Testing the Bosch Diode Board

[canuck750]

The 750 S3 battery will not remain charged overnight, even with a battery tender attached. Some component is causing my battery to drain when the bike is shut off, I thought I had the problem beat by repairing the kill switch but once again this morning the battery was dead.

I have the original diode board in the bike and the original voltage regulator.

Is it possible that a diode has failed and the battery is draining through the diode board?

I have read through Guzziology and hooked up an Ohm meter connecting the red and black terminals to the points Guzziology indicates in his chart.

Ok now I am really revealing my electrical ignorance, what setting should the Ohm meter be on, 20 ohms, 200, 2000 etc???

When I attach the black lead to the ground side of the board and the red to the leads from the alternator the meter fluctuates on two of the three terminals but I don't get the 20 ohms Richardson has charted.

Next dumb question, can a faulty voltage regulator cause the battery to drain?

[Triple Jim]

Testing a diode with an ohm meter is very iffy.  Ideally you'd need the exact model meter the guy who made the resistance chart used.  You're much better off with a meter that has a diode test function.  If you don't have one, a 12v battery and a 12v test lamp will do a very good job.  If you put the battery + on the unbanded end of the diode, current should flow to the banded end (convention current, as we think of it, that is), so a test lamp from the banded end to battery negative should light.  It should not light if you reverse the diode.

Of course you need to remove the assembly from the motorcycle, and verify that one end of the diode you're testing is not connected to anything.

[canuck750]

Thanks Jim

I found a couple YouTube videos demonstrating the Bosch rectifier - checking he diodes. I have a spare used Bosch diode board to compare against and the readings I got using the 'diode' setting on my multimeter where the same as those of a 'good' rectifier on line for the three soldered lugs on the backside of the printed circuit board for each of the three points comparing the + to - side of the assembly, all were infinity  or .512 ~ .551 on the board I had in the bike and on my spare they were infinity or .545  .548.



I did find a difference reading the positive side of the board to terminal Y, the board in the bike reads .001 on both + / - probes where as the spare board reads infinity on both +/- probes. This is the board that was in the bike,





It looks like I have a failed diode on the Y terminal and if I understand this then bridging from the positive side of the board power is 'leaking' through the Y terminal at all times.

And this is the spare board measuring the Y terminal to the positive side of the assembly





The battery is back on the charger and the spare rectifier will go back in tomorrow, I sure hope I have found the 'leak'.

[Toystoretom]

If that doesn't fix it, you could try unplugging components one at a time until your battery stays up. And/or you could pull fuses one at a time to find the offending circuit. Unhook your battery and see if the dang thing goes dead by itself overnight. Feel around the bike and see if anything is warm. If it drains a battery that fast something might be heating up. Like the alternator, the starter motor, wires that may have a partial short to ground. Is the light stuck on in the trunk (just kidding). Just some ideas....

[canuck750]

If that doesn't fix it, you could try unplugging components one at a time until your battery stays up. And/or you could pull fuses one at a time to find the offending circuit. Unhook your battery and see if the dang thing goes dead by itself overnight. Feel around the bike and see if anything is warm. If it drains a battery that fast something might be heating up. Like the alternator, the starter motor, wires that may have a partial short to ground. Is the light stuck on in the trunk (just kidding). Just some ideas....

I have checked the circuit through the ignition switch and when the key is off there is no power getting past the switch. I had a problem with the kill switch but got that resolved. Every other part of the wire harness is new, the power drain has to be from the non switched side. When I disconnected the battery it did not drain down but once I hooked it back up it was dead in 8 hours.

[Triple Jim]

Very good, it looks like you did indeed find the problem. 

Whenever there is a suspected battery drain, you can disconnect the ground battery connection with everything on the bike turned off, and reconnect it using the meter leads, with the meter in the amps setting.  That way you can read actual battery drain.  If it's near zero, you can put the meter in the milliamps setting and do it again.  If it reads more than a couple (few) mA, something is wrong.

[Kiwi_Roy]

Very good, it looks like you did indeed find the problem. 

Whenever there is a suspected battery drain, you can disconnect the ground battery connection with everything on the bike turned off, and reconnect it using the meter leads, with the meter in the amps setting.  That way you can read actual battery drain.  If it's near zero, you can put the meter in the milliamps setting and do it again.  If it reads more than a couple (few) mA, something is wrong.

:1:
Just be aware when using the milliamp range of the meter its protected by a tiny fuse e.g. 500mA, if the fuse blows the zero reading will indicate no current draw (because there is none)
The fuse will pop with the smallest load e.g. the park lights, this has caught me out more times than I care to remember.
You can check the meter by placing it in series with a 1k resistor across the battery, it should read 12 mA or 1 mA per Volt.

If the meter fuse is blown you can place a 1k (or less) resistor between battery negative and the chassis then measure the Voltage across it.

I think you have located the fault, one of the small diodes is shorted keeping the regulator powered up with the key Off

[Wayne Orwig]

Try first getting a small 12 volt dash light. Remove the battery ground, and insert the dash light in series. If it lights up, you have a drain.
It is easy to toast an ammeter it you are careful.

Maybe move to testing the power lead at the diode board. Pull that lead and try the small bulb or ammeter in series to see if the board is draining power.