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Discussion Starter · #1 ·
My Altima is a 3.5 L and I replaced the alternator and battery. After the installation I took a voltage reading at the battery and was getting 15 volts. I believe that this high of a voltage will cook my battery. Does anyone know where the regulating signal for the alternator originates from?
 

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A properly working charging system puts out about 13.2 to 15.0 volts. A battery should have a static charge of 12.3-12.8 volts when the engine is shut off. If a battery is not good, the charging system may not be able to charge properly. When the battery is fully charged, the output voltage from the alternator generally will be around the mid 13V range.

The later-model cars have adopted what Nissan calls “power generation voltage variable controls.” This system monitors battery current and varies voltage to the regulator, reducing the load on the engine and resulting in better gas mileage. By varying the voltage to the alternator, engine load due to power generation of the alternator is reduced and fuel consumption is decreased. The battery current sensor is installed on the battery cable at the negative terminal. The battery current sensor detects the charging/discharging current of the battery and sends a voltage signal to the ECM according to the current value detected.
The ECM judges whether to request more output via the power generation voltage variable control according to the battery condition. When performing the power generation voltage variable control, the ECM calculates the target power generation voltage according to the battery condition and sends the calculated value as the power generation command value to the IPDM which then converts the received power generation command value into a pulse width modulated (PWM) command signal and sends it to the regulator.

When replacing electrical components such as alternators, starters and distributors, fuel injectors and sensors, always replace with new or reman'd Nissan OEM components; aftermarket components generally don't last long, don't work right and many times are DOA.
 

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The 15V reading isn't necessarily abnormal if the battery was very weak when you started the car, but it shouldn't stay that high. Normal for Nissan charging systems is around 14.0~14.5 on internal regulation. However, your Altie has a "smart charging" setup where the IPDM uses a PWM signal to control the alternator output voltage. You should see some sort of "middle voltage" at the Gray alternator wire when the system is charging. If you see 0V or 12V then there's a problem in the IPDM or the wiring and the alternator is running on its own internal regulation. You can disconnect the Gray wire deliberately to see if the alternator is regulating correctly. The alternator should output a steady 13.9~14.5V when running unsupervised.
 

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Discussion Starter · #5 ·
A properly working charging system puts out about 13.2 to 15.0 volts. A battery should have a static charge of 12.3-12.8 volts when the engine is shut off. If a battery is not good, the charging system may not be able to charge properly. When the battery is fully charged, the output voltage from the alternator generally will be around the mid 13V range.

The later-model cars have adopted what Nissan calls “power generation voltage variable controls.” This system monitors battery current and varies voltage to the regulator, reducing the load on the engine and resulting in better gas mileage. By varying the voltage to the alternator, engine load due to power generation of the alternator is reduced and fuel consumption is decreased. The battery current sensor is installed on the battery cable at the negative terminal. The battery current sensor detects the charging/discharging current of the battery and sends a voltage signal to the ECM according to the current value detected.
The ECM judges whether to request more output via the power generation voltage variable control according to the battery condition. When performing the power generation voltage variable control, the ECM calculates the target power generation voltage according to the battery condition and sends the calculated value as the power generation command value to the IPDM which then converts the received power generation command value into a pulse width modulated (PWM) command signal and sends it to the regulator.

When replacing electrical components such as alternators, starters and distributors, fuel injectors and sensors, always replace with new or reman'd Nissan OEM components; aftermarket components generally don't last long, don't work right and many times are DOA.
 

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Discussion Starter · #6 ·
Thank you for this information. Since the battery is new and the alternator is a rebuilt I'll need to find out if its the ECM, IPDM or a poor wiring connection. I consulted the shop that rebuilds the alternator and they said to place a jumper wire from the positive terminal on the battery and terminal #3 from the alternator plug (yellow/black wire) on the diagram and then check for a change in the charging voltage at the battery. They said this wire was the sensing wire that signals the alternator if more charging is needed. I did this and the voltage fluctuated slightly between 14.5 and 15 volts. It was a minor change but I'm using a digital voltmeter and it did show a slight change. Judging from the information you gentlemen gave me I'm on the wrong track.
Is it possible to hook the car to a computer to detect if it a faulty ECM or IPDM?
 

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Jumping Y/B won't do anything, that's the field supply wire and will be 12V anyway through fuse 29 (see the diagram above). To test it you need to disconnect Gray on pin 4, that takes the IPDM out of the loop and puts the alternator on self-regulation. But if it's bouncing between 14.5~15.0 it sounds like the ECM thinks the battery is a little flat and is ramping up the charge. You might have an issue with the battery Current Sensor and not the alternator. Since the Current Sensor is on the ground cable, do a voltage drop test between the block and the negative post with the car running to verify your ground integrity. You should see no more than +/- 50 millivolts (0.05V). If it's higher then your ground path has resistance, which can fool the ECM into ordering a higher-than-normal charge rate.
 

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Discussion Starter · #8 ·
Jumping Y/B won't do anything, that's the field supply wire and will be 12V anyway through fuse 29 (see the diagram above). To test it you need to disconnect Gray on pin 4, that takes the IPDM out of the loop and puts the alternator on self-regulation. But if it's bouncing between 14.5~15.0 it sounds like the ECM thinks the battery is a little flat and is ramping up the charge. You might have an issue with the battery Current Sensor and not the alternator. Since the Current Sensor is on the ground cable, do a voltage drop test between the block and the negative post with the car running to verify your ground integrity. You should see no more than +/- 50 millivolts (0.05V). If it's higher then your ground path has resistance, which can fool the ECM into ordering a higher-than-normal charge rate.
Thank You
 

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Discussion Starter · #9 ·
The 15V reading isn't necessarily abnormal if the battery was very weak when you started the car, but it shouldn't stay that high. Normal for Nissan charging systems is around 14.0~14.5 on internal regulation. However, your Altie has a "smart charging" setup where the IPDM uses a PWM signal to control the alternator output voltage. You should see some sort of "middle voltage" at the Gray alternator wire when the system is charging. If you see 0V or 12V then there's a problem in the IPDM or the wiring and the alternator is running on its own internal regulation. You can disconnect the Gray wire deliberately to see if the alternator is regulating correctly. The alternator should output a steady 13.9~14.5V when running unsupervised.
Hi VSrar
Finally got back to checking the charging system. I took some readings and was wondering what you think.
15 v at the battery with the gray wire connected
15 v at the battery with the gray wire disconnected
- with the gray wire disconnected I took a voltage reading at the wire coming from the alternator plug, the wire that gets connected to the gray wire. The reading was 14.9 v
When I first checked the voltage at the battery before starting to take these readings I found that the battery was dead so I charged it overnight. The battery is new and the car sat for about 9 days. I think I must have a parasitic draw on this Altima also. I'll need to check that out next but I did notice some of that constant drain on the battery disappeared when I removed the 10 amp fuse located in the IPDM fuse box under the hood. With that fuse in I was drawing 0.9 amps. The alternator is a rebuilt that I just installed.
 

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That sounds like maybe you got a bum alternator which is regulating too high. That's not uncommon, I think they just wave bearing grease at some of these aftermarket "rebuilts" and call them good. But you should still check your ground integrity, the ECM is basing its charge commands on the Current Sensor which is located on the ground cable.

You have to be careful when checking for a draw, it isn't like the old days. The BCM won't go into full hibernation for 30 minutes after shutting the car off and closing the doors. The draw will step down, usually starting out at 1~2 A right after shutdown and then hovering for around 25 minutes once it reaches 100~200 mA. When it reaches full hibernation it shouldn't be higher than 40 mA, the average is probably about 13 mA in a healthy system.
 

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Discussion Starter · #11 ·
That sounds like maybe you got a bum alternator which is regulating too high. That's not uncommon, I think they just wave bearing grease at some of these aftermarket "rebuilts" and call them good. But you should still check your ground integrity, the ECM is basing its charge commands on the Current Sensor which is located on the ground cable.

You have to be careful when checking for a draw, it isn't like the old days. The BCM won't go into full hibernation for 30 minutes after shutting the car off and closing the doors. The draw will step down, usually starting out at 1~2 A right after shutdown and then hovering for around 25 minutes once it reaches 100~200 mA. When it reaches full hibernation it shouldn't be higher than 40 mA, the average is probably about 13 mA in a healthy system.
Thank You
 
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