Hi all,
Please can someone help me with my dilema.
I have a 2004 Impreza WRX STi, that exhibits not charging when on tick over and all accessories are off, the voltage is about 12.5.
However when I switch any or all of the accessories on the alternator starts charging and putting out 14.5-6 volts. All good.

I just cant underestand why the alternator doesnt charge unless there is something switched on. I have had 2 alternators within a week all do the same thing, and I got a new battery too.

I thought it was the alternator 1st of all because my battery went flat, hence from the lack of charge being given to it, the car is basically running off just the battery, when no accessories are on.

I have checked all the grounds I can find, I have checked the resistance from the alternator B+ terminal to the battery + terminal and no resistance(with the alternator disconnected). On the 3 wire plug, I have checked the 10A fuse which feeds +12v to it, to energise the rotor I suspect, thats fine. There is a feed from the ecu, but I recon that must be fine because thats what triggers the alternator to charge. The other pin is for the dashboard lamp, and that works but it doesnt display any fault with the alternator.

Apart from driving on side lights everywhere, I dont know what else to do.

My alternator is a Mitsu A2TB5391, the battery is a Bosch Silver Plus.

I do hope someone can help, my feeling is somethings going on with the ecu

Regards

Colin.

 

It does sound like an ecu fault. Can you not rig the alternator to charge all the time with one of the three wires (must need an earth I would think?)
Put it through a diode and ignition switched earth? Just an idea

 

Thats an interesting idea, I will have a think about that. The only other thing that comes to mind is that could it be something to do with it being a EDM that these things have looms for day running lights. Maybe thats something that needs by-passing or something. Dunno I am really clutching at straws.

I have got romraider logger, and had a look at the switches table. In the ecu logic it would appear that when any of these "switches" goes from 0 to 1, then the alternator charges. Its like there is either some resistance in the multi-plug that may cause some drop, to signal the alternator to charge on tick over.

I wounder if anyone knows if there is something in the logic that the ecu will output say a trigger 0.1v for standard charge. Then when each on of the switches goes from 0 to 1 then this increments, ie. 1 switch trigger is 0.2v 2 switches is 0.3v, etc. So that tells the alternator how much to put out.

Have I just tried to answer my own question...hmmmm

Any other ideas, please let me know. Im gonna pull the ecu.

Cheers

Col.

 

These alternators are designed to work that way, search for things like smart charging etc.

 

As above: smart charging.

To get it to charge run with fans on inside etc.

 

Maybe it's gone wrong though lads

 

Could be, I have seen threads where the voltage regulator has been faulty. I know from experience they're a pain in the hole for people like me who don't use the car regularly. I just take my battery out now and keep it topped up on the trickle charger.

 

Nope.

It's working EXACTLY as designed, trust me.

 

Surley the fuel pump, injectors, coils etc are a constant load?

 

Not enough, seemingly. Milli-Amps at best.