jon3929

Hi everyone, hoping you may be able to help....


My missus has a 2004 WRX STi, which we've had for a year


About 3 months ago the battery was flat after a week, did some testing and concluded the alternator was not charging properly as the voltage at the battery terminals with engine running was only 12.5 volts. No lights or warnings. Changed the alternator and it was exactly the same, didn't think anything of it and carried on as using it daily.


Then a few weeks ago the engine management light came on, got the code read and it was the coolant sensor. Changed it, fault remained, eventually traced it to a broken wire in the loom. Fixed it - sorted.


However, the same day the engine management warning light was back on this time for alternator overcharging. Static battery voltage is 12.4 volts, running voltage is 14.5 volts (yes, 2v higher than a few months ago but still well within limits). Changed the alternator back to the original and exactly the same. Checked the continuity on all the alternator charging wiring and it all checks out ok. Checked and cleaned all earths.


The yellow engine management warning light on is permanently on - so something is failing the BIT test that the ECU does every time you turn it on - ie there is a hard fault.


If this were a genuine overcharging then surely it would clear when you turn ignition on and then come back as soon as the alternator got up to a high voltage?


The red battery warning light works as expected, Ive checked those the supply voltage from the instrument cluster and that is 12v as expected.


However, I've read quite a lot about diode failure in the alternator, so did some testing and got some odd results. Statically the DC voltage between the red alternator output terminal and ground is 12.5 volts DC, and 25.6 volts AC. Engine running is 14.5 volts DC and 31.5 volts AC. I've used two multimeters and get the same result.


So thinking this might be failed diodes in the alternator I disconnected the alternator, turned the ignition on and the yellow engine malfunction warning light stays on. Engine running with alternator totally disconnected and the engine malfunction light is still on. Strangely the red battery warning light didn't come on!


So now I've apparently got AC voltage from the alternator which suggests diode failure in the alternator, but when the alternator is totally disconnected and running on battery only at 11.5 VDC the fault light remains lit, so how can it be the cause or overcharging?


Can anyone confirm AC voltages being anything like mine?


Anyone seen this before or got any bright ideas.


Thanks very much from newbie jon3929.....

IAN WR1

iTrader: (2)

Hi john i cant say what is wrong with yours but my car don't get used from one month to the next its a nice ornament to look at i went to start it last weekend and battery was dead so charged it (about three times a year tracker and alarm eats the power ) so i put the battery back on this morning and checked the voltage for you and yes mine reads the same as yours all within 0.1 volt of what you got AC and DC i must say that the power sensitive alternator reads 12.5 volt with engine running and lights off and 14.4 volt with engine running and lights on good luck and sorry im no auto electrician sound like you know more than me

johned

Smart alternators are a pain in my opinion.

XDevil666

Sounds like my car, decided I'm going to charge my battery and use the car as a daily for the Christmas week!

jon3929

Discovered a few things but not made any real progress on this yet...

I got the fault codes read and once again its was for alternator overcharging. This time the fault cleared and did not come straight back so I naively assumed it must have been the replacement alternator - Yippee!! Drove it back home all was ok. Went to start it later in the day and the light was on again as soon as the ignition turned on. Back to the garage, cleared the fault again, but this time it came back on a mile or two down the road. Bugger.

Problem is the fault light latches on after any trigger and wont go off until a reset, so you cant tell how often or how long it lasts. I've just ordered a fault code scanner so I can clear the fault myself.

My conclusion from this is that I have an intermittent wiring fault (like the original fault on the coolant sensor). I can't work out whether a short or open circuit on the alternator wires would result in this fault - but I cant think what else it could be.

So far I've checked continuity in all of the alternator wiring back to its nearest plug or connector and statically its all good, but clearly at the moment I cant tell whether the fault is there or not until I get this scanner. I've also stripped back the harness that runs across the top of the engine to the section where it disappears into the wing and externally that all good too.

So a couple of questions...

Are Impreza's prone to loom chafing in the engine bay?
Any bright ideas on how to better check continuity? other than clear the fault and manipulate the harness until it pings back on the light.....
Any other bright ideas?

Good job I love my missus.......

jon3929

Thought I'd update in Just in case anyone searches on this subject in the future.

Having got a fault code reader and being able to clear the light, I realised that the fault light came on with any reduction in the load on the electrical system - so for example turning the lights off with the engine running..... bing! on comes the light. The refresh rate of the scanner is not adequate to see an overshoot, but when I hooked up my analogue meter I could see the voltage over and under shooting whenever I turned off a significant electrical load.

Steady state with a load on, the alternator charges happily at 14.3V. Leave the sidelights or fan on and it damps down the over and undershooting and keeps it within limits.

Have driven it for a couple of hundred miles now with the sidelights on, perfect. Will order a new alternator...... (and leave the fan on a low setting for evermore to ensure it charges continually at a low rate.....).

Cheers, Jon.

scoobyskool

iTrader: (1)

I remember reading kn here not too ling ago that the alternator doesnt really go to work till it senses a load ,, ie to recharge a low battery its actually better to drive with lights on to get it to work harder

This may or may not be correct

But if it is perhaps you have a fault in the lighting circuit or elsewhere causing the alternator to believe it needs to work hard

Hopefully the above makes sense ,, as some of these electrical issues can be such a pain i thought it might be worth considering ,, ill try remember the thread i read and post up a link

the shreksta

iTrader: (20)

Will having the head unit on help