Alternator warning light

[Matt Holcomb]

My alternator warning light came on a couple of days ago during stop-start traffic. It's still on. The car runs fine, but this red warning light scares me.

Originally, it was flickering slightly, so I checked the tension of the V-belt. Perfect. Now it's just a solid, searing red. Argh!

My alternator was rebuilt under 1000kms ago; I'm going to go ape if it's the alternator.

Any ideas? Please don't say it's the alternator!


Matt Holcomb
1974 911 Carrera 2.7 RoW
1975 Mercedes-Benz 280SE
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[Talonz82]

A quick check would be to remove the negative cable from the battery while the car is running, if the car dies, your alternator needs to be removed and tested.

IMPORTANT: IF THE CAR STAYS RUNNING, DO NOT RE-INSTALL THE BATTERY CABLE, TURN OFF CAR AND REPLACE CABLE.

Reason, the alternator with attempt to go to max charging and re-connection could send a spike and short out one of the internal workings.

I have detailed ohm meter tests from Bosch, but they need to be done while it is removed for access, so since you have it off anyway, take it into your friendly checker automotive/pep boys/auto zone and they can test it for you.

you said you just replaced it, is it a Bosch? They come with a 1 yr warranty.(at least the one i just bought did)

let me know if you want the ohm meter testing procedures.

[Zendalar]

Removing battery while running the car will result in almost immediate destruction of alternator....If you trust that your alternator can take the current...well, your choice.

Diodes may sometimes turn into a pure resistors, alternator lamp is connected via a diode and if this diode has a failure, it might now be pure resistor -> Lamp burns constantly.

Diodes usually become resistors, but the other possibility is that they just burn out therefore not conducting any electricity.

[304065]

Matt:

If you have a good belt and the light is glowing brightly, that means only one of two things: the alternator has failed or the voltage regulator has failed. (There is a third possibility, that the blue D+/61 wire is shorting to ground somewhere and turning the light on but I would say that is incredibly remote)

If you disconnect the battery while the engine is running, you will immediately destroy your alternator. As soon as the battery, which is taking up the load, is removed, the alternator will go to max output and fry the diodes- it would be kinda like pushing the clutch in while hauling down a straightaway at 120 MPH.

If the light is on, that means you aren't charging, but the car will continue to run fine on the battery for a couple days, until the voltage drops to where you can't get started or the plugs won't fire.

Start the car and put a voltmeter on the battery. You should see 13-14v if you are getting a good charge. If not, then it's either the regulator or the alternator.

When your alternator was rebuilt a year ago, when you installed it, were all the connections secure? Was there a ground strap between the alternator case and one of the bolts holding the engine case together? How is your transmission ground strap? If you had a bad ground that might be the problem, even though a ground is supplied through the engine wiring harness.

I would check all the connections first before pulling it out, and see if you can swap in a known good voltage regulator.

Good luck!

[Sean Hamilton]

DON'T disconnect the battery whilst the car is running - you'll melt the diodes - you'll smell em frying! .......I'll tell you how I know this one day.

My guess is that your alternator is knackered - mine turned up it's nose after 12 months since a rebuild with the red light symptoms. I wasn't real happy about it either! (Should have got a Bosch the first time)

[Matt Holcomb]

Guys,

Thanks for your replies.

Pardon my ignorance, but how does one test a voltage regulator?


Matt Holcomb
1974 911 Carrera 2.7 RoW
1975 Mercedes-Benz 280SE
Dedicated Homepage
Engine Rebuild Homepage
Porsche Owners Gallery Profile
Pelican Gallery Profile

[CamB]

When you check the voltage (engine off) it should be about 12.5v.

When you check it engine on, it should be about 14.5v (can't remember exactly on either voltage, but the magnitude is important).

If the voltage doesn't increase to at least 13v when you start the car, the alternator may be stuffed. If it reads >15v, the regulator may not be working (and it is pretty bad for your poor battery).

On my car, the voltage is nice and steady(ish - its 30 something years old) at idle and >2k revs, but spikes in between to 15+v. I'm pretending it doesn't happen because I don't want to buy a new regulator right now... and maybe they all do that.

[davis911s]

Now I have yet another stupid question. Which one is the altenator light?

Shawn