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Old 09-27-2012, 10:24 AM   #1
OldNFast OP
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R75/5 Charging Mystery

Greetings, all:

I have a '71 R75/5 that has what seems to be a vexing problem. The battery isn't charging, but my tests are mysterious.

Using my manual, I ran the following tests:

1. I hooked up my voltmeter between the negative terminal on the rotor and the B+ terminal on the diode board. It read 14.5 volts once the generator light went off. So, it appears the alternator is fine.

2. I hooked up my voltmeter between the D+ (blue) wire on the voltage regulator and earth. It read 12.78 volts once the generator light went off.

3. I connected the two non-ground wires going into the voltage regulator. Voltage to the battery was still 12.78. I expected this, since test 2 had only 12.78 going to the D+ wire.

4. I tested the resistance at the slip rings. It seemed ok. There is no continuity to ground. Same for the stator, continuity between terminals, none to ground.

My manual says the problem is the diode board. So, off to get another diode board. It arrives, i put it in, battery still not charging. Did the same tests above with identical results. I then tested both diode boards, and both are good.

It sure seems like the alternator and board are good. I've owned this beast for 23 years, and it's never confounded me like this before. Am I missing something? Any input would be greatly appreciated.

Thanks.
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Old 09-27-2012, 10:42 AM   #2
hardwaregrrl
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How about the brushes? are they within spec? The rotor should read between 3-7 ohms with the brushes off the slip rings. The brushes should be no less than 8mm long. Be sure to fill out your profile, ie where you live.

Quote:
Originally Posted by OldNFast View Post
Greetings, all:

I have a '71 R75/5 that has what seems to be a vexing problem. The battery isn't charging, but my tests are mysterious.

Using my manual, I ran the following tests:

1. I hooked up my voltmeter between the negative terminal on the rotor and the B+ terminal on the diode board. It read 14.5 volts once the generator light went off. So, it appears the alternator is fine.

2. I hooked up my voltmeter between the D+ (blue) wire on the voltage regulator and earth. It read 12.78 volts once the generator light went off.

3. I connected the two non-ground wires going into the voltage regulator. Voltage to the battery was still 12.78. I expected this, since test 2 had only 12.78 going to the D+ wire.

4. I tested the resistance at the slip rings. It seemed ok. There is no continuity to ground. Same for the stator, continuity between terminals, none to ground.

My manual says the problem is the diode board. So, off to get another diode board. It arrives, i put it in, battery still not charging. Did the same tests above with identical results. I then tested both diode boards, and both are good.

It sure seems like the alternator and board are good. I've owned this beast for 23 years, and it's never confounded me like this before. Am I missing something? Any input would be greatly appreciated.

Thanks.
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Old 09-27-2012, 10:54 AM   #3
ignatz72
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What is the "not charging" behavior?

Sounds like the Gen light is functioning properly, and your tests seem good.

What about your battery, are you sure it's healthy? What kind is it (slosh, sealed, etc)?
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Old 09-27-2012, 10:59 AM   #4
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Originally Posted by ignatz72 View Post
What is the "not charging" behavior?

Sounds like the Gen light is functioning properly, and your tests seem good.

What about your battery, are you sure it's healthy? What kind is it (slosh, sealed, etc)?

The voltage at the battery once the generator light goes out is about 12.75v. Battery is new and healthy. It has about 12.6 volts when the machine is off.
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Old 09-27-2012, 10:55 AM   #5
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Thanks for the fast reply.

Brushes look fine, they contact the slip rings firmly. Ohmmeter wasn't stable, maybe I should have left it on longer. It gave reading jumping between 20 and 50. I figured since I was getting 14.5v+ into the diode board, I was ok with the alternator.

First alternator lasted 37 years. Hope this one isn't bad after 4.

Thx for the heads up... I completed my profile.
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Old 09-27-2012, 11:10 AM   #6
ignatz72
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I'd expect a bit more voltage in the battery at rest, but I'm used to AGM behavior.

Your rotor readings jumping around might be due to dirty slip rings. Clean them up a bit w/ fine sandpaper and carb cleaner then check again.

If it's still high and jumpy, I'd say it's the rotor. Also try testing the rotor when the motor is hot to see if your results change.
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Old 09-27-2012, 12:39 PM   #7
Bill Harris
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14.5V at the alternator is good. !2.75V at the battery won't keep it charged.

There is one /5 Achilles heel. Look at the starter relay. See terminal #30 with one large Red and two large Red wires in separate connectors going to two table on the relay? That one Red Wire goes to the diode board output. Those two Red wires go between the battery and the switchboard in the headlight bucket (the rest of the electricalsystem). If you get an poor, oxidized connection between those two tabs on the relay, the good voltage of the alternator won't get to the battery. If you have a poor, oxidized connection at any of the two connector crimps (possible after all these years) Voltage A won't get to Voltage B.

Pull the starter relay. If it is the old-style without thesoft "silicone" weather seal all on the bottom, just run a bead of solder between the two tabs. On the newer relays with the goop, justsolder in a jumper wire between the tabs.

My usual practice on "critcal crimp connections" is to put a small solder connection at the crimp. Not enough to melt the wire insulation, not enough to wick into the stranded wire (makes it unflexible and break-prone).

The electrical system and connectors work well, but after 25-30 years they get creaky, just like us. Sometimes one needs suspenders plus a belt.

Another later improvement that BMW did was to run an additional 12 Ga wire from the Diode Board output (terminal 30) back to the big M8 stud on the solenoid that connects to the starter cable. Gives an extra BIG link between the batt'ry and the alternacator.
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Old 09-27-2012, 01:11 PM   #8
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Quote:
Originally Posted by Bill Harris View Post
14.5V at the alternator is good. !2.75V at the battery won't keep it charged.

There is one /5 Achilles heel. Look at the starter relay. See terminal #30 with one large Red and two large Red wires in separate connectors going to two table on the relay? That one Red Wire goes to the diode board output. Those two Red wires go between the battery and the switchboard in the headlight bucket (the rest of the electricalsystem). If you get an poor, oxidized connection between those two tabs on the relay, the good voltage of the alternator won't get to the battery. If you have a poor, oxidized connection at any of the two connector crimps (possible after all these years) Voltage A won't get to Voltage B.

Pull the starter relay. If it is the old-style without thesoft "silicone" weather seal all on the bottom, just run a bead of solder between the two tabs. On the newer relays with the goop, justsolder in a jumper wire between the tabs.

My usual practice on "critcal crimp connections" is to put a small solder connection at the crimp. Not enough to melt the wire insulation, not enough to wick into the stranded wire (makes it unflexible and break-prone).

The electrical system and connectors work well, but after 25-30 years they get creaky, just like us. Sometimes one needs suspenders plus a belt.

Another later improvement that BMW did was to run an additional 12 Ga wire from the Diode Board output (terminal 30) back to the big M8 stud on the solenoid that connects to the starter cable. Gives an extra BIG link between the batt'ry and the alternacator.
Bill,

I'd read about this fault somewhere a while back, I think. Thanks much for the input. I leave work in 10 minutes (on my other beast ) and will try this before it gets dark.
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Old 09-27-2012, 05:23 PM   #9
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Originally Posted by OldNFast View Post
Bill,

I'd read about this fault somewhere a while back, I think. Thanks much for the input. I leave work in 10 minutes (on my other beast ) and will try this before it gets dark.
Nope, didn't work. Tomorrow or Saturday I'll get out my fancier (but more unwieldy) multimeter and get a definite ohm reading off the slip rings.

Thanks to everyone for their interest. I'll report back.
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Old 09-27-2012, 01:01 PM   #10
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May I ask you a dumb question ?
Did someone touch the starter motor recently ?
I ask this because I've seen a bike recently where the output cable from the diode board was not wired on with the cable coming from the battery but on the opposite M8 stud on the starter motor. (this stud hold the wire to the brushes of the starter motor and is the output of the solenoid relay).

As the output of the charging system is "in the open", the regulator cuts the power to the stator immediately because the voltage rises immediately above the switching point. so the charge light goes out but the battery sees no current at all...
And, by the way, I think the starter motor runs when the alternator gives it's maximum 20 amps...
An easy check...
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Old 09-27-2012, 01:07 PM   #11
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Quote:
Originally Posted by georgesgiralt View Post
May I ask you a dumb question ?
Did someone touch the starter motor recently ?
I ask this because I've seen a bike recently where the output cable from the diode board was not wired on with the cable coming from the battery but on the opposite M8 stud on the starter motor. (this stud hold the wire to the brushes of the starter motor and is the output of the solenoid relay).

As the output of the charging system is "in the open", the regulator cuts the power to the stator immediately because the voltage rises immediately above the switching point. so the charge light goes out but the battery sees no current at all...
And, by the way, I think the starter motor runs when the alternator gives it's maximum 20 amps...
An easy check...
A dumb question is, 'What year is a '71 R75/5'? But, no, haven't been at the starter motor in several years, and it starts like a champ. Thanks.
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Old 09-27-2012, 01:17 PM   #12
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An electric test meter is usually called a VOM. That stands for Volt Ohm Meter. The two primary things it checks are Volts and Ohms. Some test for Amps but only in the Milli range, very small, no practical usage here. Modern meters are auto ranging with digital read outs. They are much cheaper than they were when I was younger. A good quality VOM in the 60's and 70's would set you back hundreds of dollars. You can now get a good on for $50 or less. I have a digital read out VOM from Radio Shack I bought 6 or 7 years ago for less than $50 and it is smaller than a pack of cigarettes.

How much we have progressed I say. But there is one thing that has not changed. The Volt meter function will work with no attention from the user. The Ohm Meter needs a proper battery. If the battery is dieing or dead you will get faulty Ohm readings.

I think your meter needs a new battery. They don't have to "warm up", unless made before 1970 or there abouts.
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Old 09-27-2012, 05:26 PM   #13
kixtand
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Quote:
Originally Posted by disston View Post
An electric test meter is usually called a VOM. That stands for Volt Ohm Meter. The two primary things it checks are Volts and Ohms. Some test for Amps but only in the Milli range, very small, no practical usage here. Modern meters are auto ranging with digital read outs. They are much cheaper than they were when I was younger. A good quality VOM in the 60's and 70's would set you back hundreds of dollars. You can now get a good on for $50 or less. I have a digital read out VOM from Radio Shack I bought 6 or 7 years ago for less than $50 and it is smaller than a pack of cigarettes.

How much we have progressed I say. But there is one thing that has not changed. The Volt meter function will work with no attention from the user. The Ohm Meter needs a proper battery. If the battery is dieing or dead you will get faulty Ohm readings.

I think your meter needs a new battery. They don't have to "warm up", unless made before 1970 or there abouts.
I am afraid you are incorrect on your description of a VOM, and meter terminology in general. VOM is an acronym that stands for Volt Ohm Millammeter and, as the name implies, a VOM measures voltage (Volt), resistance (Ohm) and current (Milliamps.) One of the more common VOMs was made by Simpson, and was, of course, based on an analog meter movement. As such, meters of this type were only able to measure small amounts of current, typically in the milliamp range, as that is all the meter movements could stand.

Today's meters are typically referred to as multimeters. Like VOMs they also measure voltage, resistance and current, but may also measure other electrical parameters as well, and they also have the capability to measure orders of magnitude more current than a VOM, as the measurement technology is no longer dependent on a meter movement but, instead, is based on solid state technology. Multimeters would typically be the digital readout meters that one can buy cheaply at Harbor Freight. Additionally, whilst a VOM will indeed measure voltage and current with no internal battery assistance, a typical multimeter will not.


To the point at hand, my guess is also the starter relay. I once tried an aftermarket replacement relay on my /5 when I was having some inadvertent starter issues, and as it turned out, the supplier sent an improper relay, one which only connected the two red wires mentioned previously when pulled in (i.e. when the starter was engaged.) I had the same symptoms as you, and went a step further and confirmed that the battery was never receiving any current, but was instead current was always flowing in the other direction...

Keep us updated--

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Old 09-27-2012, 06:28 PM   #14
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If you are getting 14.5 V at the B+ terminal of the diode board the alternator is doing what it should. If you are only getting 12.78 at the battery, you have a bad connection somewhere between. There is probably corrosion in at least one of the leads at the starter relay. You may have to cut and splice the red leads; I've seen corrosion creep down the wires some distance.

The wiring harnesses for these bike are still inexpensive. If I found much corrosion, I'd buy a new harness before BMW wakes up. In fact, I may buy a couple for stock!
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Old 09-30-2012, 06:34 PM   #15
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Sorry I didn't get back sooner. I've done a bit more testing. Both red wire terminals of the starter relay give identical readings, about 12.5v when the generator light goes off. I also checked the D+ wire at the voltage regulator and coming into and out of the starter relay. It goes up to 12.8v or so when the light goes off, and gets to about 13.5v when I hit 4000 RPM.

It seems the starter relay isn't the issue, but I'm all ears.

The battery is at about 13.3 with the ignition off.

Any feedback is appreciated.
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