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Peugeot 308 alternator voltage

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Diver300

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I've been using alternator voltage as a way of determining if the car engine is running on a variety of vehicles, and it's normally fine. If the threshold is set about 13 V, the voltage is almost always above that with the engine running, and below when stopped.

On a couple of 2016 Peugeot 308s, the voltage doesn't seem to stay high all the time with the engine running, so going down below 13 V. So far, I've only noticed this on these two vehicles. On one, the voltage was dropping frequently when the vehicle was going slowly or idling, and on the other the voltage would reduce when moving as well. I don't know if there were different usage patterns between the two vehicles, like having the aircon on or off.

Is this some odd battery / power management thing going on? Has anyone else monitored alternator voltage on cars in everyday use?
 

picbits

Well-Known Member
Electric PAS can put a huge drain on the electric system and drop the voltage considerably. We've noticed variations between 12.5 and 15v on a friends Astra when making slow manoeuvres.
Could also be a knackered alternator lol. You could always look at the rectified DC on the system - there is normally a nice ripple from the three phases of the alternator which can also be used for RPM measurement.
 

debe

Active Member
Some of the new vehicles the ECU controls the altenator voltage, & can drop down to 13V. Suposedly for economy & environmental reasons. Check the voltage with the head lights on & higher engine RPM. The ECU should raise the voltage to compensate for the larger load.
 

MikeMl

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Some new cars stop the engine while waiting for a red light to change...
 

Diver300

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The expense here is the installation difficulty, so cost in time taken. I would like to use a CAN signal, but it is extra effort to locate the correct wires.

I do have provision for a separate wire to detect ignition status, but that also takes more time than just finding power and ground, so voltage sensing is often used. It works most of the time, but this particular model seems to be a problem for the most modern cars.
 
Newer vehicles are heading for power management systems that reduce power from the alternator when not necessary. There are many reasons for this, the biggest one being fuel economy. When the alternator is not being driven hard, less HP is required and therefore less fuel. There is also delayed charging to alleviate heavy loads being applied to charging systems right at start up. Loads are gradually phased in. I don't think you can get what you need from the CAN data bus either so your ignition system may be your only source.
 

arm755

New Member
Most Peugeot alternator are Mitsubishi alternator that use DFM/DF Digital Field Monitor output that will control the horsepower needed depending on the electrical consuption of the car to avoid fuel consumption when not needed. But the voltage should be always between 13.8 and 14.5 volts. If its less your alternator is damaged or your battery has a short circuit.
 
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