JMarchetto

I am having an issue with the alternator idiot light staying on. I recently removed the alternator and had it rebuilt due to the battery constantly dying. Had it tested at Advanced Auto and said that the problem was the alternator. Battery checked out perfectly. So, had a local shop repair and he found the problem to be a faulty voltage regulator. Put the alternator back in and after starting, the idiot light came back on. Talked to the shop and ran through tests and the alternator is putting out 13.85 V at idle and 13.6 V on full load at 2000 RPM. He did tell me that he did not test the diode (ugh!). I was told that the blue wire on the alternator would tell the story. Hoping to avoid removal for testing, I think I traced the blue wire to the rear electrical panel and tested the voltage output. With the key on, it registered 1.4 V and at idle 14.0 V (if I found the correct wire!). But the idiot light is still on. Can anyone suggest how to test further to see if there is a fault with the alternator, diode, wiring, etc.? I am REALLY hoping to not remove the alternator again!

pp000830

PM me with your regular email address and I will forward the wiring diagram for this to you.
Andy

Railmaster.

Why do you refer to the light as the idiot light at the same time as you need help to handle the situation involving the said light?

JMarchetto

An affectionate term that is rather specific. I would much prefer a voltmeter but Porsche did not think so.

pp000830

Nice dash volt meter:
Volt Meter eBay

JMarchetto

Thanks. This is a nice item and must have for the garage queen.

pp000830

If your car reads between 13.5 and 14.2 volts with the engine running and between 12.5 and 12.8 a few seconds after you turn it off your alternator is most likely working correctly and is most likely wired correctly.
I looked at the wiring diagrams and the wire from the battery light in the dash clock is a shared circuit that goes to the alternator, alarm, abs & central informer controllers so it is not a simple system.

This leads me to the idea of simply disconnecting and reconnecting the battery as it may reboot/reset some internal software switch in one of the controllers related to this circuit.

An idea I had was that the alternator diodes are part of what is called a rectifier bridge circuit where sets of diodes are set up like one-way valves to redirect the alternating current from the alternator rotations to direct current. For example, something called a full wave bridge design using more diodes produces a smoother level of DC current, a half wave bridge design produces what looks like pulsed DC current. Conceivably if one or two of the diodes in the rectifier circuit internal to the alternator are bad creating a pulsed DC condition this may be causing the warning light controller to turn the light on. From the wiring diagram, the alternator has multiple diodes in it that suggest it is designed to produce very quiet electrical power. The good thing is that this condition can be check without removing the alternator as it presents as a noise in the alternator circuit. Any qualified technician should be able to tell you if this theory applies to your car.

A very very long shot is that the solder joints in the clock are known issues in our cars and to pop out the clock and reheat the internal solder joints on the board is relatively easy, I did it at the kitchen table years ago and seem to recall the solder joints to the clock motor need to be released to get the board out. Then make sure the bulb connections are good.

Some folks replace the some of the dash display lights with LED bulbs. These are not resistive loads like the original bulbs and also act as one-way electrical valves, this may be part of the issue if you have replaced any of your display bulbs.

Finally, do you know why the alternator failed in the first place as this root cause may still be present? Have you had any servicing done recently, just before the alternator issue cropped up?

Hope this all sorts out. The upside is you can still drive your car.
Andy

drodg

Has anyone bought one of these and used it ? Seems pretty cheap and a good idea.

pp000830

Yes, they work really well and look nice. I have purchased several and handed them out to my auto driving family members.
To shut the meter off I just pull it part way out and leave it in the socket.
Andy

IXLR8

Install one and connected to where it should be connected, directly to the battery.

This one is "on" all the time as it draws less than 2 mA.

This reading is at idle...

drodg

Thanks

JMarchetto

Thanks for the multitude of suggestions and your in-depth perspective. I will try the easiest first and work down the line. I will let you know of my progress and pose questions as they may arise. Thank you for taking an interest and I will get a voltmeter mounted.

mike cap

If you use a radar detector, they may have a selection to display bus voltage when the unit is in passive mode. My Escorts have this function and I use it for the 993 and 997. So, you may already have a voltmeter installed.

pp000830

Very slick lookin'!

IXLR8

Got one on my motorcycle as well. Simple to build if the person is the least bit handy. One BUD potting box and a Datel meter.

You always know what is going on with your charging system.