A simple series of tests for your rectifier

[Luddite]

Ok, so this isn't a Ducati component, (it's actually from my Gilera), but the tests can be used on most rectifier units, including Monsters. Since Regulator/Rectifier (R/R) problems seem to appear quite frequently on the forum, I thought I'd share these tests as someone else might find them useful.

For identification purposes, this is a three-phase, full-wave, bridge rectifier using six diodes to convert the AC voltage from the alternator to DC voltage to the battery.


Simplified rectifier diagram


Gilera 5-wire rectifier

All of the current Monsters should similarly be three-phase (with three yellow wires), but the early versions had single-phase alternators (two yellow wires) and so will have a different R/R to suit. The Monster R/Rs also have dual positive (red) and negative (black or green) wires, (making seven in total), which are sometimes siamesed together as here.


Typical Monster 7-wire rectifier

My problem
I'd noticed that my battery was going flat despite being boosted by an Optimate.

Preliminary checks
The first suspect was the battery itself. Although it's only a couple of years old, it doesn't get used much so could have deteriorated. However, after fully charging it, it held 12.8V for a week, so then the alternator seemed a likely culprit.

Checking the voltage at the battery with the engine at 3,000 revs gave only a maximum of 12.5V when you'd expect high 13/low 14V, which again suggested the alternator.

An (external) check of the alternator with a multimeter suggested that it was working normally, so that just left the R/R, (all connecting wiring appeared fine).

I'd not really considered the R/R as usually, when it fails, it seems to result in overcharging rather than undercharging.

To check the rectifier component of the R/R, there are four tests to be made on each of the three alternator connections (12 in total). So first turn your meter to the diode check facility:

[Luddite]

Test 1: Forward Bias of Positive Circuit Diodes
Put the meter's negative lead on the positive (red wire) terminal of the R/R. Then, put the meter's positive lead on all three terminals in turn of the alternator (yellow wire) connectors.

The meter should show a forward voltage drop of around 0.5V, which indicates that each diode is allowing current to pass (forward bias), and is therefore working properly. All three results should be approximately the same. (Note the variance from the left hand connector, which could indicate a problem.)


Test 1 results

Test 2: Reverse Bias of Positive Circuit Diodes
Put the meter's positive lead on the positive (red wire) terminal of the R/R. Then, put the meter's negative lead on all three terminals in turn of the alternator (yellow wire) connectors.

The meter should give a reading of "1." (on my meter), or "OL", (which stands for open loop), on some other meters. This shows that the diode is preventing current from passing back through (reverse bias), and is therefore working properly. All three results should be the same.


Test 2 results

Test 3: Forward Bias of Negative Circuit Diodes
Put the meter's positive lead on the negative (black or green wire) terminal of the R/R. Next, put the meter's negative lead on all three terminals in turn of the alternator (yellow wire) connectors.

The meter should give a positive reading as in test 1, which indicates that each diode is allowing current to pass through. Again, all three results should be approximately the same.


Test 3 results

Test 4: Reverse Bias of Negative Circuit Diodes
Put the meter's negative lead on the negative (black or green wire) terminal of the R/R. Next, put the meter's positive lead on all three terminals in turn of the alternator (yellow wire) connectors.

As in test 2, the meter should give a reading of "1." or "OL", which indicates that each circuit is open, and the diode is working properly by preventing current from passing back through.

If any of the forward bias test readings fail to show a voltage of around 0.5V*, or if any of the reverse bias test readings show a voltage, the diode has failed.
(*For comparison,I tested the new R/R and all six forward bias readings were identical at 0.520V.)


Test 4 results

In my case, the reverse bias test on the left contact showed a voltage, rather than the correct open circuit, indicating that the diode was shorted and therefore faulty.

A new R/R was fitted and, 'eccola!', back to a healthy 14.3V charging current.

So, if your battery is not being fully charged, don't automatically blame the alternator - check your rectifier too.

[350TSS]

Brilliant explanation - please can we make this a "sticky" - Thanks

[jerry]

Thankyou 350SS what a wonderful tutorial

[the lodger]

Thank you. Very helpful . Thinking of other bikes, how do you tell if you have single phase or 3 phase, eg are most modern Jap /German/British bikes 3 phase ?

[Wrider]

This is awesome thanks

I was looking at getting one of these

https://www.rmstator-europe.com/en/7...54040111c.html

[spuggy]

Originally Posted by the lodger

how do you tell if you have single phase or 3 phase, eg are most modern Jap /German/British bikes 3 phase ?

I believe you count the wires coming out of the alternator stator, and 2 == single phase, 3 == 3 phase.

[spuggy]

Originally Posted by Wrider

This is awesome thanks

I was looking at getting one of these

https://www.rmstator-europe.com/en/7...54040111c.html

I bought 2 used Shindengen series R/R (one of which was removed from a 2019 Panginale) for just over half of what they're asking for that MOSFET shunt R/R. Look for an SH821AA; fitted to many post-2011 bikes, including Ducatis.

In fact, I just checked Flea Bay, and the same seller (james_sherlock) I bought the other two from was listing one removed from a Yamaha for 29 squid, free postage. Just bought that too, so now all my Monsters will get one...

(A series R/R draws only the current from the alternator that the bike is consuming. A shunt runs the alternator 100% duty cycle 100% of the time - and shunts any excess it doesn't want back into the stator windings, making them hotter and the wires crispier).

Adding in the most expensive Triumph link cable I bought (30 quid) still comes in cheaper than a shunt from an unknown manufacturer. Or you could have superior technology made by the Japanese OEM supplier to almost every motorcycle manufacturer on the planet. Your choice...

Great tutorial Vince, once again I am doffing my cap at your knowledge. I would like some more advice if you can oblige. So I am.....well a Luddite () when it comes to electrics....sorry about that....so I went to start my new addition earlier(S2r 1000), and battery was low. Plugged in the optimate and we are showing 11.3 v ,89amp. I let it do its thing for an hour then hey presto we are back in the game.

With the engine running we are now showing 14 v( I think this indicates alternator is doing its bit ).......engine off and we are showing 12 v.an hour later 12.1v.

So here are the questions, if it's a 12 v battery,with the engine off it will never be more than 12 v will it as that's the capacity of the battery????

If the answer to above is , correct it will max out at 12 v, then why is the optimate charge indicator still showing "low".

Please give your answers at "thicko" level

[Luddite]

Hi Mike. Sounds like your battery needs replacing to me. Although, nominally, they are 12V, each of the six cells that make the battery are 2.1 - 2.2V so, fully charged, the battery should read 12.6 - 12.8V. Anything less than 12.4V and you'll probably have trouble starting, especially with a Ducati.

To check, disconnect the battery, fully charge it for 8 hours then measure the voltage after an hour (this allows the 'surface charge' to dissipate for an accurate reading). Let the battery stand for another 24 hours and see what the voltage is - if it shows 12.4V or lower you've got a problem. As you say yours is only showing 12.1V just an hour after charging, I think it's Motobatt time!

As an example, the battery on my Gilera dropped to 12.3V when it failed to start. After I charged it up it read 12.8V and held that charge for a week while I waited for the new R/R to arrive.

Hope that helps.

PS I found this table online, which is quite useful. You can see that, although your battery has only dropped maybe half a volt, it's actually holding less than 50% of the charge of a healthy battery.

Hi Vince, thanks for the advice,I will follow your instructions to the letter and let you know.The seller said he had replaced their battery recently , I was worried it was slowly draining, faulty earth or similar, will let you know ,cheers.

Mike

[Darren69]

It could be draining through the reg/rec if the reg/rec is faulty. It's rare but it's always connected across the battery even when bike if 'Off'. Just some more food for thought.

[Luddite]

Originally Posted by Moco1961

Hi Vince, thanks for the advice,I will follow your instructions to the letter and let you know.The seller said he had replaced their battery recently , I was worried it was slowly draining, faulty earth or similar, will let you know ,cheers.

Mike

If you charge it off the bike, (or at least disconnect it), and it still drops to 12.1V, then you'll know it's definitely the battery and not a parasitic drain or faulty wiring. If, on the other hand, it holds its charge, let me know and I'll show you how to check for a parasitic drain.

Originally Posted by Darren69

It could be draining through the reg/rec if the reg/rec is faulty. It's rare but it's always connected across the battery even when bike if 'Off'. Just some more food for thought.

.....thanks Darren, will look out for that one too, sounds like I may be embarking on a voyage of discovery here !

[Darren69]

Originally Posted by Moco1961

.....thanks Darren, will look out for that one too, sounds like I may be embarking on a voyage of discovery here !

If you're chasing down a parasitic battery drain then Lud's diode tests should show that as a potential issue. you can disconnect the black plug from the reg/rec and there should be no drain on the battery or there will be if the diodes in the reg/rec are failing.