[Avernus]

Hi all, my STi Limited has barely 500kms on it and it has failed to start up one hot afternoon. (then it miraculously works that night and till now)

On that day, it could crank but nothing would fire up, so the usual suspects would be fuel pump, spark plugs or alternator, am I right? It shouldn't be the battery since it can crank.

Normally, my battery reads 12.6v (measured using a multimeter) when the car is totally switched off and around 14.0v when the car is running, which should be the alternator charging voltage.

I've caught it doing 12v+ (on the Apexi Timer that came with the car) while the engine was on, and managed to catch it on video :-

YouTube - WRX STi 12v reading when engine on - Alternator bust?

Is my alternator faulty? And since it only happens sporadically, will my dealership be able to detect a faulty alternator if the fault doesn't manifest itself on the day I bring it in? Is there some load test or other test that can detect a faulty alternator (assuming that is the problem)?

I don't wanna be caught out somewhere with a car that won't start, or worse, have the engine die mid-way while driving! Any pointers would be greatly appreciated!

Add 24/04/08 : Here's a shot of the car working fine today. The Apexi reads 14.2v when the engine is on.

[VSGTS14]

you can't read the full voltage off of a turbo timer, it's only getting 12v sent to it.
14v would be at the source, which is straight off the battery.

you can do a few tests to see if you're having voltage drops across the starter circuit and alternator circuit. they will let you know if you're starter is faulty or the alternator.

[Avernus]

Hmm.. the Apexi reads 13.8 - 14.0v normally when the car is running, and occasionally drops to the 12v as per the video. If it's getting 12v sent to it all the time, it shouldn't show 14.0v normally?

I used a DMM connected to the batt direct and got these readings :-
- 12.6v (car off)
- 14.0v (car on at idle, nothing else on)

From what I've read, a 12.6v reading when the car is off is a good bet for a healthy batt, and 12.2v and below indicates a bad batt. Of course, a proper load test would be more accurate.

Upon the engine running, the voltage is read as 14.0v due to alternator charging.

I could be totally wrong but thanks for your input! Really hope they can test out if it's the starter or alternator.

[Avernus]

Added pic of Apexi reading under normal operation. Any other thoughts or comments are appreciated!

[TAPOUT]

I would bet your Apexi has a bad ground or poor connection. I wouldn't trust the voltage of an added part nearly as much as I would trust the reading of a multimeter across the battery terminals. Are all of your connections soldered, and your ground free of paint and locked in tight with a lock washer? Check your connections first. I seriously doubt that if your battery had enough juice to run the starter it didn't have enough to fire the spark plug or run the fuel pump.