Charging system problem - Experts, help keep Porsche practical! (photo)

[KobaltBlau]

You guys all know how practical a 911 is:



But it has to keep running on the whole thanksgiving road trip!

Scenario:

"battery not charging" idiot light comes on with ignition on and engine off, but always off when engine is running. However, battery is clearly not charging. after 1.5 hours of driving I killed the engine in stop and go traffic and it would not restart. Quick tow to wal-mart (open on thanksgiving) and a new battery later I was back in business. Instant start. New battery fully charged before 3.5 hour drive about 1 hour of which I used the headlights. I now get the following readings from the DMM at the battery:

1. engine off 12.8 Volts

2. engine running no accessories on 14.8 Volts

3. engine running lots of accessories running (headlights, rear defroster, radio, heater fan) 12.4 Volts.

The bentley manual says that if #2 is much over 14.2 volts you probably have a bad regulator. However, it also says that if 2. and 3. are the same you probably have a bad regulator, but if they are different it is probably a bad alternator instead.

I guess I don't know for sure that the new battery won't charge and function normally, but the voltage readings seem to suggest a problem, and the old battery was eventually able to charge from a plug-in charger as well.

So, to the question: Which is it, a bad alternator, or a bad regulator, or how can I test further to help you experts diagnose? The car has 30k original miles and original engine. Thanks in advance!!

[Joeaksa]

Alternator is probably gone.

Get somewhere you can or will stop for a day or two, find a "mom and pop" alternator overhaul shop and take yours in. I did the same with my '85 model and $75 bux later it was like new.

Might call them before and ask "can you overhaul a xxx alternator" first. Try not to say Porsche or 911, just give the name of the alternator if possible. Some places see $$$ when they hear Porsche, while some are fair and just overhaul the bits.

Does not matter alternator or regulator, as the regulator is internal and you have to take the puppy apart to get to everything other than the brushes. While in there you might as well put new brushes and bearings.

The "mom and pop" shop that I went to had a new regulator on hand, as I guess they are used on other cars as well.

Joe

[KobaltBlau]

Thanks, Joe.

I wasn't clear about this before, but the car is home now and I can fix it at my leisure.

Actually, the car has an external regulator because it is pre 82 and original, so therefore it does matter to me whether it is the alternator or regulator. I probably won't update to the internal regulator even if the alternator is gone. I have done that external->internal update before on another car, and I think it sucks without getting the newer shroud.

I do see that a mom and pop alternator shop could figure this out, but because I can fix it at my leisure and I am handy with a Multimeter, I thought perhaps I could do the diagnosis myself.

[Elombard]

Check in 101 projects too, Wayne has some clear advice on testing the alternator. I kinda think you are OK??

[KobaltBlau]

Erik, you mean you think my readings indicate no problems?

We are living in an apartment between houses so I think my 101 Projects is in long-term storage, but I can look on my (shadow of what it once was) bookshelf tonight just in case. Alternately, if someone wouldn't mind giving the gist of the test scheme I would appreciate it.

Cheers,

[Elombard]

I dont have it here, I will try and look tonight. Yah I think the readings indicate no problems.

[Joeaksa]

Andy,

Your readings indicate that either the regulator or alternator is tits up. It should hold 13.5+ with a load on it, not drop back down where it is now under load.

Joe

[KobaltBlau]

OK, is there a way to:

check voltage at the alternator with the engine running and all of the accessories running, or is this a bad idea?

or check resistance across some poles of the external reg to indicate failure?

[CliffBrown]

Originally posted by Joeaksa
Andy,

Your readings indicate that either the regulator or alternator is tits up. It should hold 13.5+ with a load on it, not drop back down where it is now under load.

Joe

Joe

I agree with your statement but want to make a suggestion.

It might be possible that one of the accessories (rear defroster, heater fan, etc..) could be causing a larger than normal drain (higher amperage) and overloading the alternator (75 amps when it was new?)

A poor electrical connection to any of the accessories could cause a higher drain.

A poor electrical connection at any point in the charging circuit could cause a symptom being experienced.

Andy

To eliminate any of the problems I'm suggesting you could easily check the battery connections in the trunk and the starter and the ground straps (tranny). Unless you've cleaned these connections previously then it would be a good preventitive maintenance job to do this on any vehicle after many years in service.

[KobaltBlau]

Thanks, Cliff. The battery is new and the terminal clamps are clean, though I did not metal brush them. The car has 30k miles in dry climates (just moved to the NW) but of course it is 28 years so I should check the tranny ground straps!

[CliffBrown]

Andy

Also make sure to check the battery ground at the chassis in the trunk.

Did anyone ever remove or clean the alternator ground strap from the alternator to the motor?

[KobaltBlau]

Cliff, the alternator ground strap has probably never been removed or cleaned. this car is pretty dang original.

[Joeaksa]

Also check the 14 pin connector and make sure that the pins are clean and making a good connection. Believe that some of the alternator wiring goes through that connection.

Checking all the connections and grounds is a very good first step but it sounds like you have done this. As Cliff said it could be using too much power when the defroster or like is on but still it should be a lot closer to 13 volts and not 12.4.

Assume that you are using a digital VOM and not an old fashioned one? The digital ones are more accurate and gives you a better idea of what is going on.

[KobaltBlau]

Yes Joe, I am using my digital multimeter that I have a track record with.

I looked for my 101 projects but it must be in long-term storage. I don't expect to be able to access it for months, so I would be interested in someone's interpretation of the diagnostics. Erik, maybe you would have time

I am still interested in direct diagnosis techniques for the alternator and/or external regulator that anyone has. Thanks for all of the responses so far!

[Grady Clay]

Wayne’s diagnostic guidelines for troubleshooting the alternator (from 101, Project 20, pp. 61-63) are:

Alternator indicator lamp is on with the ignition key off.

• Alternator has failed and diodes have shorted out.Replace alternator , and in the meantime disconnect the battery to preventing it from becoming completely discharged.

Alternator light does not come on when the key is turned on.

• Alternator bulb has burned out.
• Regulator has failed.
• DF wire is disconnected or broken from alternator.
• Alternator has failed, and internal windings are open.
  

Indicator light remains on after engine is started and above 2000 rpm.

• Regulator has failed.
• Battery lead to alternator has been disconnected.
• Alternator is not firmly grounded.
• Alternator internal bushings are dirty or worn.
• Alternator is bad.

Indicator lamp is dim after starting and gets brighter as engine rpm increases.

• Battery lead to alternator lose or bad.
• Ground connections are bad.
• Alternator is bad due to open diode failure.

Indicator lamp is dim after start-up and gets dimmer and goes out when rpm increases.

• Low charged battery.
• Poor connections to battery.

Indicator lamp is dim after start-up, and gets dimmer as engine rpm increases, but never goes out.

• Regulator has failed.
• Alternator is failing.

This diagnostic tree works OK if there is one problem. It is all to common for there to be multiple problems, particularly with intermittent connections or one problem causes another.

The first place to start with the diagnosis is always to clean and check all the connections.

With the often neglected transmission ground strap, there can be a problem when starting with a poor ground connection. In this situation the entire engine and transmission becomes connected to the positive battery terminal (however briefly). This can cause serious problems in other circuits, not infrequently the charging system.

There are many grounding points on 911s. They vary by year/model. It would be useful to post a catalog of all those pints. Each needs occasional service.

A unique feature of Porsche’s charging system is the fact that the warning lamp is an integral part of the system. It isn’t simply a warning light. The wattage of the bulb is important.

On to Andy’s issue. It may turn out that both the alternator and regulator have failed, possibly one leading to the other. Identifying that doesn’t lessen the necessity for insuring all the connections and grounds are clean and proper.

What Andy needs right now is test procedures for the external voltage regulator when disconnected from the 911. Warren?

Best,
Grady

PS, Andy, I emailed you the three pages from 101.
G.

[KobaltBlau]

Wow Grady, thank you for going to this effort. this is my situation:

Indicator lamp is dim after start-up and gets dimmer and goes out when rpm increases.

the light is bright when the engine is off and the ignition is on, and when running the light is very dim near idle, and dark everywhere else.

Here are my latest measurements with my trusted multimeter at the battery:

1. engine off: 12.9V
2. engine running, no accessories 14.8V
3. engine running, heater fan, headlights, radio, rear defroster 12.2
4. same as 3. but turned off defroster 13.1
5. defroster still off but turned on air conditioning 12.2
6. accessories back off, engine running 14.8V again
engine approx 1500 rpm during this test

I'm not certain of the problem now, I hooked up a charger after driving approx 3.5 hours including 1 with headlights, and the charger went to "charged" almost immediately. Most of the readings look pretty good. However, it seems like the reading with A/C or defroster on at 12.2 is too low, though I would very rarely have that many accessories on.

If I had a way to test the alternator separately (maybe I should take it out and look at the brushes as in wayne's pages), and/or the regulator (as Grady said, Warren? ) I would really be able to narrow it down directly.

I can certainly clean the battery terminals and find the ground strap and clean and re-attach those connections. what else does "clean and check" entail, and which particular connections could cause these problems, other than the transmission ground strap and battery terminals?

Thanks in advance!

edit: I should also check that my fan belt isn't slipping, though it isn't making noise.

[Early_S_Man]

Alternator light does not come on when the key is turned on.

•Alternator bulb has burned out.

•Regulator has failed.

•DF wire is disconnected or broken from alternator.

•Alternator has failed, and internal windings are open.

Just for future reference ... there is an important possibility MISSING from Wayne's list, namely:

•Brushes worn down too far, causing intermittent contact with slip rings!!!

[KobaltBlau]

Hi Warren, that is represented in the text of 101 projects, Grady emailed me some scans (my copy is in storage), but indeed it is not in this list.

Warren, do you know a method to test the external regulator separately? Could the brushes be worn down to far after only 30k original miles?

[Grady Clay]

Andy,

Your diagnosis sounds like the alternator output is satisfactory (14.8 V is slightly high) when at light load. It looks kike the output is current limited (it drops to 12.2 V) as the load increases. The question is why.

I can think of a few possibilities: One or two of the 3-Ø windings could have no output. This could be a brush issue or diodes. An oscilloscope can tell.

Poor electrical connections could be at fault. This could be grounds or in the B+ circuit.

The heater blower at the engine may provide some info. When the output is low (12.2 V), measure the voltage at the disconnected heater ground wire relative to the chassis. It should be zero. Any voltage indicates a bad ground. Aside from the transmission ground strap, IIRC a possible place is the ground terminal to the chassis behind the fuel filter & accumulator. The other circuits that share this common ground are; D- from the voltage regulator, D- from the alternator heated rear window and CIS air meter contact. There is a small braided ground strap from the alternator to the crankcase but you can’t get to that without removing the alternator.

In the B+ circuit two possible places are at the starter battery terminal. That is where both the 25 mm big battery wire connects and the red 2.5 mm wire from the alternator B+ terminal.

You can measure the voltage at the alternator B+ terminal at the rear fuse box fuses for the rear window defroster and heater blower. If there is a difference between the voltage measured at the fuses and at the battery terminal, then there is a poor B+ connection somewhere.

I hope this helps. Some of this is from my lame memory so if someone has corrections, please offer them.

Is the Current Flow diagram, Type 911SC USA, Model 78 posted on Pelican?

Best,
Grady

[KobaltBlau]

Thank you again Grady, this gives me a bunch of things to try. I have the bentley manual which may have a satisfactory current flow diagram. Of course I will post what I find out!

Best Regards,