Red Light on Dash on, Need Help!

nick325i · 09-12-2009, 01:15 PM

The lower red light on my dash is on. It doesn't seem like the alternator is charging the battery either.

Do I just need a new alternator or is there something else that could be wrong. I just got the car running for the first time in a couple years so if anything could happen from sitting then that could be it.

Thanks in advance for the help.

Por_sha911 · Por_sha911's Garage

It could be an assortment of things. First, make sure the fan belt is still on. Then, put a voltmeter on the battery and see what you get with 1) the motor off 2) with it running 3) with it running with everything on (lights, AC, etc).

pete3799 · pete3799's Garage

If the car's been sitting for two years i'd charge the batt. and drive it a bit.
The alt. might come into it with a little use.
If the light dosen't go out after a while then it may be alt. time.

lr172 · 09-12-2009, 08:22 PM

Quote:

Originally Posted by pete3799

If the car's been sitting for two years i'd charge the batt. and drive it a bit.
The alt. might come into it with a little use.
If the light dosen't go out after a while then it may be alt. time.

I wouldn't drive it too long, as that light on most cars usually means a voltage below 13 VDC, which means no charge. The car will probably only run an hour or two on the battery charge, depending upon the condition of the battery.

The alternator is not likely to start working again without intervention. I would first verify the voltage at the battery while running. If it is below 13, it's likely the alternator or voltage regulator which is a component of the alternator. If the fan is turning, the alternator is turning.

It is possible that other odd things could cause this, such as shorts, but alternator is top of the list. Most manuals will give instructions for testing the alternator.

If the measured voltage is around 14, indicating a charging state, you will need to dig harder on the cause of the light and beyond my expertise with these cars.

Good luck,

Larry

nick325i · 09-13-2009, 10:58 AM

Thanks for all the help guys. I really appreciate it.

I had the alternator tested at the local auto parts store and they said it is charging. The battery tested bad, but I have since tried a few different batteries that I know are good and it still has the light on.

I've also checked all my wires and cleaned all my grounds. They appear to be good.

Could the voltage regulator be the problem? I have a little experience and usually a bad one causes over-charging, not under-charging, but that is not on a Porsche so I don't know.

Paulporsche · 09-13-2009, 01:15 PM

Have you checked the tightness of the fan belt? Is your idle speed correct? Does the light go out when you up the rpms?

docrodg · 09-13-2009, 01:32 PM

Other items to consider: Are the battery connections clean and tight? Take them off and give them a good cleaning, same with the wires to the alternator. Check the wiring with an ohmmeter for shorts or bad spots. Check the voltage at the battery with ignition off and again with engine running even though the parts store checked it you should check in the car. If it is low with engine running then you have a bad alternator or a serious drain.

To really determine the problem we need to know the voltage at the battery in all conditions. Also, consider that a bad battery will suck up all the output and drag the voltage low as it eats amps. If you have a good battery in another car that is fine try hooking that in and taking the same readings if light stays on, similar readings point to the alternator.

nick325i · 09-13-2009, 05:43 PM

The fan belt is tight. I've removed the alternator and cleaned or replaced every connection. The light stays on at all rpms.

One weird thing I noticed is the light gets brighter the better the battery. The battery in the car was a little low so I attached another one to help get the car started. The light got brighter when I hooked the other battery.

I am going to put the car back on the lift tonight and follow the wiring back through the car and check it out.

Does it make sense that it could be a voltage regulator? Could that cause I under-charging situation? If I need an alternator then so be it, I just don't want to spend the money if I'm not sure. Thanks again for the help.

nick325i · 09-13-2009, 05:50 PM

Also, I know my car originally had an external voltage regulator. Where would it be if my car still had one? I am trying to figure out if my alternator has been replaced with an internal regulating model. It does have the replaceable brushes if that helps. Thanks again. Y'all are great.

docrodg · 09-13-2009, 05:54 PM

Could just be a blown diode too. Certainly a regulator can fail and cause an undercharge if the failure mode is correct, but that is unlikely in the regulators used in the early cars. If I had to bet, based on your description, you have one blown diode, and that causes a slight "bucking" of the full voltage.

nick325i · 09-13-2009, 06:00 PM

Isn't a diode an internal part of the alternator? So that would me I need a new alternator or is that something I can replace separately?

docrodg · 09-13-2009, 06:03 PM

Separate part for the rectifier assembly. Plus, if you have a meter that can test diodes you can even find the faulty one and then rather than spend 60 bucks on the whole assembly you jsut get a 1.50 part.

nick325i · 06:20 PM

How would I got about changing a diode?

Also, What is the difference between the top light (battery light I think) and the bottom light. The bottom light is the one that is on in my car and to clarify, it should go off when the car is running?

nick325i · 10:38 AM

So, I finally got my multi-meter to work. It's reading 12.4 v with car turned off at the battery and 12.15 with the car running at idle and 2000 rpm.

jstobo · 09-14-2009, 01:40 PM

With that voltage your alternator is not charging. The shop you took it to is nuts. You need to search the posts. If the alternator has been upgraded to one with an integral voltage regulator you may have to add a resistor to the light. The factory manuals showed how to do this and it is in the board's archives. Also make sure the alt light is the correct one. The light is an integral part of the charging circuit. Essentially if the battery voltage is higher than the alternator side the light comes on. I forget all the details. Do a search. You will find a wealth of information.

docrodg · 09-14-2009, 01:47 PM

Yep... Time for an alternator.

nick325i · 02:31 PM

Ok guys, looks like I need to break down and get an alternator. Or see if I can have mine rebuilt. I think it's a bosch model with a external bosch regulator. I'll start the search.

Is there anyway to make absolutely sure it isn't the external regulator before I spend money on an alternator?

Thanks for all the help.

Walter_Middie · Walter_Middie's Garage

Quote:

Is there anyway to make absolutely sure it isn't the external regulator before I spend money on an alternator?

Yes - remove your alternator and take it to a reputable repair shop. They will test it before rebuilding. I've had mine rebuilt twice, as I have been resisting the switch to an internal voltage regulator. I like my external one - no reason, I just do. I think, if you purchase a new or rebuilt alternator, it will be superseded by one with an internal regulator, and you will have to convert your car over. Lots of info on here regarding that. Do what you think is best for your circumstance.

nick325i · 09-14-2009, 02:46 PM

Ok, ordered a new alternator. Another Bosch unit with the external regulator. I think is the problem and otherwise it won't hurt anything.

teenerted1 · 09-14-2009, 02:57 PM

Quote:

Originally Posted by nick325i

How would I got about changing a diode?

Also, What is the difference between the top light (battery light I think) and the bottom light. The bottom light is the one that is on in my car and to clarify, it should go off when the car is running?

on tach
the upper green light is your low oil pressure warning light.
the lower red light is you alternator warning light.

my green oil idiot light isnt wired so it never comes on
but my red alternator light stays on at start up till i rev past 2000. then it is fine(off) if the idol stays above 900 rpm.