# check engine light + battery light



## borsa_boy (Apr 7, 2005)

Nissan Altima 2001 GXE: I was driving away from a parking lot. Raining, humid outside. After a few minutes I get check engine light on; after another two minutes I get battery light on.

I measure the battery: 14.4V while the engine is running, 13.6V with the engine off. I assume that is enough charging.

What could be wrong?

Thank you!


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## ssaemployee777 (Jan 8, 2006)

How old is the battery? Is you breaklight on the dashboard also coming on?


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## gfriedman (Oct 12, 2005)

Check the alternator


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## borsa_boy (Apr 7, 2005)

gfriedman said:


> Check the alternator


Isn't the 14.4V voltage an indication of the alternator working properly?

Does anyone know what is the threshold voltage at which the battery light goes on?

I don't get the brake light on.

I don't knowthe age of the battery. Car is resale. Looks original though (NISSAN sticker).


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## Darktide (Jul 29, 2004)

if your reading on the battery when off is actually 13.6v then you have a problem. Though I'll just assume that you ment 12.6v and then yes that would indicate a healthy battery. Was there any abnormalities in the engine? surging or what not? It is quite possible that your belt is slipping or was slipping due to the increased humidity. Try some belt conditioner or replacing the belt (always a good thing to do anyway). The battery could very well be original. You can pick one up at Advance Auto Parts for $50 for their silver class (mid grade). I just did that myself actually. Its another worthy investment. 

If your actual reading is 13.6V then either your altenator is seriously over charging or your battery is having issues and needs to be replaced. but the battery in this case would also not be able to hold a significant charge and you'd be left dead when you try to start up so I doubt this is the case.


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## borsa_boy (Apr 7, 2005)

Thank you!

The voltage was over 13V dropping quickly to 12.5-12.6 with some load on it (dome light). If overcharging was the problem I would see a 15-16V on the voltmeter with the engine running, wouldn't I?

Why is the check engine light on? Is the ECM also monitoring the alternator/battery?

The belt is less than 1 year old...


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## Darktide (Jul 29, 2004)

if the battery hold onto the 12.6V for a while its good. but the battery should be at 12.6V not 13.6v. If the battery drops below 11V your battery is probably having issues. 

The check engine light will show up if it feels something is wrong with the emissions system. I can only think of a loose wire. Everything seems to be working well? no surging? 

I would also check into a tune-up I suppose. I really don't know. I would actually PM KA24Tech or Asleepaltima as they are the two most knowledgable people on the boards. 

Good luck


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## KA24Tech (Feb 2, 2004)

Sent a reply to your PM. Battery has a bad cell and needs to be changed.

Troy


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## borsa_boy (Apr 7, 2005)

Just an update: changed battery - lights did not go off. Checked alternator - good.

After exactly two weeks both SES and Battery light went off. The same day, PM, both lights came on again - at the same spot (left hand turn) where they first came on.

Two weeks later both lights went off again (today). They are still off. Weird!


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## captain_shrapnel (Sep 10, 2004)

Have your charging system checked by Advanced auto or Autozone, they do for free. Just because your alternator is putting out 14.4 volts doesn't means its right. It could be putting down just 1 amp! It needs to be tested under load with the proper equipment.


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## borsa_boy (Apr 7, 2005)

captain_shrapnel said:


> Just because your alternator is putting out 14.4 volts doesn't means its right. It could be putting down just 1 amp!


I think clarifying some issues on how the current flows between battery and alternator will help many of the less electrically inclined readers (I am an Electrical Engineer). I appreciate everyone posting on this thread and no pun is intended.

If the alternator cranks out a voltage that is higher than the battery voltage (this can be measured when the engine is off) the alternator is not dead. As long as the battery is functional (no dead cells which could break the circuit continuity) it is a load for the alternator. If the alternator is not able to push out enough charging current the voltage would drop accordingly (Ohm's law I=U/R). However an alternator cranking enough juice might still have a defective regulator. That will show on autoshop testers as a high ripple reading. If you can borrow an oscilloscope you can measure that ripple at home (you will need the knwo to correct value to compare to).

Good luck!

P.S. In Toronto area they don't do charging system test for free.


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## captain_shrapnel (Sep 10, 2004)

Thank you for clarifying that. I knew that an alternator could be putting out what looked like nominal voltage, but could still be dying. I have been fooled in the past by my multimeter, so I wanted to bring that up. Obviously, you know what your doing, so disregard my post.

BTW, I always heard that the battery was the regulator for the alternator, and that it cleared up the ripple just by being in the circuit. Is this not correct? I ask because my father told me this and he is an EE as well (fixes lasers and stuff for high end lab equipment used in hospitals).


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## borsa_boy (Apr 7, 2005)

captain_shrapnel said:


> I always heard that the battery was the regulator for the alternator, and that it cleared up the ripple just by being in the circuit.


That is correct. However that might not be good for the battery. A large ripple (AC component) translates into high charging current spikes.

I tried to keep things simple in the previous post and did not go into the details of the stator 3-phase winding and rectifier diodes. This design will give minimum ripple with minimum number of parts (which for the currents we are dealing with tend to be expensive and large). The regulator (separate entity - yet integrated in the alternator body) actually controls the output of the alternator by injecting more or less current into the excitation winding - this is the feedback.

So busted diodes will give you large ac component, while busted regulator translates into little or no feedback (overcharing - undercharging).

I am not an car passionate, these statements are based on my knowledge of electrical systems not experience in automotive repairs.


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## SuperSentra4203 (Dec 17, 2002)

Odds are you may have part of the diode circuit in the altenator fail (YES, part of it can fail), causing both DC and AC voltage to be introduced into the electrical system. However, an occiloscope is not required, remove the altenator, take it to Autozone or Advance (who i used to work for) and have them bench test it. If excessive ripple current is outputted, then it will fail. Note, make sure you find the most compotent person to do the test. I unfortunately had a few employess who would still pass a bad altenator becuase they didn't pay attention to the diode indicator.


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