR) removing fans and belts and drain cooling liquid.

Here is an easier fix, by just replacing rectifier, carbon brushes and regulator whilst the alternator is still mounted in the car:

1. Get a spare Alternator that fits your vehicle. You may also try other Delco alternators, the XKR’s LNC1800AA rectifier and carbon brushes matches with those of a (cheaper) Delco DAN671
2. Disassemble the rear of your donator alternator to get an idea of where nuts and bolts are located. Procedure 414-2 of the repair manual describes this very well, or have a look at steps 6-11 below.
3. Disconnect the battery (minus pole)
4. Optional, to get access from above and better lighting: Remove the air filter box (disconnect the connector from the air flow meter, lift off the air pipe, remove one screw that holds the filter box and pull the filter box out)
5. Optional, loosen the driving belt and turn the pulley of the alternator. If you hear grinding noise from the bearings or feel play – swear quietly and prepare to replace the alternator altogether.
6. Remove the alternator cooling duct from underneath the car
7. Disconnect the power cable from the alternator using a 13mm wrench, also remove the second nut and the plastic isolator (if you get fancy sparks while touching ground with the cable you did not follow step 3!)
8. Disconnect the control connector from the alternator (use a small screw driver to push the lock in while pulling on the cable)
9. Using a 8mm wrench, remove three nuts from the rear of the alternator, also remove the bolt which secures the ground tag
10. Lift off the metal cover from the back of the alternator, catch the ground tag and the carbon brush rubber cover which are likely to fall into your face when the metal cover comes off suddenly
11. Using a Phillips Screw Driver N°2 remove the two long bolts 1 & 2 holding the regulator (see pic). The tricky part is to remove the smaller bolt 3, because you can’t quite see what you are doing. Make sure the screw driver locks well in the head, push against the screw while turning the screwdriver to avoid damaging the head. If you damage one of the heads, quietly swear and prepare to remove the alternator altogether. Don’t worry about 4 & 5 right now, these are not connected to the alternator housing (at least this was the case in mine)
12. Using a ratchet with a Phillips N°2 insert remove screws 6-9 to disconnect the rectifier plate
13. Lift off the rectifier together with carbon brushes and regulator. Should come off easily. If not, do not apply force, but check if you got the right bolts out.
14. Get back from under the car and offer yourself a beer, the hardest part is done now.
15. Re-installation of the new parts from your donator alternator goes vice versa. Carefully lift the carbon brushes one by one over the cleaned shaft.
16. Do not forget the plastic insulator on the main terminal
17. When done, check all connectors again before re-connecting the battery. Start the engine, make sure you have no battery warning light on

If you own a Multimeter, measure the voltage. Measuring close to the alternator you want to see 14.3 – 14.4V, in the trunk at the battery you will get 0.1-0.3V less. With all electrical users on (lights, radio, seat heating, airco and ventilation at full speed) you can expect something in the 13.6-13.9V range.

Tools needed:
8mm wrench or ratchet
13mm wrench or ratchet
Phillips N° 2 Screw Driver
Phillips N° 2 insert for ratchet
Small screw driver
Beer / any other adult beverage
Optional: Multimeter

Attached are pics of the new rectifier in place, and one of the old rectifier with two burnt diodes.

Cheers
Chris

 

Thanks for that. Good information and an easy to follow guide.