If you like our site, please consider joining our club!
By joining you will help ensure that we can continue to provide this service
JOIN HERE!

R75/5 Alternator / Voltage regulator

User avatar
cbclemmens
Posts: 216
Joined: Sun Apr 25, 2021 2:21 pm
Location: Apollo, PA
Been thanked: 4 times

R75/5 Alternator / Voltage regulator

Post by cbclemmens »

My R75/5 is giving me a new problem. When I put the key in the red ammeter light does not come on. The oil light and neutral light come on. I chased the wiring around and checked the voltage regulator. If I hold the switch down on the regulator, the light comes on. But I also checked the output of the alternator I'm getting 2 VAC. I think that means a bad regulator but I'm thinking the alternator should put out more, checking directly at the brushes, or does the output depend on the regulator?

Craig

User avatar
schrader7032
Posts: 9016
Joined: Fri Oct 27, 2006 3:00 am
Location: San Antonio, TX
Has thanked: 2 times
Been thanked: 29 times

Re: R75/5 Alternator / Voltage regulator

Post by schrader7032 »

Can you give more of a description of your 2 VAC measurement? Typically you only measure volts DC not AC. Was the engine running?

First initial test, is to start the bike and use a voltmeter across the battery terminals. What does it read at idle? And more importantly, what does it read at say 3000 RPM. At this high RPM, you should be seeing well north of 13.5 vdc and actually better if you saw 14 vdc.
Kurt in S.A.
'78 R100/7 '69 R69S '52 R25/2
Fast. Neat. Average. Friendly. Good. Good.

User avatar
srankin
Posts: 1054
Joined: Fri Jul 14, 2017 8:45 pm
Location: Spencerport, NY USA
Been thanked: 14 times

Re: R75/5 Alternator / Voltage regulator

Post by srankin »

As Kurt says, voltage past the diode board is in DC current, the diode board converts the AC current generated by the alternator to DC for the system.

That said, you get a voltage at the battery the way Kurt explained. At 3K RPM, you should be seeing DC voltage between 13.5 to 14.5. If you are not seeing this, you can pull the plug off of the voltage regulator and bridge the plug at the blue and I believe the black wire. At this point if your voltage goes up, it will read higher than or around perhaps 20 Volts DC. If it remains lower than that or less than 13.5 to 14.5 at 3K RPM, the problem is in the alternator. St.
Owner of a 84, R80RT and 78, R100RS

User avatar
cbclemmens
Posts: 216
Joined: Sun Apr 25, 2021 2:21 pm
Location: Apollo, PA
Been thanked: 4 times

Re: R75/5 Alternator / Voltage regulator

Post by cbclemmens »

Thanks guys for your input. I do have a basic understanding of electricity/electronics. Alternators put out AC voltage/current that is converted to DC by the diode block (a wheatstone bridge). If there is no AC voltage at the brushes, it can't be converted to DC to read at the battery. My concern is whether the alternator output is entirely controlled by the voltage regulator. I'm thinking that it has to be, so the problem has to be the regulator. But an additional concern is that the regulator could be working fine but if the alternator is burned out there would also be no voltage. I was hoping to find a way, or if someone knew a way. to distinguish between those two conditions.

Craig

User avatar
srankin
Posts: 1054
Joined: Fri Jul 14, 2017 8:45 pm
Location: Spencerport, NY USA
Been thanked: 14 times

Re: R75/5 Alternator / Voltage regulator

Post by srankin »

Look, my answer to you was in regard to the comment (I'm getting 2 VAC. I think that means a bad regulator but I'm thinking the alternator should put out more, checking directly at the brushes, or does the output depend on the regulator?)

None of the troubleshooting information I have seen has ever given a voltage at the brushes. So I know nothing about if 2VAC is good or not? The only time any trouble shooting is done at the brushes has to do with the contact of the brushes to the slip ring on the rotor, Rotors are know to fail internally and open up, meaning they will show open on a continuity check with a meter. You can check this by pulling the two spade connectors at the brushes and measuring continuity from the terminals bromone brush to the other. This not only shows a closed or open circuit in the rotor but could show a bad brush connection to the slip ring. Usually this brush thing is a visual inspection for excessive wear on the brushes. You don't mention how many miles are on the bike?

Like I said, bypassing the voltage regulator should give you full output of the alternator at 3K RPM, this should be about 20VDC. If you don't have that, then look at the brushes, check the rotor for continuity, check the stator windings for continuity, and if all that looks good, then check the diode board. If you think you are getting AC current at the battery, then check the diode board.

If you have 20 or so VDC when the regulator is bypassed, then the problem is the regulator, put a new one in. Am I making sense? If you don't understand, I would suggest finding a Clymer manual or better yet Rick Jones's book on airhead charging trouble shooting. Perhaps Snowbum has a trouble shooting guide? If you are nearby Rochester NY, feel free to stop by and I will help you out. St.
Owner of a 84, R80RT and 78, R100RS

User avatar
schrader7032
Posts: 9016
Joined: Fri Oct 27, 2006 3:00 am
Location: San Antonio, TX
Has thanked: 2 times
Been thanked: 29 times

Re: R75/5 Alternator / Voltage regulator

Post by schrader7032 »

A little more than a guide, but Snowbum has this page on troubleshooting the alternator. If you can read through this, the answer will be there.

https://bmwmotorcycletech.info/trbleshootALT.htm
Kurt in S.A.
'78 R100/7 '69 R69S '52 R25/2
Fast. Neat. Average. Friendly. Good. Good.

User avatar
srankin
Posts: 1054
Joined: Fri Jul 14, 2017 8:45 pm
Location: Spencerport, NY USA
Been thanked: 14 times

Re: R75/5 Alternator / Voltage regulator

Post by srankin »

The voltage regulator has failed. To test for that, either install another one; or, UNPLUG the VR, & in the PLUG, connect a paper clip or other item, between the opposing female connections. DO NOT connect to the solid BROWN wire! If the charging is now good, and voltage keeps rising with increased RPM (do NOT let it go over ~14.6 or so, absolute maximum 14.9), then the VR is almost for sure bad.

This can be checked without removing the front cover, it takes just a few minutes to do. The rest of Snowbum's article is worth the read if you need to go further in testing.

If you are lucky, it will be the regulator and you won't have to mess with taking the cover off and dealing with further troubleshooting. Not that taking the cover off or doing any repairs to the charging system are all that hard to do. But personally I would rather keep things simple and not do anymore work than has to be done. I would rather be riding than working on a bike. Let me know what voltage you get when the regulator is bypassed. St.
Owner of a 84, R80RT and 78, R100RS

User avatar
cbclemmens
Posts: 216
Joined: Sun Apr 25, 2021 2:21 pm
Location: Apollo, PA
Been thanked: 4 times

Re: R75/5 Alternator / Voltage regulator

Post by cbclemmens »

I have ordered a new regulator so we will see where that goes.
To answer your questions; The bike has 40K miles on it, I have a Haynes repair manual (2 actually, but they are duplicates.), I show 190 ohms through the armature winding (brush to brush) (I think that's probably normal), and 2 ohms from the brush terminal to the slip ring. (I think good).
The alternator AC output should be about double the DC voltage because to make DC from AC, the diode block takes the negative swing of the AC, inverts it and adds it back to the positive swing, so I would expect to see about 30 VAC at the brushes.
I will check bypassing the regulator, but I will have to put the tank back on to do that.
So Thanks for your advise and comments. It really helps even when it just confirms what I was thinking.

craig

User avatar
srankin
Posts: 1054
Joined: Fri Jul 14, 2017 8:45 pm
Location: Spencerport, NY USA
Been thanked: 14 times

Re: R75/5 Alternator / Voltage regulator

Post by srankin »

LOL, I am sorry I was such a pain. Next time, bypass the regulator first, it is as I said a whole lot easier to do than to pull the front cover off. Oh, there should be just enough gas in the float bowls to run long enough to check.

I appreciate the lesson you post in regard to the double voltage, and stuff. Like I said, I have never used the voltage at the brush method. In all my years, if there is continuity throughout the brushes, slip ring and rotor, that system is OK. Same goes for continuity in the stator loops.

The biggest problems with airheads I have experienced have been open rotor, bad voltage regulator, or bad diode board. BMW had a lot of problems with a few years of diode boards but cleared it up. Some of the aftermarket boards are better than the OM. So diode board problems have taken a back seat to rotor and V reg. LOL, even the rotor situation has improved so now, the v regs are the number one problem I see or dead battery.

Thanks for letting all of us know the outcome and such. St.
Owner of a 84, R80RT and 78, R100RS

User avatar
schrader7032
Posts: 9016
Joined: Fri Oct 27, 2006 3:00 am
Location: San Antonio, TX
Has thanked: 2 times
Been thanked: 29 times

Re: R75/5 Alternator / Voltage regulator

Post by schrader7032 »

That reading brush-to-slip ring I'm not sure what that tells us. According to the Motorrad booklet, you want to slip a business card or the like under each brush...this lifts them off the slip rings. Then measure the resistance across the slip rings. You should have near 6.9 ohms for a 1970-1973 system. I also don't know what 190 ohms tells us.

Most people don't think about AC and making measurements. It's really all about DC measurements at the battery and if that's not correct, then working backwards. As ST says, bypassing the regulator with engine running helps eliminate one side of the system.
Kurt in S.A.
'78 R100/7 '69 R69S '52 R25/2
Fast. Neat. Average. Friendly. Good. Good.

Post Reply