7AFE Alternator Q - drawing current when engine is off?

[AndyTTR]

Hi all, I've searched but haven't found the info I'm looking for.

I've been trying to diagnose a problem with my sister's AE102 Corolla (built Feb '95).

After replacing the alternator, the battery drains when the car is off and the + pole of the alternator is connected.

When the pole is connected, there is a 110mA (0.11A) draw on the battery with the engine and all accessories turned off. When the pole is disconnected this drops to 8mA (0.008A) which is essentially zero.

Is this normal, or is the new alternator a dud? Are the diodes in it no good?

Or am I completely off the mark and the problem is elsewhere in the electrical system?

Background info:

Last week the alternator died (charge light was on), the brushes had worn down below their minimum length. It is a Bosch alternator with the brushes and reg in one unit. The regulator is no longer available, so I bought a complete alternator (BXT1254).

After replacing it, the battery charges correctly but the alarm kept going off after a few minutes of being armed. It was being triggered by the battery voltage dropping. I figured that the battery was stuffed and replaced it. The problem still exists.

Before we replaced the battery the old one was stone cold flat, struggled to even jump start the car off the Supra.This would suggest that the current keeps draining right till the battery reaches zero volts...

It's doing my head in. Any advice would be greatly appreciated.

Cheers!

[StuC]

thats weird as dude. Is there a fusible link or inline fuse in the +ve line? Have you checked for continuity through the wiring? Possible short?
Dont know if that could really cause a current draw but its worth having a look.

[oldcorollas]

does the alt have the same circuit for the charge light? ie, providing either 12V or ground at the alternator side?

[toymaz]

What sort of alternator is it ?
Is it a bosch with a reg hanging off the back with an injector type plug on it?

[AndyTTR]

StuC - Yep, checked continuity through the wiring. I noticed that the old alternator has no continuity from its + post to its body whereas the new one does. This may just be because the old one has worn bushes though... The other (used but working) alternators I have around here have continuity, but only just...

OC - There's a plug on the top of the reg, but it only has 2 pins instead of the usual 3. I'm guessing they're for the L and S lines, it doesn't make a difference to the current draw if they're plugged in or not though...

You might be on to something though. I don't have the diagrams here to check, but maybe the power for the memory functions is drawn through the ALT fuse and there's a diode to prevent power being drawn directly from the battery when the ignition is off... *shug*

Toymaz - I attached a picture from the catalogue. The original alternator looks identical, but has a different part number. The old reg looked similar, but had a different case. The plug was the same though...

Does anyone know what the 'normal' current an ECU and Radio will pull when turned off?

It shouldn't be this complicated, its a bog stock Corolla dammit!

I've left the battery hooked up today and it doesn't seem to be getting drained any more. There is still a ~0.2A current draw though...

[Hiro]

I've got the AE102 Gregorys manual if you need anything checked, plus the wiring diagrams hosted on my Photobucket account.

[oldcorollas]

100ma could be within spec.. = 2.4 Ahrs per day but still seems a bit high for just a radio and ECU..


http://www.aa1car.com/library/battery_runs_down.htm
at the bottom, check AC voltage of alt when charging.. it could be a leaky diode int he new alt.

[CILAchainsaw]

If it wasn't draining current with the old alternator, even before it stuffed up, but it is now, that would be an indication that either the "new" alternator is stuffed, or something went wrong during the removal of the old alternator or the installation of the new one. Did you disconnect the battery before you took out the old one? You may have shorted something and it's going all weird on you. For example, I know when I take the fuse out for one of the head lights in my AE101, they both go dim, rather than one of them going out altogether. They're weird like that...

[AndyTTR]

Thanks Hiro, that might come in handy later!

OC - That link is gold, you're clearly the highest rep'ed member for a reason.

It's showing ~0.3VAC when the motor is running, not sure if that's within spec or not. After reading that site I realised that 'ripple' is related to the AC -> DC conversion in the alternator. I checked the charging system with a battery diagnostic tool yesterday, it said the alternator ripple was within acceptable limits, but I wonder what their acceptable threshold is?

Also, the stuck relay thing occurred to me too.. we've been getting a lot of cold and wet weather lately, I wonder if moisture has gotten in somewhere it shouldn't be.

Scarface - lol It must've been Toyota Australia that designed the wiring for these cars, I've never had a problem like this with any of the others. You're right, the problem may have been there all along... its my sister's car and its fairly neglected.

Anyway, its booked in for a service at Toyota on Monday. The battery seems to be holding charge OK now, will be interesting to see what they find.

I have a feeling that if not the alternator, the problem is the alarm (for which she's thrown out the wiring diagram I gave her when I installed it 2 years ago). Fun times ahead!

[toymaz]

One thing to try, is just unplug the reg and see if the currant draw stops, sometimes when the rectifier / reg stuffs up you can even hear a little hummm from the alt, untill reg is unplugged.

[Nug]

Andy hi,
Just reading your issue with interest, i can assure you that 0.2amps current draw is fine.
If you really want to; remove the fuses for each of your systems i.e alarm, clock and measure each current draw with your meter you will soon see after adding them all up where your 0.2amps comes from. Another quick method would be to place your ammeter in series with the battery neg post and earth strap then pull each fuse in turn and record the drop. stay cool mate

Regards
Nug

[Casey-G]

I have the same issue except with a replacement denso unit for a 4agze.. obviously i couldn't get a bolt in replacement, so i used a honda accord (or integra? cant remember) alternator in place, it works and charges as it should except i have >1A draw with the key off..



I haven't worried about it as im going back to a 4age alternator setup ect on my new motor, but If i disconnect the + terminal and hold a ohmmeter on the -tive terminal and the +cable i get a circuit so something isn't happy either a short or something is wired incorrectly internally to suit the corolla, i suspect the charge light circuit or the likes is to blame and may be different on the honda replacement one as to what is built into the corolla from factory.

interesting to see if yours shares similar symptoms to mine.

[CrUZida]

100mA draw is still pretty high.

Even with stereo/clock/ecu/immob it should be around the 40mA zone.

An alarm with a bonnet pin sensor might draw a bit more when the bonnet is open though...?

[AndyTTR]

So the car came back today and apparently the electrical system is 100% ok!

Toyota did the 100,000k service (60,000k's late, but hey its a Corolla - they don't need services) and gave it a thorough going over. The current draw is within the acceptable threshold and the alternator is fine.

I guess the current sensing part of the alarm must've gotten fried when the original alternator died? Shame, it was a pretty expensive unit back in the day!

Toymaz / nug - yeah, I used those techniques in the first place (good old haynes manual to the rescue) which is how I narrowed it down to the current draw originating through the alternator. I can only assume there's some funky wiring going on in the loom and this is where the power is supplied when the key is off?

Casey - On the brand new alternator there was full continuity between the +'ve output pole and the alternator case which is grounded via the engine. On the older (but working) alternators there was less continuity but more resistance. On the completely dead alternator there was no continuity at all. I'm guessing there's some arrangement of diodes between the output pole, the brushes and the stator inside the alternator that allows this to happen when the alternator is not generating power (which seems a bit counter-intuitive).

Cruz - I agree! Battery seems to be holding charge fine now though... The bonnet pin was disconnected because it was giving me the shits having to hold the pin down every time I re-connected the battery...

Overall, I think this whole ordeal has proved I don't know as much about alternators as I thought I did. There's a whole lot of black magic going on in that little unit!

Thanks for all the input fellas.

[CILAchainsaw]

No worries mate, and don't forget to Rep appropriately.