Quote:
Originally Posted by sleek lemur
Still stuck!
New Charge Regulator fitted and....no progress. AMP light remains lit with engine running and voltage reading at battery remains stubbornly at 12.5.
1. I have 12 from the AMP light via the yellow cable to the Charge Regulator. I measured this with the yellow cable disconnected from the Charge Regulator.
2. When I reconnect the yellow wire to the Charge Regulator, the voltage reading drops to about 1.6.
3. I get zero reading at the DF connection on the alternator where the green cable plugs in.
I have checked both connections from the Charge Regulator to the alternator and also double checked the earth. All good. Alternator was bench tested by professional and is fine. I also previously very briefly connected the DF terminal to 12v and the alternator put out 14.5v at the battery.
Am "rather frustrated"!
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Indeed frustrating.
I have attached the internal wiring diagram for a Bosch 35 amp alternator. I am going to assume that your Bosch 55 amp alternator is similar or identical.
The voltage regulator supplies voltage to the Df terminal on the alternator and controls this voltage to control the output of the alternator. You have done the full field test (apply 12v to Df ) which indicates that the field winding / rotor (5 in the diagram) and the brushes are OK. The fact that your full field test and the test by the 'shop' worked suggests that the stator winding (1) and the main rectifier diodes (2 and 3) are OK. When the shop tested the alternator and pronounced it OK, I presumed that this also confirmed that the magnetizing diodes were OK.
The voltage regulator gets its power to control the alternator field and measures the alternator voltage via terminal 61/D+ on the alternator. Terminal 61/D+ on the alternator should be connected to the same terminal on the voltage regulator as the yellow wire from the charge indicator light. When you first start up the car, a tiny amount of current flows through the filament of the charge light in the dash to the regulator. The regulator applies this small current to the Df terminal of the alternator which causes the alternator to start producing a small voltage which appears at alternator terminal 61/D. This supplies additional current to 61/D on the voltage regulator which then turns around and supplies this additional current back to the Df terminal on the alternator which raises the voltage on 61/D and around and around we go until the voltage hits around 14 volts and the regulator says 'enough' and starts limiting the current to Df. This is called field flashing or boot strapping the alternator.
In order for the alternator to boot strap itself, it needs that initial little current from the charge light. If you change out the charge light for a LED this will kill the bootstrap current and stop charging. If you change out the charge light for a filament bulb with a lower wattage this will reduce the bootstrap current and boot strapping can become hit and miss. The fact that you measure 12 v at the regulator end of the yellow wire when it is disconnected suggests that the charge light / boot strap circuit up to the voltage regulator appears to be intact.
In order for the alternator to complete the boot strap process, it needs the feed back from the alternator 61/D terminal back to the regulator 61/D terminal. So, three possible problems:
1- The wire from D61 on the regulator to 61/D on the alternator is open circuit. When you checked and got 1.6 volts on the end of the yellow wire connected to the voltage regulator check to confirm that you are also getting 1.6 volts at the alternator. This would indicate that the wire from 61/D on the alternator is connected to 61/D on the voltage regulator
2- There is a broken internal connection between the 61/D terminal on the back of the alternator and the internal magnetizing diodes (4 on the diagram). I would have expected that the shop would have confirmed that this circuit was working when they did their tests.
3 - one or more of the internal magnetizing diodes is dead or has a bad internal connection. I would have expected that the shop would have confirmed that the diodes were working when they did their tests.
Note, that if the yellow 61/D wire ever came into contact with ground when the engine was running this would have created a dead short across the magnetizing diodes likely cooking them.
Since you are already familiar with the full field test where you apply +12v briefly to the Df terminal on the alternator, I am going to suggest that you repeat this test. However, this time disconnect the wires to both the 61/D and Df terminals on the alternator. Rather than measure voltage at the battery, this time set up your voltmeter to measure the voltage between 61/D and ground when you do the full field test (with the engine running). When you apply 12v to Df, you should see the voltage rise on the 61/D terminal just like the battery voltage / B+ terminal did. If the 61/D terminal voltage does not rise that is an indication that there is an internal problem with the 61/D circuit in the alternator. If the terminal voltage on 61/D does rise up then I really don't know what is going on and I am going to stop talking!