911 not running so good....
 

I have a 74 911s with a 75 2.7 engine. It has just been running a little strange lately. Bogging a little at low rpm. Not as much power. I took it into a shop today to have them check it out and they told me that I need new alternator and that it is going to cost me 400 bucks for a rebuilt one. Does the alternator have anything to do witht the way the engine runs? Doesnt it just charge the battery. How do you know if your alternator is bad or not?

PLease help,

I do not want to pay for this if I do not have to.

THanks,

JP

I am trying to think of a way that a alternator can affect the performance of a motor in the way you described.

I'm no CDI guru, and i'm sure someone will chime in about the alternator issue.

I'd suggest you perform some very basic checks first, before forking over any amount of cash.

If you do end up needing a new alternator. Screw, the shop... put it in yourself. Pelican sells alternators for much less and installation is not that hard with basic hand tools. There is even a tech article on this site on the same subject I believe.

A fully charged battery should be putting out 12V or a little higher.
With the car running...a properly operating alternator should be putting out between a little over 12 to 14V.
You can check this with a cheap $30 voltmeter.

Is the charging light coming on on your dash? (small red light)?

In regards to your engines performance... have you changed the fuel filter/accumulator lately? It might be time for a new filter.

How about the distributor cap and rotor?
Might want to also pull the spark plugs and check their condition.
This should give you some idea of how your fuel and ignition system are operating.

There are a ton of people here who will guide you through anything you are unsure of...

best of luck,
Lee

I'm with Lee; pull your alternator and have it rebuilt. It will cost all of $150.00.

That is, IF that is the problem. You can use a voltage meter to check what the alt. is putting out at the battery terminal. With fully-charged battery, check for ~12volts. Start engine, check again; should be roughly the same. Rev engine and note if voltage increases: should be around 14.5 or so, but not higher (not much anyway).

You want to know that the alt. is putting out enuf juice, but not so much that your battery is boiling from overcharge.

Your car has an external voltage regulator, behind the small black panel on the driver's side of the engine bay. If it is simply the regulator, you can swap that out pretty easily.

Finally, welcome to the "He has a Porsche and so his wallet must be huge" syndrome. Until you meet a tech you trust, you are at the mercy of these shops.

John

If the alternator was indeed causing the igntion system to act poorly AND you went ahead and fixed it yourself, why not compensate your tech for giving you valuable diagnosis information? I'm sure he'd feel better providing you free service if it appeared you were grateful for receiving it. If he was incorrect, then prehaps find a repair shop you can reciprocate with.

Have you ever gone into a retail store to demo a camera or some piece of audio equipment, received the undivided attention and time of the salesperson, then walk out and order it through a mail order discounter? Put yourself in their place. I think there is an ethical issue here. It would be with me.

Just my $.02
Sherwood Lee
http://members.rennlist.org/911pcars

Excellent point Sherwood!

I typically bring a 6-pack of beer with ONE non-alcoholic root beer of a high-quality nature for obvious legal reasons! I always try to curry favor with the mechanic because they don't like to be used. For John Walker, as an example, I enjoyed putting together Grateful Dead bootlegs in nice CD cases with cool covers. He now has the entire '77 week at Barton Hall, etc. in NY. Good times, good times...

I used to repair bicycle frames via the backdoor for those racers who would show up on Monday with broken forks and down tubes and dropouts. They paid in beer and pizza for labor, parts at cost. These guys are living out of VW buses, but the bikes rival our Porsches...

John

To answer your question the alternator can affect the performance.

I had the alt on my '77 wear the brushes down to nubs. Sometimes they made contact, sometimes they didn't.

When they didn't make contact, the car ran off the battery. And when it get's low in charge, the ignition doesn't perform well.

Then the brushes might make contact and charge it back up. This made for some strange performance and diagnosis.

That doesn't mean you have a bad alt, just that it can cause some unpredictable behavior if it is not working.

I had mine rebuilt and it cost about $150.

When was the last time you replaced the points?
They might need to be cleaned with some 600 grit sand paper or replaced.

The alternator does affect performance because if the electrical system is not seeing at least 12 Volts, then it will act a little funny...you won't get a fat spark like the engine wants, etc.

Check the voltage at the battery, check to see if the battery needs to be filled with De-ionized or distilled water, check the voltage with the car running at idle, check the voltage with the car not at idle, check the voltage with the lights on plus anything else you can think of to draw power like the radio.

The alternator should put out 13 volts MINIMUM, otherwise it won't be able to charge the battery. Not sure what the specs are for your alternator, but I would say it is supposed to put out around 14 to 14.5 volts normally.

Paul

That reward thing has its merits. My 944 guru (out of the repair business now..retired) had all the tools you could wish for but one day he was at my house doing some work and I handed him a flexible stubby to use from my toolbox....he loved it. So as a gratuity I got him one. I suck at diagnostics when it comes to engine problems but he was forever willing to diagnose (free) and let me fix it. Every Christmas he got a bottle of fine German wine which he loved. Got a head gasket replacement done at his shop with my labor and his expertise for the cost of the parts and a case of that fine wine (definitely not screw top) We still reamain good friends.