Anyone up for a little diagnostics?

[mark 74 carrera]

I have a 74 911 carrera with a 2.7 and webers. I drove my car to work (round trip 42miles) which is long enough each way to get to operating temprature. On the way home it was raining and I ended up leaving the lights on all night.

I got up the next morning found the battery to be completely dead. I thought that it was odd that it would not even light up the CD player or the any of the switches. I checked the water level in the battery and it was almost completely dry.

I filled up the battery with water and completed a 2.5 hour charge. I got in the car and it fired right up and drove it back to work again. It ran great. On the way home I decided to take the long way home (bad choice). I was sitting at a light and the engine just turned off (at idle).

Pushed it to the convenience store at the intersection. I attempted to start it and turned over again and again. Just when I thought it was completely dead it fired up. I pulled to the intersection again only to have it cut out again (at idle).

I backed it up again and then started checking everything. I did find one blown fuse (replaced it & nothing changed) I checked the fuel filter, distributor cap, electrical connections and everything looked fine. As I attempted to start it I realized I might be flooding it so I sat for a while and but still nothing.

To this point the battery seemed to be holding up fine. Just when I was going to give up and call someone, it fired up and ran great for about 3 miles. Then in 5th gear at speed, it cut off (at speed). It just died. I called afriend and he came and gave me a jump start. (this took about 40 minutes) It started up and he followed me home. Again, it ran strong.I turned it off in the garage, and its completely dead again.

My gut tells me it's not fuel related. My questions are:

Does the car run off the alternator? Does anyone think the car was running off the battery and the altenator is dead?(the light at the dash is working and showing that the alternator is working) Would a battery take a charge but then not take a charge from the alternator?

I am at a loss. Any ehlp would be appreciated. I know, why did I let the battery run out of water? I'm an idiot and I work too much? No that no excuse. Actually, I have no excuse. See Ya, Mark

[jabb]

Do some basic checks..... Check the voltage across the battery
with the car off should be about 12.2. Check the voltage with at idle should be about 13.8 check at 3000rpm should stay at about 13.8 or just a bit higher. You can also get the battery load tested........ start there than post your findings sounds electrical or a charging problem

[jhh967]

Sometimes you have to go against your gut. Fuel pump, fuel lines, fuel filters and then look for metallic stuff in the carb float bowls.

This type of problem is the hardest to diagnose. Good luck!

[mark 74 carrera]

Oh, I forgot to say, fuel pump humming and clicking (normal) but I will check the other stuff.

[al lkosmal]

Althogh it doesn't fit the dead battery scenario, the coil in my 77S caused intermittent problems just like that until it finally died. Do you have a spare coil to try???

Al

[pjv911]

This is just a guess . You burned the points by leaving the ignition on while working on a starting the car. Also check the CDI box making sure you hear a high pitch whine. And last but not least check the primary neg wire that goes to the distributor.
You should rule out any fuel system problems because of the instant cut out. Carbs always bog and sputter when leaning out due to loss of fuel.
My suggestion would be to take a timing light and attach the inductive end to the secondary coil wire and watch the flashes while its idling. Also wiggle the wires around. BTW did you notice if tachometer dropped when it died in fifth gear ?

Kurt Williams

[Early_S_Man]

Mark,

Hard to say with so little data about the battery and system Voltage when it dies ... but, with a dying/dead battery, and a normal aging alternator that probably doesn't produce enough power at idle ... I don't really see anything abnormal happening in your description!

The combination of being low on water, and running thew battery down completely ... probably killed the battery. And, if the 2.5 hour charge was with a normal charger ... then you never gave the battery a full charge! It takes overnight, starting at 10-15 Amps, and tapering off to 4-6 Amps ... for a FULL 12-14 hours to charge up a dead battery without overheating and causing it to gas and boil off more water! And, the water shouldn't be completely 'full' when attempting to charge a dead battery ... or the gas produced during charging will cause it to overflow!

So, charge the battery overnight, build a set of leads to check the system Voltage while driving (from 12 V. power plug in cig. lighter socket to DMM input jacks) ... and try it again tomorrow.

... or, just get a new battery,m and charge it overnight ... just to be sure! And, try it again, tomorrow!

[Early_S_Man]

Kurt ... you can't burn out the points in a CDI-equipped car by leaving it on a day, night, or for a week! The maximum current through the points is 140 mA +/- 10%!!! That is 0.140 Amps!

And, just for the record ... a CDI-unit needs MORE than 11.0 Volts to operate correctly, so DON'T go off on the 'bad BOSCH' wild goose chase!

[pjv911]

O.K. I stand corrected about the points I knew that from being corrected by you in the past but forgot. I dont fully agree with you as far as the charging system causing the car to stall at speed (but I am still learning about these porsche cars). If the alternator is charging and the battery still has enough for a restart then it shouldnt die. But then if the cdi requires more than 11 volts(new thing to remember) then I suppose if the alternator has a bad diode this could be the cause. Also charging a dead battery with low water level puts an extreme load on the charging system since its always puting out peak amps to recharge the voltage sucking battery.
Also I would figure the only way the dash light could be out is if the alternator has a bad diode. If it were a faulty regulator then the light would light up ...NO ?
Tell me more Warren I would like to understand better.

Kurt Williams

[Early_S_Man]

Kurt,

Lacking data on the Voltages at which various events occured at ... leads to a certain amount of postulating ...

The 'dying at speed' phenomenon is outside the parameters of what I described above ... and may well be the result of a Voltage dropout caused by a shorted cell ... or an overloaded Voltage regulator, fluctuating, and possibly failing. Or, possibly a bad solder joint at one of the main power rectifier diodes ... causing a sudden drop in current supplied by 1/3.

The alternator has a parameter called 'Zero-Ampere RPM' ... and as automotive electrical systems age ... cumulative Voltage drops tend to raise that specific rpm at which no useful current can be produced, and that is why many older cars alternator warning lamps glow partially or fully at idle. An idle speed significantly less than 900 rpm can accentuate the problem!

My assumptions for the 'dying at idle' scenario include any or all of the following:

1. A battery with a 'surface charge' that was able to start the car, but not able to accept a full charge.

2. An intermittent shorted cell in the battery.

3. A hot alternator, which reduces the regulated output Voltage by 0.6 Volt to 1.0 Volt below the level of a cold, freshly started engine.

4. A possibly low/marginal idle speed after the engine is hot.

5. Elevated 'Zero-Ampere RPM' due to aging of the electrical system.

And then, when all else fails ... take a SWAG!

[Superman]

Well it seems to me that if the starter merrily turns the engine over then there is enough voltage for the ignition system. At least, starters are what seems to need 12.5+ volts in order to "go." So it'd be my guess that there is a connection in the ignition system, or a component, that is working intermittently. I agree that voltages should be looked at. You need AT LEAST 12.2 volts or better just to work the starter. Once it is started, you should have 13.5 to 14.5 volts with lights and things off.

Also, when it is refusing to start, hook a timing light to it, or use a spare spark plug to look for spark. I'll bet there's no spark when it refuses to start. It is true that a fuel starvation problem will not cause the engine to suddenly die. Instead, fuel starvation problems on a carbureted car will sputter first. There is no question this is electrical.

[dickster]

mmmmm, i like these threads.

my money's on a new battery (you got dead cells).

let us know.....................

[Zendalar]

Just have to add, if the battery and alternator are ok. Then it "must" be the CDI box.

Try putting on the power and go "listen" the CDI-box. Listen for a while and if the sound is not "constant" give the CDI box a little smack(s) with your fist. If the sound changes you have a loose connection inside the CDI box. You could smack it even if the box sounds healthy, this way you see if vibrations cause something in the box.

My car did exactly the same, it just died. Turned out to be loose solder joint at the big capasitor inside the box.

[dean]

Get a new battery. That one is cooked

[mark 74 carrera]

Man this is good stuff!!! Each post has given me ideas about the fix. I have to say though that Warren's observation about my particular old car are eerily right on the money.

First, I have noticed before that my idle seemed to be low and Im sure it is not in the 900 range. More like 650. Point number three is very possible, while the car was not running too hot, it was up there since I was running it hard. (80-85 mph). Point four - the car did cut out twice at idle. Point five with the Zero Ampere RPM level increasing is my car. Sometimes when I start out in first, I look down to see the red glow is still there, then just when I think there is a problem, the RPMs get up and the light goes out. (Does that mean my alternator is going?)

So, I am going to try the charge over night. Hey that reminds me, I didn't charge the battery without water, but I did just charge it for two hours.

I suspect it is a combination of many things but mostly that I have a 28 year old car that was driven with minimal preventitive maintenance and was fixed only when something broke. (Can't say that I've done much better over the last year) I gues what I'm trying to say is that it may be time to pay the fiddler.

But guys, I really appreciate the help. Introduction to Electricity almost prevented me becomming a civil engineer. I am OK with things I can see. Water runs downhill - Check! Structures fail under loads-Check! Now let me tell you about a capacitor - What? Well a capacitor is something that stores energy and then releases it over time. OK in simple speak - its a pool of water with a waterfall. Check! (Look at the sparkling water- it's pretty. I like shinny things)

See Ya, Mark

[Superman]

Of course I have the greatest respect for Warren's automotive understanding, but I am going to place my betting chip on a different square. It seems to me that the story involved running the starter quite a bit, with the battery holding up, and the car not starting. That suggests the battery is making plenty of voltage and amperage. Also, the report is that the car just dies. Suddenly and without warning. My understanding of the story is that it dies as though the ignition was just shut off. I think it's because the ignition is shutting off.

If not, I'll buy Warren and Mark both a beer. Here.

[Early_S_Man]

Mark,

Have you taken any Voltage readings while the engine is running?

I don't claim to have a crystal ball, or to 'KNOW' what the problem is ... but I think an intermittent shorting cell in the battery is most likely. Also, the alternator may be overcharging the battery, and that may be why the water was low. High Voltages above 16 Volts will ALSO cause the CDI-unit to cut off!

It would be nice to know if the alternator is putting out 13.5 Volts to 14.5 Volts while the engine is running above 2000 rpm.

And, yes, the CDI-unit cutting out is undoubtedly why the engine is dying, BUT, the real question is whether it is a fault inside the CDI-unit ... or fluctuations in the system Voltage levels that are the culprit??? An analog expanded-scale Voltmeter sure would be useful in this situation!

And, NO, an alternator that blinks or shines the warning lamp at you at idle is not necessarily failing in a 29-year old car! This has been discussed by Bruce Anderson in his tech column in Excellence.

[mark 74 carrera]

I just went out and checked the battery. Is it possible that a 12 volt battery would put out 14 volts? I took Warren's advice and gave the battery a full charge and it is reading 14 volts. I only have a needle reading volt meter but it is definetely reading 14 volts.

How do I read the volts from the alternator when the car is running?

[surflvr911sc]

I have to agree with Early_S_Man, and not just 'cause he tried to give me a virus.

The combination of being low on water, and running thew battery down completely ... probably killed the battery.

The Alternator does not power the car. The battery powers the car, and the alternator powers (Charges) the battery. I have money that a new battery will put everything right. Get that old one out before it cooks your alternator or voltage regulator too!

[dickster]

Is it possible that a 12 volt battery would put out 14 volts?

yes, and it doesn't mean the battery is ok!

get it checked out (the shop will load check it)

btw - how old is it?