Alternator Overcharging

[Malcolm]

Right, I'm running an alternator from a ST185 Celica GT-Four in the MR2.

Before the HG blew the alt was overcharging, to around 16 volts. The alternator is in otherwise good condition because it was rebuild by nippondenso and hasn't had much use since (brushes are barely worn).

Anyway I swapped the regulator for one from an ST165 Celica alternator and now it's charging to 18v! Great for car audio I suppose, but don't feel like frying the ECU!

So, my questions are:

Is there anything else in the alternator that can cause overcharging if fault, other than the regulator?

Anyone know a good source for brand new regulators (going to email Warwick about genuine ones tomorrow)?

[matt dunn]

Is there anything else in the alternator that can cause overcharging if fault, other than the regulator?

Anyone know a good source for brand new regulators (going to email Warwick about genuine ones tomorrow)?

Yes, if there is no power to the voltage sensing wire will cause overcharging.
What makes you say it is charging at 18V because that seems way to high for an alternator to get too even if it has a faukty regulator?
Try a new battery in your multimeter.

Genuine will be way more expensive than aftermarket.

I can probably do a guaranteed s/h or an aftermarket one if you give me the alternator number you want it to fit.

Matt

[Alex B]

Hm electronics teacher seemed to think an ECU could quite happly work with 24V and much higher.

[Malcolm]

I'll check continuity on all the wires running to the alt, that is a possibility as I had to extend the wiring and might have a bad joint or something.

The factory voltmeter and my multimeter confirm it's 18volts.

What sort of money would I be looking at for an aftermarket regulator anyway?

Thanks for the help.

[matt dunn]

Hm electronics teacher seemed to think an ECU could quite happly work with 24V and much higher.

and he teach's electronics ??

I am actually suprised that if it makes it to 18V it has not fried things already.

About 110ish for an aftermarket?

From memory only complete alt's available through toyota
but would pay to check anyway.

[Akane]

I have encountered such problem before, it was a dodgey battery, nfi why it was doing it.

The PowerFC registered 15.99v, and I'm sure it's WAY over 16v, my headlights was REALLY bright.

I think the ECU has some sort of internal voltage limiting circuit.

But yes, a new battery did plenty good.

[Stealer Of Souls]

I've had my stock ECU survive short spikes (well actually long spikes of upto around a second or two) of 17-18V when my alternator went fluff...

[Malcolm]

I have encountered such problem before, it was a dodgey battery, nfi why it was doing it.

The PowerFC registered 15.99v, and I'm sure it's WAY over 16v, my headlights was REALLY bright.

I think the ECU has some sort of internal voltage limiting circuit.

But yes, a new battery did plenty good.

Well I definitely hope it's not the battery, the one I have wasn't cheap and I think the 3 year warranty might be nearly up

[deaf_rattle]

i wouldnt have thought it would be the voltage sense wire, that would more likely cause it not to charge than over charge.

have you got another battery to try?

[Malcolm]

it would make sense that the voltage sensing wire could cause it, as the regulator would probably stop the alt from generating current once the voltage reached a certain point. If the wire was broken at some point it would see 0 volts and assume it needed to charge more.

Haven't had a chance to look at it since the original post, probably wont for a few days yet. Matt Dunn I'll be in touch if I decide to swap the regulator toward the end of the week

[Akane]

Well my battery was a repco $99 jobbie, it died after 1.5years (1 year warranty).

I'd be hurry to get your battery replaced before the warranty is out (if that was the case).

Maybe our batteries are getting cold and lonely all the way up front and it packs a sad?

[Malcolm]

right, so here's what happened today:

Checked voltage at voltage sensing wire at the alternator, with key ON but engine not running;
9.8V when the rest of system was around 11.8-12.2V, looks like a probable cause

Checked voltage of same wire at the begining of the engine harness, where it plugs in to the chassis loom;
9.8V

Trace wire back to fuse box, pull AM2 fuse and check voltage on battery feed side of the fuse;
12V

A ha! So it's losing voltage from the fusebox to the end of the chassis loom? Check resistance of wire, $&#$% all, around 0.1ohm

So I check resistance of the fuse - no reading at all, not even on the highest scale on the multimeter (200M!)

So I swapped in a new fuse (old one was wrong rating anyway, 15Amp when it should have been 7.5Amp), and viola, no more overcharging!

This is the offending fuse, it was transmitting current, it's clearly not blown, but somehow it was causing a massive resistance

[Akane]

hardout.

[johntramp]

well i assume if you had the correct fuse in it would have blown before it did any damage to your electronics (hence the fuse rating is 7.5A not 15A)

[Malcolm]

nah because it's only used as a sensing wire, and the current would have been very low due to the high resistance so it probably wouldn't have blown, even if there was a smaller fuse (like 5amp) in it. And if the fuse HAD blown, the alt would've seen 0 volts and overcharged to like 24V or something!