Battery not charging problem

Today after stopping at a rest stop on the way back from a trip my bike would not start. I got a jump start and got a few miles before the bike died. Got another jump and was close enough to get my bike home. I just got done charging the battery and went for a test drive. After about 45 mins the bike died again. Just got done taking my bike to battery warehouse and the tech tested my battery. The testing equipment told him that I had a bad diode. Does this mean my alternator is bad? Or something else wrong with it. I am not real good working on bikes but I can do brakes and small stuff like that. I'm hoping it is not my alternator but It looks that way to me? I was hoping someone could point me in the right direction on getting my 1991 st1100 back on the road again!!

Thanks so much!

Bob

Bob, what year is your ST1100?

I re-read and noticed that it's a 1991 which has the oil cooled 28amp alternator unless it was already changed to the air cooled 40 amp. If you have a bad diode then I believe the alternator will need to be removed. To remove the 28 amp alternator the rear wheel, shock, final drive and swing arm have to be removed first.

In the past most guys would upgrade to the newer air cooled 40 amp alternator however the availability of those alternators seems to be drying up at this point. The engines on e-bay used to come with the alternators but now the sellers are not including them.

You can always post a Wanted on the forum, maybe someone has a working replacement in their spares.

Hi Bob,

I can't see how testing the battery would lead the tech to believing you have a bad diode. The diodes are in the VRR (voltage regulator/rectifier) mounted on the large aluminum footrest plate just below the battery. The VRR would have to be disconnected to test the diodes. Did he do that? If he did, then you may be lucky with getting away just replacing the VRR.

However, there is definitely something wrong somewhere in the charging system and the most common problem lies in one or more coils in the alternator burning and shorting out. There is a complete diagnostic test for the alternator in the Honda Service Manual and you would be best to check the alternator out first, before investing $$$ in a new VRR.

If you have a voltmeter, a quick test of the alternator output can be done by disconnecting the red 3 point connector going between the alternator and the VRR. From the alternator side, with the engine running, you should see at least 25 or more AC volts between each of the three wires and none of the three should be widely different than the others. Make sure the meter is reading AC volts and connect the probes alternately between the three wires.

ST11Nut said:

Bob, what year is your ST1100?

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It is a 1991

Thanks
Bob

The guy put some kind of testing equipment on the battery and had me start it and let it run for a bit. It showed on his display bad diode. and said something about pulling less then 30 amps or was it volts?? Is the VRR the square looking thing like below the battery to the left? It has a white connector on it. and a green wire to the right. It would be great if that was all it was. I do remember him saying that the diode controls AC to DV or something like that...

Thanks

Bob

robertp039 said:

The guy put some kind of testing equipment on the battery and had me start it and let it run for a bit. It showed on his display bad diode. and said something about pulling less then 30 amps or was it volts?? Is the VRR the square looking thing like below the battery to the left? It has a white connector on it. and a green wire to the right. It would be great if that was all it was. I do remember him saying that the diode controls AC to DV or something like that...

Thanks

Bob

Click to expand...

Yes, the VRR is as you described.

The alternator puts out three wires of AC voltage to the VRR, where the 6 diodes inside convert that AC voltage to DC voltage, which is what the bike battery is charged with and also outputs to the electrical system. One bad diode will reduce the charge going to the battery.

Hey Bob, here's a waveform to explain what the diodes do in converting AC to DC.

Since AC voltage has a positive and negative half cycle you need diodes to convert the positive/negative sine wave cycle into two positive 'humps'. Then, because the time between the humps is too long, you use 3 phases spread equally in time to make the gaps closer together. Then filter between humps, and you've got DC. When one or more of your diodes burn out, you lose the contribution of the half phase when that diode is conducting.

Here's a picture of the waveform. the red/blue/yellow are the 3 AC phases, the black ripple at the top is the result of the 6 diodes working with the 3 AC phases. Each diode contributes its own little hump, note that there are 6 of them in a complete cycle. Don't know if his diagnosis meant you literally had one bad diode, or if it meant you had at least one and possibly more.

robertp039 said:

It would be great if that was all it was.

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One hump or three?

Yes you want all three. Once upon a time the alternator in my Chevy wasn't charging properly and was diagnosed with a bad diode. I was able to order a replacement (set of three IIRC). I found the bad diode but replaced all three. It was fun but I'd probably just get a rebuilt alternator if it happened today.

This has no bearing on your situation. Just a random trip down memory lane. And that if it's the VRR you'll probably have an easier time than I did with my Chevy.

So, do you guys think that replacing the VRR will solve my problem? I sure hope so! If this is the problem where would you suggest I buy one at?

Thanks for the information! The guy who tested my bike was trying to explain something like what you talking about to me. So do you think replacing the VRR will fix my problem.

It would seem a strong possibility but should be easy to confirm by testing it specifically as previously outlined.

I'm guessing the diagnostics performed recognized a 'deficient' waveform and concluded there was a bad diode. Test or get the VRR tested and you'll now for sure.

I found this VRR on Ebay. Should I buy this one? https://www.ebay.com/i/331906389777?chn=ps&dispItem=1

robertp039 said:

So, do you guys think that replacing the VRR will solve my problem? I sure hope so! If this is the problem where would you suggest I buy one at?

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The OEM (Honda) VRR would be the way to go, but they aren't cheap ($185.00 from Partzilla), so I would strongly suggest you perform that dynamic AC output test on the alternator, which I mentioned in my earlier post, before you buy a new VRR, which may not be the problem.

robertp039 said:

I found this VRR on Ebay. Should I buy this one? https://www.ebay.com/i/331906389777?chn=ps&dispItem=1

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Well, the price makes it worth a try I guess, but it is obviously a Chinese (or similar) knock off and the internals may not be of much quality.

Bush said:

so I would strongly suggest you perform that dynamic output test on the alternator, which I mentioned in my earlier post, before you buy a new VRR, which may not be the problem.

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Strongly recommended. It's not hard to do and however performed your first test might be able to sort the VRR.

If it's the VRR my money would be on OEM unless this knock off has a decent track record.

Bush said:

The OEM (Honda) VRR would be the way to go, but they aren't cheap ($185.00 from Partzilla), so I would strongly suggest you perform that dynamic AC output test on the alternator, which I mentioned in my earlier post, before you buy a new VRR, which may not be the problem.

Click to expand...

I found one on ebay for just 15 dollars. I will try the test you posted before. I hope that i can do it correctly.

robertp039 said:

I hope that i can do it correctly.

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Start with engine off. Disconnect that red 3P connector. Follow the wires from the VRR to it, to find it, but it is right there in the open. Set the voltmeter to read AC volts, something above a 20V AC scale, if so equipped. Start the engine. Put one voltmeter lead onto one terminal in the ALTERNATOR side of that 3P connector and the other lead on any of the other two, Check the reading, Move the leads to each terminal, such that you have measured all three against each other. All readings should be very similar. Shut off the engine and reconnect the 3P connectors. Report back.

Thanks I will try this...