Testing the charging system.

[SteveD]

Airhead Charging Circuit - Faulting.txt Issue: 15/06/2016

Author: Ced Mungall (Galactic Greyhound) with thanks to NSGearhead [BMRC].
Date: 29/05/2014
Ref: http://www.bmbikes.org.uk/Forum/viewtop ... 15&t=23365

PROCEDURE: AIRHEAD CHARGING CIRCUIT - FAULTING.
See the attached diagram; 'Airhead Charging Circuit.jpg' which accompanies this article.

CHARGE FAILURE - EMERGENCY GET YOU HOME INSTRUCTIONS:
A good, fully charged bike battery should give you around 3 hours running on Ignition ONLY without being charged by the Alternator - do NOT use the Starter Motor (bump start) or switch on any lights etc.

If travelling some distance, consider purchasing or acquiring a second FULLY charged small car battery, wire for longer battery leads, a cheap mains charger (if the journey will take longer than a day) and some straps or rope.
The second small car battery can be securely strapped to a pillion seat or rack and connected with longer battery leads when required for use (disconnect the original bike battery). The car battery will likely have a higher Ampere-hour capacity than a bike battery, be cheaper and so last longer before going flat.

Two fully charged batteries will give you at least 6 hours running (Ignition only) per day which could be 300 miles or more.
The charger gives you a means of recharging the batteries if mains is available. A bike battery will normally require at least an 8 hour charge to reach a fully charged condition from flat.
NEVER leave a battery in a discharged state for more than about 2 days or it will sulphate and be ruined.

The Airhead Charging process is as follows:

1. Ignition On, Charge Light passes 200 m/A through the Regulator and the Rotor to the Rotor earthed brush and the Charge Light illuminates.
2. Engine started and run at 2500 rpm - voltage across each Stator winding (U-Y, V-Y and W-Y) is 3.0 VAC initially with the 200 m/A Charge Light current but rapidly rises as the charging process begins.
3. The Stator Voltage is rectified by the Diode Board and applied to the Regulator D+ terminal which increases the Charge Light current through the Regulator and Rotor by a small amount.
4. This increased Rotor current produces a higher magnetic field which increases the Stator Voltage further and produces a further increase in the Rotor current which again increases the magnetic field.
5. This process rapidly escalates into 'avalanche' mode whereby each increase in the Rotor current produces an even higher Stator voltage which further increases the Rotor current until the Stator is eventually producing around 14.0 VAC at 2500 rpm.
6. At this point, the Rotor current will be around 3.0 Amps, the Stator windings around 14.5 VAC and the voltage at the Diode Board D+ and the B+ terminals will be around 14.0 VDC thus charging the Battery.
7. The Diode Board D+ terminal voltage being around 14.0 VDC will cause the Charge Light to go off as there will be around +12v on both sides of the light.
8. The Regulator will sense the Diode Board D+ terminal voltage of around 14.0 VDC and, at this point, will limit the current flowing through the Rotor to prevent any further increase in Stator winding voltage thus limiting the charging voltage at D+ and B+ to around 14.0 VDC.

1. CHARGE LIGHT ON CONTINUALLY:
The charging circuit components which may be causing the non-charging problem are:
1. The Alternator Rotor.
2. The Regulator.
3. The Alternator Stator.
4. The Diode Board.

ROTOR CIRCUIT TESTS:
The Charge light is illuminated from the Battery 12v+, through the Ignition Switch, Charge light, Regulator (D+ & DF) and the Alternator Rotor to earth. However, any short to earth on the Rotor side of the Charge light will cause the light to remain lit continually and prevent the Alternator from charging the Battery.

WARNING: Do NOT remove the front engine cover without first disconnecting the battery earth terminal. This is because the Diode Board has a metal heatsink connected to the +12v which can be shorted to earth by removal of the front cover.

TEST 1A:
To determine if there is an earth short on the Rotor side of the Charge light which is causing the problem, remove the front engine cover to access the Alternator and perform the following test:
1. Switch On the Ignition (engine not running) and observe that the Charge light is lit.
2. Disconnect the engine earth from the Rotor by lifting the earthed 'D-' brush off the slipring.
Note: This brush earths via the brush holder mounting to the Stator casing and a Brown wire from the terminal supplies the earth to the Regulator 'D-' terminal.
The brush can be held off the slipring by securing the brush side against the holder with the brush spiral spring.
3. The Charge light should go out - if it does so, then there is NO earth fault on the Rotor side of the Charge light.
4. The Charge light should go out - if it does NOT, there is an earth back towards the Charge Light.
5. However, the other thing to consider in the Rotor circuit is that the Rotor has gone short-circuit. This will prevent the Alternator from charging. Lift one brush clear of the Rotor sliprings and then measure the Rotor winding resistance across the sliprings - the Rotor resistance should be around 3 ohms.
6. If the above tests are OK and the Charge Light goes out when the Rotor bottom earthed brush is lifted off its slipring and the Rotor winding resistance is OK, then see the sections REGULATOR TESTS and STATOR TESTS below.

TEST 1B:
If the Charge light does NOT go out when the Rotor bottom earthed brush is lifted off its slipring, perform the following test:
1. Switch On the Ignition (engine not running) and observe that the Charge light is lit.
2. Disconnect the DF wire (Blue/Black wire) from the insulated Rotor brush connection.
3. The Charge light should go out - if it does, there is an earthing fault in the DF insulated connection or in the Rotor winding.
4. The Charge light should go out - if it does NOT, there is an earth on the Blue/Black wire back towards the Charge Light.

TEST 1C:
If the Charge light does NOT go out when the DF wire (Blue/Black wire) is disconnected from the insulated Rotor brush connection, perform the following test:
1. Switch On the Ignition (engine not running) and observe that the Charge light is lit.
2. Disconnect the DF wire (Blue/Black wire) from the Regulator DF terminal.
3. The Charge light should go out - if it does, there is an earthing fault in the DF Blue/Black wire to the Rotor.
4. The Charge light should go out - if it does NOT, there is an earthing fault back either in the Regulator or back towards the Charge Light.

TEST 1D:
If the Charge light does NOT go out when the DF wire (Blue/Black wire) is disconnected from the from the Regulator DF terminal, perform the following test:
1. Switch On the Ignition (engine not running) and observe that the Charge light is lit.
2. Disconnect the D+ wire (Blue wire) from the Regulator D+ terminal.
3. The Charge light should go out - if it does, there is an earthing fault in the Regulator.
4. The Charge light should go out - if it does NOT, there is an earthing fault back towards the Charge Light or Diode Board.

TEST 1E:
If the Charge light does NOT go out when the D+ wire (Blue wire) is disconnected from the Regulator D+ terminal, perform the following test:
1. Switch On the Ignition (engine not running) and observe that the Charge light is lit.
2. Disconnect the D+ wire (Blue wire) from the Regulator D+ terminal AND disconnect the D+ wire (Blue wire) from the Diode Board.
3. The Charge light should go out - if it does, there is an earthing fault in the D+ wire or the Diode Board.
4. The Charge light should go out - if it does NOT, there is an earthing fault back towards the Charge Light in the Instrument Binnacle or its connector.

TEST 1F:
If the Charge light does NOT go out when the D+ wire (Blue wire) is disconnected from the Regulator D+ terminal AND the Diode Board D+ terminal, perform the following test:
1. Switch On the Ignition (engine not running) and observe that the Charge light is lit.
2. Disconnect the connector to the Instrument Housing Binnacle.
3. The Charge light should go out - if it does, there is an earthing fault in the D+ wire to the Binnacle Connector or in the Connector.
4. The Charge light should go out - if it does NOT, there is an earthing fault on the Rotor side of the Charge Light holder in the Binnacle.

REGULATOR TESTS:
In case the Regulator is faulty and preventing the Alternator from charging, perform the following tests:

TEST 1H:
1. Disconnect the D+ and DF wires from the Regulator and join these wires temporarily together so as to bypass the Regulator completely.
NOTE: Check the wiring at the Regulator connector for corrosion or overheating - gently pull the wires to check they are secure.
2. Start the engine and see if the Charge light goes out - if so, the Regulator is likely faulty.

STATOR TESTS:
In case the Stator is faulty and preventing the Alternator from charging, perform the following tests:

WARNING: Do NOT remove the front engine cover without first disconnecting a battery terminal. This is because the Diode Board has a metal heatsink connected to the +12v which can be shorted to earth by removal of the front cover.

TEST 1J:
1. Disconnect the Stator U, V, W and Y leads after noting down where they are connected to the Diode Board.
2. Measure the resistance between each stator winding (U, V & W) and the Y connection. The Stator windings U, V and W should have a resistance of around 0.65 ohms each. All windings should show the same reading.

TEST 1K:
1. Disconnect the Stator U, V, W and Y leads after noting down where they are connected to the Diode Board.
2. Switch On the Ignition - the Charge Light should illuminate sending 200 m/A through the Rotor winding - start the engine.
3. With the engine running at about 2500 rpm and your voltmeter set to read around 30v AC [Note AC not DC], measure the voltage across each Stator winding (U, V and W) to the 'Y' lead i.e. U-Y, V-Y and W-Y.
You should see around 3.0 VAC on each winding and the voltage should be the same for each winding.

TEST 1L:
1. Disconnect the Stator U, V, W and Y leads after noting down where they are connected to the Diode Board.
2. Disconnect the Blue/Black wire at the Rotor DF terminal.
3. Use a 10A fused jumper lead to connect from the Battery Positive (+) terminal to the Rotor DF terminal (this will cause around 4 Amps to flow through the Rotor winding).
CAUTION: A 10 Amp fuse is used in this jumper lead in case it is earthed accidentally or there is a Rotor earth fault which can cause the Battery to be short-circuited resulting in a meltdown or fire.
4. Switch On the Ignition - the Charge Light will NOT be lit - start the engine.
5. With the engine running at about 2500 rpm and your voltmeter set to read around 30v AC [Note AC not DC], measure the voltage across each Stator winding (U, V and W) to the 'Y' lead i.e. U-Y, V-Y and W-Y.
You should see around 18.0 VAC on each winding and the voltage should be the same for each winding.

DIODE BOARD TESTS:
In case the Diode Board is faulty and preventing the Alternator from charging, perform the following tests:

TEST 1M:
1. With the engine running at 2500 rpm, measure the voltage at the D+ and the B+ terminals to earth with the wires kept connected to these terminals.
The voltage measured should be around 14.0 VDC from each terminal to earth.
2. If the voltage is NOT correct then test the Diode Board as per the link below.
3. If the Diode Board tests OK, then carry out the Charging Circuit tests as given in this procedure.

See the link below for an article by Rob Frankham on how to test this component. First check that all the wires have been correctly connected before you start removal and testing particularly the 'Y' earth connection. Keep a note of what wire is connected where:

http://www.frankhams.freeserve.co.uk/te ... _board.htm

TEST 1N:
1. Disconnect the Rotor DF Blue/Black wire from the Rotor DF terminal.
2. Disconnect the wires to the Diode Board D+ terminal (Blue wire) and the B+ terminal (Red wire).
3. Connect a fused (10A) jumper lead from the Battery Positive terminal to the Rotor DF terminal (this will cause about 3 Amps to flow through the Rotor winding and maximise the magnetic field around the Rotor).
4. Switch the Ignition ON (the Charge Light will NOT light).
5. Run the engine at 2500 rpm.
6. Measure the DC voltage to EARTH at the disconnected Diode Board terminals D+ and B+. The voltage should be 25 Volts DC.

2. CHARGE LIGHT DOES NOT ILLUMINATE:
The Charge light is illuminated from the Battery 12v+, through the Ignition Switch, Charge light, Regulator (D+ & DF) and the Alternator Rotor to earth.

TEST 2A:
To determine if there is an open circuit on the Rotor side of the Charge light which is causing the problem, remove the petrol tank to access the Regulator and perform the following test:
1. Switch On the Ignition (engine not running) and observe that the Charge light is not lit.
2. Disconnect the D+ wire (Blue wire) from the Regulator D+ terminal and connect the wire to earth.
3. The Charge light should illuminate - if it does, the break is further on towards the Rotor.
4. The Charge light should illuminate - if it does NOT, the break is back towards the Charge light. Check the Charge light bulb is not blown, the multipin connector to the Instrument Binnacle and the bulb holder connections.

TEST 2B:
If the Charge light illuminates when the D+ wire (Blue wire) is disconnected from the Regulator D+ terminal and earthed, perform the following test:
1. Switch On the Ignition (engine not running) and observe that the Charge light is not lit.
2. Disconnect the DF wire (Blue/Black wire) from the Regulator DF terminal and connect an earth to the Regulator DF terminal.
3. The Charge light should illuminate - if it does, the break is further on towards the Rotor.
4. The Charge light should illuminate - if it does NOT, the Regulator is likely faulty - disconnect and touch the Regulator DF Blue/Black wire and the Regulator D+ Blue wire together - if the Charge Light illuminates the Regulator is faulty.

TEST 2C:
If the Charge light illuminates when an earth is connected to the Regulator DF terminal, remove the front engine cover to access the Alternator and perform the following test:

WARNING: Do NOT remove the front engine cover without first disconnecting a battery terminal. This is because the Diode Board has a metal heatsink connected to the +12v which can be shorted to earth by removal of the front cover.

1. Switch On the Ignition (engine not running) and observe that the Charge light is not lit.
2. Disconnect the DF wire (Blue/Black wire) from the insulated Rotor DF brush connection and connect this wire to earth.
3. The Charge light should illuminate - if it does, the break is further on into the Rotor.
4. The Charge light should illuminate - if it does NOT, the break is back towards the Regulator DF terminal and is likely a broken Blue/Black wire.

TEST 2D:
If the Charge light illuminates when an earth is connected to the Rotor DF Blue/Black wire, perform the following test:
1. Switch On the Ignition (engine not running) and observe that the Charge light is not lit.
2. Disconnect the engine earth from the Rotor by lifting the earthed 'D-' brush off the slipring.
Note: This brush earths via the brush holder mounting to the Stator casing and a Brown wire from the terminal supplies the earth to the Regulator 'D-' terminal.
The brush can be held off the slipring by securing the brush side against the holder with the brush spiral spring.
3. Tap an earth onto the Rotor earthed brush slipring.
4. The Charge light should illuminate - if it does then the earth connection through the earthed brush (D-) is faulty.
5. The Charge light should illuminate - if it does NOT then the path through the Rotor is faulty - check that the brushes are making contact with the slip ring, the slip rings are clean and the Rotor winding resistance measured across the sliprings is approximately 3 ohms.
Disconnect the Rotor DF Blue/Black wire - Measure the resistance between the Rotor DF terminal and the Rotor D- terminal. This measurement includes the Rotor brushes, the Rotor winding and the brush contact resistance on the sliprings - it should measure around 5 Ohms.

Diagram: Airhead Charging Circuit

Airhead Charging Circuit

REFERENCES:
A. Airhead Charging Circuit - http://bmwmotorcycletech.info/trbleshootALT.htm
B. Testing the Diode Board - http://www.frankhams.freeserve.co.uk/te ... _board.htm
C. Bosch Charging Systems - http://www.buchanan1.net/charge.shtml
D. Testing Voltage Regulators - http://bmwmotorcycletech.info/testingvo ... lators.htm
E: Rotor, Stator & Brushes - http://bmwmotorcycletech.info/altbrushrotor.htm

[Airbear]

Thanks Steve. Ced is a great asset - that looks like the full story - more than you'd ever want to know. I've saved it as a PDF to go on my travelling tablet. I have a similar but shorter version there which must have been published on Boxerworks a few years ago - it contains additional notes by GSPD on fault finding when one has 'known-good' parts at hand:

GSPD’s comments –

I've been trouble-shooting electrical systems for decades, Audi, VW, Porsche, BMW, etc. and more recently, domestic and asian cars.

When it comes time to find a fault with a airhead charging system, I've developed a fail-proof method that saves a lot of diagnostic time ($ for the customer); I can tell him very quickly exactly what it's going to cost.

My method may not be 'correct' in the eyes of many electricians, but it sure saves a lot of shop time.

Before doing anything I load test the battery and inspect the terminals;
You won't get a proper charging voltage reading if the battery has a bad cell or is deeply discharged.

If the battery is OK...

... I simply swap the regulator with a known good one.
It's the easiest component to access and swap, and a very common fault.
For this test, you don't have to install the replacement, you can just plug it in.
I check the bulb circuit by grounding the blue regulator wire (if the bulb is not working at all)

Next I go to the rotor, easy to test for open or short without removing anything, and I get to check the brush condition at the same time.

Next the stator, fast and easy to replace, but RARE failure,
I've seen a few stators 'test' good, but still cause the light to flicker or glow dimly.
Takes less than 5 minutes. NOTE: Not all stators are interchangeable.

Then, without removing or disconnecting anything, I check for power (on the bottom half) and ground (on the top half) of the diode board, and do a visual inspection of all its wires.

If none of the above fixed it, I can confidently inform the customer that he NEEDS a diode board, and mounts, etc. and give him an exact estimate.
Total shop time for diagnosis; 20 to 30 minutes

The reason I leave the diode board for last (IT IS A VERY COMMON failure), is that it's sometimes timely to remove and test.
Often, the rubber mounts break, then the starter cover, air filter, etc. needs to be removed to replace them.

The only thing that should preclude readers here from adopting this method may be restricted access to spare parts.

[SteveD]

Good stuff Charlie. That lot should just about cover everything.

[Airbear]

It's important to have this knowledge at hand. I was wondering why I had never had a charging problem until recently, when I noticed that my volt meter was reading oddly low.

I used the trouble shooting guide I had along with my cute little travelling mulitmeter ...



... just AU$13.78 with free shipping ...

... to ascertain that the rotor on my old dear was open circuit when 400kms from home. FWIW, I got home easily with lights turned off - even using the starter a couple of times along the way.

[CWRoady]

Old post that helped me solve an issue I had been scratching my head on. No idea if the OP is still active here, but many thanks. Found a shorted wire on the backside of my diode board on test 1E. Thank you Steve!

[SteveD]

Old post that helped me solve an issue I had been scratching my head on. No idea if the OP is still active here, but many thanks. Found a shorted wire on the backside of my diode board on test 1E. Thank you Steve!

[120Nm]

Thanks @SteveD, helped me find a poor ground from the D- brush due to the cable being soldered too short and holding the brush off the rotor.

Posts like this save a lot of time and money on troubleshooting and help us getting our bikes back on the road where they should be. Thanks a lot for taking the time to do the write-up and sharing your knowledge.

/Doug
‘91 R80GS, Stockholm, Sweden

[SteveD]

Thanks @SteveD, helped me find a poor ground from the D- brush due to the cable being soldered too short and holding the brush off the rotor.

Posts like this save a lot of time and money on troubleshooting and help us getting our bikes back on the road where they should be. Thanks a lot for taking the time to do the write-up and sharing your knowledge.

/Doug
‘91 R80GS, Stockholm, Sweden

thanks for the feedback.