An Apparent Charging Problem

[guenner]

Did you watch the voltage between D- and DF at the alternator? How does it change from idle to 4k RPM?

Might be time to put a scope on D+/D- and DF/D- at the regulator. If these voltages are oscillating the voltmeter set at DC would not show this.

Here's good description of the charging system:

http://www.buchanan1.net/charge.shtml

I would swap the diode board for a test.

/Guenther

 

[MonoRT]

Glorious news comrades!

I received my adjustable (made in China sigh) Transpo voltage regulator from Rocky Point yesterday. I loosened the VR harness so that I could zip-tie the new VR to the right-hand throttle cable alongside the starter cover and get at that tiny pot to experiment with different settings.

With the bike sitting in the driveway and the throttle locked to give a steady 4K RPM, the as-delivered VR setting gave me voltage in the low 13V range ÔÇô just like my 3 other VRs. I slowly dialed the VR pot up to the full-on Chernobyl setting and managed to get a bare 14 volts. Not great, but encouraging. IÔÇÖve learned that the charging systemÔÇÖs driveway behavior is quite different from its on-the-road behavior, so I went for a ride, armed with my Datel voltmeter (reading off the battery terminals) and a small screwdriver.

Right away, I started seeing up to 14.75V in the 3K RPM range. So, I pulled off and dialed the VR back a bit. I did a bunch of riding up and down PCH where you can get anything from gridlock to 60 MPH . I was still seeing voltages over 14.2 at times and I did not want to get carried away, so I made a few very small downward adjustments. When I got home and turned off the bike, the batteryÔÇÖs resting voltage was 13.25 ÔÇô not bad at all.

Between last nightÔÇÖs run and my ride in to work, I noticed that the highest voltage (14.25V) came on in the mid to upper 3K RPM range. Above 4K, voltage drops a bit ÔÇô I was seeing 14.0V at 5K. By running an adjustable VR that is set ÔÇ£highÔÇØ, I suspect that IÔÇÖm treating a symptom here, but not knowing what the root problem is, IÔÇÖll have to settle for the cranked-up variable VR fix for awhile. IÔÇÖll have to keep an eye on the meter, just in case something starts working right ÔÇô I donÔÇÖt want to fry my system.

I wish that I had made the VR bypass test right off the bat and that IÔÇÖd paid more attention to the results. This test can eliminate many potential failure points from consideration if it produces a bunch of voltage. If you can get 16V at 2K RPM, you know that the battery is not acting like a black hole for electrons. You also know that the wiring to the battery is not adding in a bunch of resistance due to corrosion or bad connections. This test also tells you that the D+ and DF wires are good and that D+ on the diode board is capable of producing more current than is really needed by the rotor.

I am still wondering how 4 VRs could give similar (low) results in the face of that bypass test. It seems like the alternator and diode board are willing to produce big power (for an airhead) if they are allowed to. The adjustable VR also seems to support this conclusion. The question is: why wonÔÇÖt the VRs (4 of them!) let the voltage come up like they should? Is there a quality issue with the D+ power? It is certainly good enough to energize the rotor, but perhaps it has a screwy waveform that is hard for the VR to regulate? Guenther suggests a scope or an AC check on DF/D- I'll have to try that sometime. I do have my tired, but not broken Wehrle diode board handy, but I sure do hate removing and replacing those diode boards - however, a swap does seem like a logical test.

Over time, Ill probably learn more That article about tin whiskers was interesting/alarming, but given the age of most of my components, I doubt that they have pure tin plating inside. I have noticed that the ring and spade lugs Ive bought recently are bright tin plated. Kurt, if I had an adjustable DC power supply, I would have tried Snowbums VR cutoff test. I tend to collect tools, but I dont have one of those. Yet.

 

[Stan_R80/7]

Glad to hear you have a fix. I expect your charging woes are over. Thanks for sharing your results.

My opinion on the BMW charging system is that the voltage regulator is of poor design. After reading about charging issues over the last 20 years, I followed Lew's thread (forums.bmwmoa.org/showthread.php?t=60764) and replaced my 1978 original voltage regulator with a BWD-R588 from Advance Auto. I looked into the NAPA part, but this was more convenient.

I told the parts person a voltage regulator was needed for a 1968 BMW 2002 and they were happy to order the BWD-R588. What was received was the Intermotor part R588. The replacement bolted up in the same spot as the original with the same electrical socket. My only addition was a couple of zip ties for the socket wires.

 

[guenner]

I speculate that with the new adjustable VR you could compensate for a problem in the rotor or the diode board and the previous VRs are all good.

Btw. I have a calibrated builtin digital voltmeter with direct wiring to my battery and I get 13.50-13.65V at 2,800+ RPMs. The lower voltage when the system is hot.

/Guenther

 

[MonoRT]

I was able to get in a little more testing while cleaning up the wiring this weekend. I do NOT see fluctuating DC voltage between D+ and DF, but I do get half a volt of AC with the meter in AC mode. Again, that makes me wonder about the small diodes that provide positive current to the D+ circuit. I see the highest alternator output at around 3.5K RPM, from there, voltage drops a bit as RPMs go higher ÔÇô some sort of weird frequency interference? IÔÇÖll have to keep an eye on the situation and be ready to put in a new diode board if things go further south. I doubt that the situation will rectify itself over time.

I had forgotten that my original (spare) diode board was retired over poor output, making the diode board swap test difficult.

My OEM diode board has a good design where all of the leads had been formed like nail heads with some sort of die after being pushed through the board. That allowed a nice big puddle of solder under the nail head so that the connections for the primary power outputs and the D+ diodes were all still intact. However the add-on design of the center tap Y circuit was either given to an inexperienced junior designer or to an experienced fool. There was not a lot of board real-estate open because of the need to allow access to the mounting bolts  I think they should have used jumper wires or long Y diode leads and bypassed the board altogether. The add-on Y circuit has too-small traces and solder connections which melted and fried the board connections to the Y diode leads. So, my back-up diode OEM board has a lower output already due to the loss of the center tap output. I could probably fix my old board with jumper wires...