GART
Well-Known Member
My Johnson sailmaster outboard has a rectifier to charge the battery, I get 14v AC from the charging coil at a fast tick over, but only 6-7v DC out from the rectifier is this correct? Or do I have a faulty rectifier?
Outboard rectifier question
- Thread starter GART
- Start date
VicS
Well-Known Member
Sounds like a faulty rectifier. But does the volts out from the rectifier increase well above 12 when you rev it. It wont charge unless you get up to around 14v
It is easy to check the non regulated ones with a multimeter provided it is an old analogue one or a digital one with a diode test setting.
You dont say exactly what model but since you have mentioned testing the Ac output from the stator coil I assume its the three wire type ( two yellows and a red) but the same procedure applies even if its the 4 wire type ( three yellows and a red)
(ignore any blue or grey stripes on the yellows)
Disconnect it and set the meter to the diode test setting.
Take a reading between one of the yellow wires and the red wire, now reverse the meter and read again.
One way you should get no reading ... ie the same as open-circuit or infinite resistance the other way you should get a reading of around 700 ( which is the forward voltsdrop in mV)
Now repeat from the other yellow to the red
( and between the third yellow on the red if its a four wire one)
Now repeat the whole procedure between each of the yellows in turn and the case.
Between each pair of wires ( or wire and case) you must get one infinite reading and one reading around the 700mV mark
If you have an analogue meter use an ohms range and expect one low resistance reading and one infinite resistance reading for each diode.
If you have a digital meter without a diode test range use the highest ohms range available ... but be suspicious if you get all infinite readings .. it might be an unsuitable meter.
Tell me the outboard model number and/or the HP and year and I'll modify the above if necessary.
The biggest likely cause of a failed rectifier is connecting the battery with the polarity reversed ... instant toast ... ring any bells.
Disconnecting the battery while the engine is running may well also damage the rectifier.
It is easy to check the non regulated ones with a multimeter provided it is an old analogue one or a digital one with a diode test setting.
You dont say exactly what model but since you have mentioned testing the Ac output from the stator coil I assume its the three wire type ( two yellows and a red) but the same procedure applies even if its the 4 wire type ( three yellows and a red)
(ignore any blue or grey stripes on the yellows)
Disconnect it and set the meter to the diode test setting.
Take a reading between one of the yellow wires and the red wire, now reverse the meter and read again.
One way you should get no reading ... ie the same as open-circuit or infinite resistance the other way you should get a reading of around 700 ( which is the forward voltsdrop in mV)
Now repeat from the other yellow to the red
( and between the third yellow on the red if its a four wire one)
Now repeat the whole procedure between each of the yellows in turn and the case.
Between each pair of wires ( or wire and case) you must get one infinite reading and one reading around the 700mV mark
If you have an analogue meter use an ohms range and expect one low resistance reading and one infinite resistance reading for each diode.
If you have a digital meter without a diode test range use the highest ohms range available ... but be suspicious if you get all infinite readings .. it might be an unsuitable meter.
Tell me the outboard model number and/or the HP and year and I'll modify the above if necessary.
The biggest likely cause of a failed rectifier is connecting the battery with the polarity reversed ... instant toast ... ring any bells.
Disconnecting the battery while the engine is running may well also damage the rectifier.
Last edited:
William_H
Well-Known Member
Johnson battey charging
Just a few points to add to Vic's excellent story. The system is designed to be used to run lights so essentially 12v AC. If you hope to charge a 12v battery you need around 13.5 voltsDC. The rectifier will provide more than this but only on the peaks of the pulses of DC derived from the AC.
Using a digital meter to check DC volts is fraught with problems. The digital meter takes a snap shot of the DC over a short period (relative to the pulses) and takes a longer time to process so you don't know just where on the pulse you have measured. This usually shows itself as constantly varying voltage readout.
An analogue (pointer) meter will give an average voltage which again is not necessarily any use. It is the peaks that charge the battery if only for a short period of the cycle.
The best measurement is to use an analogue amp meter and just see how much current you can get into a reasonably charged battery. I reckon mine will give about .3 amp.
Replacing the diodes with Schotky type with a lower voltage drop may enhance this charge current.
good lcuk olewill
Just a few points to add to Vic's excellent story. The system is designed to be used to run lights so essentially 12v AC. If you hope to charge a 12v battery you need around 13.5 voltsDC. The rectifier will provide more than this but only on the peaks of the pulses of DC derived from the AC.
Using a digital meter to check DC volts is fraught with problems. The digital meter takes a snap shot of the DC over a short period (relative to the pulses) and takes a longer time to process so you don't know just where on the pulse you have measured. This usually shows itself as constantly varying voltage readout.
An analogue (pointer) meter will give an average voltage which again is not necessarily any use. It is the peaks that charge the battery if only for a short period of the cycle.
The best measurement is to use an analogue amp meter and just see how much current you can get into a reasonably charged battery. I reckon mine will give about .3 amp.
Replacing the diodes with Schotky type with a lower voltage drop may enhance this charge current.
good lcuk olewill
