I've had an intermittent massive drain for a while - every so often, no real pattern to it, I'll go out to the car and it'll be sitting about 9 volts. I figured it was the usual battery drain issue but unplugging the keyfob receiver didn't make any difference.

Yesterday morning was a lovely bright morning and reasonably cool, and wouldn't you know it, the battery was flat, down to 7 volts. I opened the bonnet to jump start it, and realised that the battery was warm to the touch (it's not a great battery, it's one I was using with a solar panel to play with inverters, but I thought I'd swap it just to see).

Not only was the battery warm but the alternator was warm. Nothing else under the bonnet was, and the last time it had been driven was about 6 hours previously so while it wasn't exactly ice cold it wasn't residual heat. There was a hell of an "inductive-looking" spark when I put the jump leads on too.

I replaced the alternator recently, but it was doing this before - I changed it because the voltage was swinging up and down quite a bit and have the old one sitting in my shed waiting to be rebuilt.

Later on yesterday after it had been sitting outside work all day, even with the sun shining on it the alternator was cold.

So, why would it be doing this? RAVE says that it senses voltage on pin 15 of the BECM which is what I'd think of as the D+ terminal, but does that get powered by the BECM when you turn the ignition on? Could this be something like a relay in the BECM sticking on?

I haven't got a power board kicking about to check but I bet it's the sort of thing that Marty knows... ;-)

Marty is in the UK at the moment but flying back to NZ in the next few days. However, I was at his place a couple of days ago helping him with jobs and clearing his garage. He gave me a number of BeCM power boards. There's one that I know is faulty as I tried it in my car (to get rid of the Fuse 20 blown message I've had for about 8 years) but the others are suspected as good.

Massive drain is usually a short circuit rectifier in the alternator but that would be there all the time and not intermittent, so your theory might be correct. You could try disconnecting the D+ wire and seeing if there is power on it?

Alternator feed to the BECM is an input only and goes to the logic board, so no power flowing back the other way, no relay etc - it goes through some input filtering components on the logic board and then somewhere into one of the processors.

Interestingly, when I was looking through the BECM SID, There was a note in there under battery drain that (the document was dated 1998, so GEMS era), that "the alternator has been found to cause a drain of 4 to 10 Amps, and the 2 fuse boxes have a combined drain for 1A"

The combined drain of 1A would be what we normally would see when BECM is awake, but interesting that there's actually a note there about the alternator... I would imagine that if it was drawing 4 to 10A it would get somewhat warm... I haven't really dug much into what could cause it in the alternator though. could be anything from diode pack to voltage regulator, or I guess even the brushes on the slip ring for the exciter.

Will be interesting to see what you find - but I highly doubt BECM involvement, since the alternator D+ is just listed as an input.

A large drain through the alternator is usually a short circuit rectifier. As they are 3 phase, it will still charge but only from 2 of the 3 phases. But that wouldn't be intermittent, it would be permanent.

I just completed a full rebuild on 2 150 amp units here.
The one on the Borrego had a rough small end bearing, and worn brushes, but was charging fine.
My "spare" one only appeared to need new brushes as it had gone intermittent so got swapped a couple of years ago.
Brushes were indeed shot and slip rings were worn beyond service life.
I went ahead and did a full rebuild since I had the bearings, slip rings etc, and found upon testing that it was drawing 4.6 amps
on my 12v bench power supply, so a rectifier shorted to earth.
Ordered a new diode bridge and soldered it in. Tested under load at 165 amps cold, so a successful repair!
For reference, A good alternator draws zero amps when power is applied to the output stud and earth.
Oh, yea. I was able to purchase all parts from Maniac Motors here in the US and all parts supplied were top quality and made in USA.
A pleasant surprise!

That's great. I've also used Maniac, but they don't ship to Canada, so it's a bit of a hassle getting their parts here. We don't seem to have a similar supplier on this side of the border.

It really annoys me when companies won't ship internationally!
Living in Hawaii for nearly 30 years, it was astonishing to find so many places that would not ship because it was "overseas"
and they had no way to fill out the customs documents!!
Just had stuff sent via free shipping to mate in California, and he just pasted a new label on and sent it to me.
Great trick if they sent via priority mail, he just put on my "Forwarding address" and it was sent on to me for free!
In any event, I am happy to forward parts up there for you if needed.
Just let me know!
Cheers,
Tom

Thanks for that offer Tom. I may take you up on it sometime.
Harv

Customs documents?

I thought Hawaii was a part of US?

th.

You are entirely correct. The 50th state as a matter of fact.
Thus my astonishment on the incredible stupidity of some companies that I had to do business with!
Trying to convince them that the US postal service would be happy to deliver to Hawaii for absolutely no
Additional charge was an uphill battle!

Thanks again for helping me out by sending me the boat trailer wheel bearings @Bolt :)