MR16DDT Alternator Function

The Nissan Juke (f15) and Nissan Sentra Turbo (B17) use an alternator system that is becoming more and more common in today's market.

Instead of an internal regulator doing all of the voltage regulation for the vehicle, the ECM and IPDM are the primary method of duty cycle demand.

I will not be explaining the mechanical function of an alternator here, for information on how they work head over to youtube and watch a few videos.

This article is to help shed some light onto why so many people seem to have alternator and battery issues with these vehicles, and if someone is recommended multiple alternator replacements, this may explain why that is.

Information on upgrading the alternator is after the diagram photos. If this does not interest you, you do not need to read past there, just to save you, the reader, some time.

What is duty cycle; on the most basic level, it is the speed at which a circuit is turned on and off within a minimum and maximum range of function.
So, the higher the duty cycle in this case, the more electricity is generated by the alternator.

Most vehicles operate with a ~14v charging voltage. This means that when the engine is running the alternator is putting out ~14v. Now. Cars are actually 12v systems. So why 14v? Well, it allows the alternator to re-charge the battery after it is used to start the vehicle, and also gives some head room for additional electric loads that may be added as the vehicle is running. Turning the AC on or rolling a window down are a few examples of this. If the car operated at strictly 12v, the sudden draw of current would mess with the operation of other electronics.

So, for years, the alternator of a vehicle would just constantly crank out ~14v, with a small regulator in the alternator controlling this. This was all fine and good...until emissions regulations started clamping down. HARD. Manufacturers were looking for any way to save any amount of engine load they could to increase economy and reduce emissions. When the alternator is generating that 14 volts, it creates a parasitic draw on the engine. To use some arbitrary numbers as an example; with the alternator off, the engine has no extra load on it, with the alternator on and active at 50% duty cycle (typically used to maintain a 14v charge in most alternators), there would be a 4% parasitic draw on the engine. So the engine now has to use 4% more energy to maintain its current rpm or load. Again, 4% is just an example number, the actual draw depends on a lot of factors. This is the same reason your air conditioner compressor turns on and off.

So what was the solution for this. Well. Don't make the alternator run so hard or so often to reduce its draw. How did Nissan accomplish this? By having the ECU calculate an estimate demand on the electronics of the vehicle, you could run the alternator at its absolute minimum more often. Thus, creating less draw and more efficiency. Typically, an alternator gets a voltage reading from the charging system and an internal regulator calculates how much more power the system needs. Nissan has the ECU calculate a voltage to SHOW the alternator to make it think its making a certain amount of power that then tells the alternator if it need more or less power. We spent a long-time logging voltage output from the ecu into the alternator using a scope, and the stuff the ecu does to change alternator demand is wild.

Its a great idea. If the alternator was sized correctly. As most Juke owners will know, if the car is sitting at an idle and you try to roll down a window, the car basically stalls out. This is the alternator being shocked into adding more power for the drop in voltage the ECU has detected. The ECU has no way to predict if you are going to do something like roll a window down, so it cannot pre-emptively increase alternator output, so you get a drain on the system. Now, for things like fans or radio functions, the ECU gets a moment's notice to increase alternator output. This is why when you turn the fans on full blast, or turn the radio on, its not all at once. The ECU take a split second to increase alternator duty. As mentioned at the start of this paragraph, this would all be well and good if the alternator was sized right. What Nissan basically did was put the smallest damn alternator in the car as they could. The OEM alternator is only rated at a maximum output of 110amps. Which sounds like a lot. Until you start adding up all the systems on the car and the draw they all have. THEN add on the extra load the engine creates when the ignition system is running at 7000 rpm.

If you search this and other forums you will find stories of people CONSTANTLY needing new batteries or alternators. This is why. The system is operating on the bleeding edge basically at all times. Sudden increases in output demand are very bad for the alternator, mechanically speaking, and the battery ends up picking up some of that amperage sag when the alternator cannot keep up. The first Juke I ever tinkered with had 4, YES 4, alternators replaced in a year. And two batteries. This was in 2012, and the car has 8k miles on it. So Nissan replaced it all under warranty. The car was stock at this time.

Below is a wiring diagram of the alternator, showing how the IPDM and ECM communicate a duty cycle to the alternator via can-bus.

Rectangle Font Parallel Slope Number

Here is the diagnostic aide for the charging system on the car. As you can see, Nissan says it is completely normal for the charging system to be running at only 13v. Not leaving a whole lot of room for error.

Rectangle Font Slope Parallel Pattern

So. How can this be rectified. Well. You can adapt on a fully internally regulated alternator. It has been said in the past that you cannot do this because of how the ecu controls the voltage. This is not true at all. As the alternator is not an OBD monitored system, aside from providing power, it has no connection directly to the ecu. So, as long as the IPDM and indicator light on the dash are seeing voltage in a specific pre-determined range, you can run any alternator you want. Here at FastReligion we did just this. We adapted a chevy LS alternator onto a Juke. Completely bypassing the ecu controls. The cars run at a near perfect 14.2 volts constantly, and even in our highest horsepower vehicles that are revving out to over 7000 rpm, the voltage for the entire car sits near level the entire time. A large reason for this is the fact that the LS alternators have a 140-165 amp range, depending on the model you get. This leave PLENTY of headroom for the alternator to operate in. Reducing the odds of failure and reducing over-all component wear. There are also LOADS (no pun intended) of upgraded LS alternators on the market for those of you who may need EVEN MORE POWA!!! 
We have done this swap to over 10 vehicles here at the shop, and have shipped out quite a few conversion kits, and all of these vehicles have a must smoother operating electrical system. Windows no long nearly make the car stall out, batteries are dying on a yearly basis (we have several local customers who had this issue, yay new england cold), and over-all performance is more regular. No voltage drop.