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Hi I was looking at a Alfa Romeo Mito 2012 1.3 diesel with a flat battery and thought it would be a straight forward diagnosis. What I found: no battery light with key on or engine running, no fault codes stored, battery 10v when running, 10v at alternator, only one thin wire at alternator with a CAN signal when engine off/running. Looking at manufacturer description on the charging system there is little detail but just says the thin wire is between the body computer and alternator is to monitor the system, no other detail. I am interested in finding out how this system works and generally for more info on this type of system. Has anyone seen anything like this or have experience with it? One funny thing I came across in the manufactures description is that the battery light only comes on once the battery is below 4.5v! :lol:
Can't be a CAN bus signal that requires 2 wire CAN hi and CAN lo.
Are you sure its isn't a simple Pulse Width Modulated signal back to the engine computer to allow it to adjust idle air control as an anti-stall.
It is common with Nippon Denso and Valeo alternators to output to the ECU roughly a 50% duty cycle PWM signal at idle with major electrical loads off down to almost 0 at full electrical load.
The alternator will charge normally without this output, and the ECM will simply increase idle speed slightly to compensate for the extra mechanical power going into the alternator.
" We're trying to plug a hole in the universe, what are you doing ?. "
(Walter Bishop Fringe TV show)
Andy.MacFadyen wrote: Can't be a CAN bus signal that requires 2 wire CAN hi and CAN lo.
Are you sure its isn't a simple Pulse Width Modulated signal back to the engine computer to allow it to adjust idle air control as an anti-stall.
It is common with Nippon Denso and Valeo alternators to output to the ECU roughly a 50% duty cycle PWM signal at Iidle with major electrical loads off down to almost 0 at full electrical load.
The alternator will charge normally without this output, and the ECM will simply increase idle speed slightly to compensate for the extra mechanical power going into the alternator.
I am not sure what it is. The manufacturers data says it is a CAN line to monitor the alternator. Voltage is rapidly between 0v and battery voltage. So yer that doesn't make sense for it to be a CAN line, it does look more like a PWM signal and it comes from the bodycomputer to the alternator. Is there any info anywhere on exactly how these systems work? Would be interested to know more about them.
I am pretty sure the manual actually means the charge warning light will not illuminate below 4.5v because this the minimum voltage the dash electronics will work at.
On non-smart charge systems ay charge warning lights I have encountered work on the same principle they go out when the generator output matches or exceeds the battery voltage. So if you have a battery sitting at 12.5v and alternator outputs more than 12.5v the light goes out. If the battery is sitting at 11.8 volts and the alternator outputs 11.9 volts the light will still go out.
Of course on many recent models the ECM and/or Instrument may independently trigger a low battery warning on the dash.
The alternators and batteries on common rail diesel engines have a really hard time especially immediately after start up. In addition to a starter current up to 3 times that on the equivalent petrol engine the electric fuel pump(s) glow plugs take huge current and with a cold start the glow plugs generally continue working for anything up to 20 seconds after start up to reduce white smoke. You can usually observe this by watching the battery voltage from a cold start, immediately after start up at idle speed the battery voltage can take 30 seconds to pop up from say 12 volts to a more normal 13.8 to 14.8v volts
Before testing an alternator you need to be 100% sure the battery has a decent charge and has been tested before you start testing the alternator.
" We're trying to plug a hole in the universe, what are you doing ?. "
(Walter Bishop Fringe TV show)
I don't have any service info or wiring diagrams to refer to on this one. :silly: But the 12V signal you're describing sounds more like a LIN network. It's commonly used on several different manufacturers (Honda, Ford, Mercedes off the top of my head) for alternators and other low data rate actuators/sensors. The network 'rests' at battery voltage, and pulses the network low to send messages.
There's also 0-5V single wire CAN networks, but your observations rule that out. Does disconnecting the alternator make any difference? One common theme I've observed is that most LIN alternators will default to a set charge rate if the network communication disappears. If they continue to not charge, it suggests a bad alternator.
