battery or alternator: Please help - 986 Forum - The Community for Porsche Boxster & Cayman Owners

Steve Tinker · 03-08-2010, 02:02 PM

As an addition to JFP's advice, if you disconnected the battery with the engine running as per your item#

4.) Car stops the second you take off the negative terminal.

there's a good chance that you have blown the diodes in the alternator......

mptoledo · 03-08-2010, 02:08 PM

Funny you should mention that. I read that in one of my searches after I had already done it. Live and learn. I guess 9 years on an alternator, I got my moneys worth.

Why would my oil check guage not work? that is what I can't figure out. It just blinks and doesn't do anything else

ARModen · 03-08-2010, 02:31 PM

The battery light is really an alternator light, which generally indicates that there is something wrong with the alternator or the voltage rectifier/regulator on the output. The ones I've taken apart sense voltage at the diodes, so the light coming on indicates the alternator is no longer rotating, or the diodes have burnt out. So it could be a pulley issue, but you get a lot more lights if you throw the belt.

Most of the electronics in the car will run down to 5-8VDC. Below 8VDC you start losing sufficient potential to generate an arc at the plugs and run some larger loads effectively. If you're alternator isn't working then you have 12.5VDC from post to post on a fully charged battery. Battery voltage drops as load increases and the battery discharges so 10-11VDC is reasonable while driving. Any sort of grounds or dirty connections or large loads (headlights) can drop portions of the electrical system another 2-4VDC. This is fine if you're starting at 14.5VDC but enough if you're at 11VDC, which is probably what you're seeing. 5VDC is probably enough to still run the instrument cluster, but not enough to run the oil level sensor. My old 944 had dirty battery terminals that were dropping ~6VDC. They were hot from dropping that much power.

Bottom line, fixing or replacing the alternator and then driving the car around a bit will probably fix your problem.

mptoledo · 03-08-2010, 03:29 PM

No new noises or anything pully related. I pay close attention to sounds in my car after the "ims" scare of 2009 on here.

I think when I arcd my battery installing it, I zapped the alternator. I new I did a bad thing when I saw a fews blown and my rear spoiler had gone to the up position.

I am just thinking of taking the alt in tomorrow, but I will decide that after I attempt to take it off. What I have read it isn't that muchof a big deal. Cant' be as hard as when I replace my alt on my girlfriends Taurus. Some genius really screwed the pooch when they desinged that engine.

Lil bastard · 03-08-2010, 04:40 PM

I suspect you've had a bad alternator all along which is why the other battery failed to start the car. The arc is surprising unless you connected the negative (-) cable first then the positive(+), or with the key in the ignition - big NO-NO that.

When connecting the new battery, you likely blew another set of diodes (3 prs. total).

Also, did you charge the battery prior to installing it? FYI, batteries straight from the store are not at 100% charge due to shelf life. It's always best to charge the battery independently first and not rely on the alternator - they are not intended or really designed to fully charge a battery. Their function and design is to run the car and it's accessories while topping off the charge of the battery dissapated by the start-up.

If your alternator were newer, I recommend just swapping the diode pac - available sparately. But since it's 9 yrs. old, I'd opt for replacement so you have fresh bearings, brushes and windings, it's the best way to go.

BTW, a battery in top consition should show no less than 12.8 volts and more like 13.0-13.2VDC. And for the Bosch alternator, the output spec is 13.5-14.5VDC.

Cheers!

Paul · 03-08-2010, 07:17 PM

The battery idiot light will glow when the battery is almost dead even if the alternator is OK. My 01 did this on Saturday after sitting for 6 weeks as soon as I put the key in the ignition and before I turned it.

vipola · 03-09-2010, 06:28 AM

Quote:

Originally Posted by mptoledo

4.) Car stops the second you take off the negative terminal.

I would NOT do that, even if it was a good test a while ago, with today's fancy electronic, you don't want to unplug the battery on a modern car/motorcycle while it run. The battery help to keep the voltage stable and prevent it to spike.. it's not intent to run without a battery.

Quote:

Originally Posted by ARModen

The battery light is really an alternator light, which generally indicates that there is something wrong with the alternator or the voltage rectifier/regulator on the output.

I learned this one the hard way, been a motorcycle guy for ever, when I saw the battery warning light up I tought it was a low voltage indicator and I went for a ride to 'recharge' the battery.. and got stuck when the battery died in the middle of nowhere 15 min later LOL..

mptoledo · 03-09-2010, 01:25 PM

UPDATE:

Ok, i got checked the battery with a meter 12.5 turned the car and on and it was at 11.9, turned the lights on and it started dropping to about 11.7. Took the alternator out(20 minutes). had it tested and the test machine immediately showed an error. Ordered a remanned bosch $208. I will have it in about 2 days and will update you on the results.

Side note: I would not test the alternator by taking off the neg battery cable. I have heard from too many people that this is a no no!!

JFP in PA · 03-09-2010, 02:22 PM

Quote:

Originally Posted by mptoledo

UPDATE:

Ok, i got checked the battery with a meter 12.5 turned the car and on and it was at 11.9, turned the lights on and it started dropping to about 11.7. Took the alternator out(20 minutes). had it tested and the test machine immediately showed an error. Ordered a remanned bosch $208. I will have it in about 2 days and will update you on the results.

Side note: I would not test the alternator by taking off the neg battery cable. I have heard from too many people that this is a no no!!

"Just like advertised"............for future reference, you can use a volt meter to get a "go/no-go" reading on the alternator; if you really want to be able to fully evaluate the entire charging system, consider getting a load tester. Besides being the best way to test the battery, it can also test the alternator output under light and heavy loads, putting the diodes and regulator thru their paces as well. Cost about $50-60 bucks, works on any 6 or 12 V system........and won't kill anything in the process................

vipola · 03-10-2010, 05:30 AM

Quote:

Originally Posted by JFP in PA

"Just like advertised"............for future reference, you can use a volt meter to get a "go/no-go" reading on the alternator; if you really want to be able to fully evaluate the entire charging system, consider getting a load tester. Besides being the best way to test the battery, it can also test the alternator output under light and heavy loads, putting the diodes and regulator thru their paces as well. Cost about $50-60 bucks, works on any 6 or 12 V system........and won't kill anything in the process................

Ehh.. battery tester need to match the amp of the battery.. if you test a 600amp battery with a 50amp tester, you 'may' not have a valuable result :P From my experience, those 50$ tester are best suit for small motorcycle battery :P