i turned my car on the other day, and i noticed that my battery gauge reading like 9 to ten volts. My car was idle was terrible. i think it might be that my alternator is bad, and my battery cant power the fuel pump oil pump etc. That is what i think, but if anyone has a different thought please post it. I also tried to drive my car at night, and when i got to a light, my car was struggling to stay alive. Could this be my alternator. thanks

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Steve
88 Firebird TBI,TH700R4 - Open Element, Cat Delete, K&N filter 14*4 element, Dynomax Catback Exhaust
1/4: 16.1@85.7

 

I would disconnect the battery while the car is running (one lead). If the car dies its the alternator.

 

Disconnecting the battery can damage the ignition module, computer, and alternator (which sounds shot already)

Test the terminal at the back of it. You should be getting 14.4v

It's probably gone though. I just finished replacing mine which overcharged when it did charge. At idle it didn't charge at all. Finally it started making funky noises (kinda like a supercharger) and my voltmeter was bouncing. Figured I would replace it before i stranded me. When coming to a quick stop voltage would drop off and sometimes it would stumble/stall.

I got a duralast one this time and it stays rock steady at 14.4

 

yeah disconnecting the battery w/ the car running will send voltage spikes to all of your delicate electronics...VERY costly and NO fun to track down all the problmes. as stated, check your output...it can be done easier at the battery terminals with the same results, anyway it should be 13-mid 14 volts, any lower and it needs to be replaced

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350, Comp 262, Edelbrock Performer intake 600 cfm carb and exhaust, headers
-JVC, Eclipse, Pioneer, MTX, Phoenix Gold, Planet Audio

 

I haven't had any problems with spikes or any other electrical problems when I did this. O'well. You can take it to Auto Zone to test it out too.

 

Just like if you ever have to jump start somebody, leave your car off. Leave the jumper cables attached for 5 minutes until the person gets started, then disconnect the cables and finally you can start your car. Leaving your car running while jumping could damage electronics also.

 

Coulda, woulda, shoulda! It's a bunch of myths just like lots of things about our cars!! I have ALWAYS disconnected my battery to check my alternator!! If you're not comfortable doin' that just take the thing off the car and run it down to the local parts store and have them test it! Or run the car to the parts store and take the alternator off in their parking lot!! I have also ALWAYS left my car running while jumping another car....no problems from that either!! Myths, myths, and more myths!!

 

To answer the original question, I would say yes, it could be a bad alternator, but I would suspect a bad battery or both. One may have damaged the other.

Check the basics first. Make sure all the electrical connectors are okay (fusible link from the alternator output to the battery still intact?). Is the belt tight to the alternator?

As mentioned above, the easiest method may be to take both the battery and alternator to a parts store where they can both be tested - often free.

If you want to test them yourself, I would test with a fully charged battery and then see if the alternator is working. With the car running at 2000RPM for more than two minutes, most cars will have 13.0 and 15.1 volts, with 13.8 – 14.8 volts being common on a warm day. The reason for the fluctuation is because the alternators’ output is temperature compensated (lower ambient temperature = higher voltage output to compensate for the increased battery resistance) as well as many other factors such as the battery’s condition, its state-of-charge, load, etc.

At 75°F, a six cell automotive battery (engine off and not under load) will put out 12.65+ volts at 100% charge (or 1.265+ average cell specific gravity). 0% state-of-charge would be ~11.89 volts (or 1.120 average cell specific gravity). If you’re only seeing ~9 volts, your battery is not technically “dead”, but is so low that starting the car would be questionable. With the alternator working with an output of only 9-10 volts, I would suspect that it is putting out little-to-nothing, unless the battery is creating such a load to the alternator that it cannot produce more than 10 volts. You’d need to take a current measurement on the alternators’ output to confirm.

To address the two other points posted (jump starting with the engine off and removing the battery with the engine running), I offer the following.

It is not necessary to have the donors’ car engine off to jump start another vehicle, but it may be better. With the engine running, you’ll have a couple more volts and a lot more amperes available to the receiving car, making it easier to start. But if I were the donor car, I would prefer to jump start with my engine off (unless I was jump starting to only charge the battery of the receiving car). The reason being is that with the alternator operating, as soon as the starter is engaged, 100-300 amperes (depending upon the vehicle) will be suddenly applied as a load to the alternator (called “shock load”), even though much of that will be absorbed by the battery of the donor car. While the chances of damaging the alternator are probably small (afterall, it’s done every day), I personally would not want to take the risk. I question whether this could degrade a component in the alternator (e.g. the regulator). In other words it would not destroy it immediately, but only degrade it to a point that it will still work, but over time it will fail sooner than its normal life expectancy.

I do not have any data on the cause of failures to automobile alternators to support my suspicions. But until I do, I’ll error on the side of safety (I do not give jump starts).

Regarding the removal of the battery while the alternator is running and connected. I would not recommend it. It is certainly not a myth. It is not something magical or mysterious, but well documented (at least in my experience). I’ve know many people (especially in Central America where a lack of resources is the norm) that would test the alternators’ output by doing just as you suggested. Sure it works, but every time I see it done I cringe. Indeed maybe most or maybe even all have never experienced any problems afterwards - at least that they could attribute to disconnecting the battery. But that doesn’t mean that it is safe or without risks.

The output of the alternator is pulsed DC. It has three fields rotating at a couple thousand rpm’s at idle, so the output frequency will be high (>6Khz). With the battery connected, it acts as a filter to smooth this pulse, or ripple for a cleaner voltage. That itself is not the problem. The real potential for a problem comes from what is known as a “load dump”. Any electronics engineer designing circuits for the automotive environment worth his salt should be well aware of this. What happens is an energy surge comes from disconnecting the battery while being charged by an alternator. The alternator can only respond at a rate of 40-400msec. Removing the battery while the alternator is generating current will suddenly have no where for it to go, thus forming an energy surge. (Actually, it does have somewhere to go – through any electrical device which is electrically connected!) Think of a title wave moving across the ocean and then it suddenly crashes into the beach with “no where for it to go”. The resultant overvoltage (exponentially decaying positive voltage) is the most formidable transient encounted in the automotive environment. The actual amplitude depends on the alternator speed and the amount of current/voltage the alternator is outputting (field excitation) and can exceed 100 volts.

What makes one transient more dangerous than another transient is not the amount of voltage, but the amount of energy. A 600V 1msec transient has much less energy than a 100V 400msec surge.

With good engineering there are methods of clamping these overvoltages or surges. For lights and motors, this is of little concern. But for electronics it is of great concern. Even the best engineered system will not be 100% effective 100% of the time.

I’m not saying that removing the battery will cause damage. There can be a lot of explanations why it would not. Perhaps all the electronic components were well engineered, each with a good power supply. Maybe good fortune plays into the equation. Maybe (as stated above) no immediate damage was done, but an electronic component was degraded. But over time it may very well fail far short of its normal lifetime. Attributing a failed electronic component (i.e. the ECM) to disconnecting the battery with the engine running which occurred a long time ago I would imagine rarely happens if at all, and would be difficult if even possible for the layman to link the two events together. The protection circuitry, if it exists, may offer protection for the first surge/spike. But after that? 100%? Less? None? How many surges/spikes can it protect against? At what intensity? There are just too many variables.

It’s an unnecessary risk that I would not take.

[This message has been edited by Stuart Moss (edited April 09, 2001).]

 

a good place to start is to buy a voltmeter/ohm meter, not only will it help diagnos this problem but many other electrical problems even starters. first a voltage test of the battery if the voltage is less than approx. 9.6Volts the car will probably not start. then start the car and check battery voltage, if the alternator is good you should get around 14 Volts. testing with a volt meter eliminates the chance of toasting any wires or solid state computer parts.

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you can never have too much horsepower