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Nissan came in with a no charge condition. Was from out of state and had the alternator replaced about a year ago out west. Napa reman alternator.
I did the usual stuff, tested battery, charge voltage, voltage drop on hot and ground sides. Warning light circuit, always hot feed to reg. Everything checked out ok so I had them order an alternator. This is a 3.5 engine so the swap took quite some time. (I pulled the front end off, I think it made it a little quicker) Got it all back together and guess what? Still no charge! This time though, the battery light wouldn't come on with key on engine off. I could use my test light to turn the battery light on but the new reman alternator wouldn't turn it on. Tested and retested everything according the service data on Pro Demand. The IPDM on this car can control the regulator according to service info. I scoped the control circuit and it had basically 0 volts Service data said it should be 5v and a pulse width modulated signal. I got nuthin.....so I went down that rabbit trail for another hour or so. Tested the circuit for a grounded out wire. Had an identical car in the next bay with the same IPDM so I swapped them. Still no charge. I finally took a weak 9 volt battery and powered the wire that the IPDM controls and the alternator charged fairly normally. As I read service data, it seems that the internal voltage regulator should work with or without computer input. I took the old alternator and used jumper wires to supply power and ground the hooked my test light to hot and touched the terminal for the warning light circuit and the light lit up indicating the circuit was good. Did the same on the car in the next bay and got the same results. Ended up ordering another alternator but have to wait till tomorrow for it to come in. Do you guys see anything in this long-winded mess that I overlooked? I'm pretty sure of my diagnostic skills but this thing has left me looking and feeling like a rookie!
Oh by the way, my Snap On Modis doesn't have PIDS for the alternator control or bi-directional controls for it either. The shops Bosch and Launch scanners both have those features but it seems like their functionality is a little wonky.
I figured it out finally. The last shop that worked on it supposedly replaced the connector with a used one. I pulled a wire in two while disconnecting it and replaced it with a new aftermarket part. The stupid thing still wouldn't turn on the battery light KOEO. While I was planning on loading up my tools and driving away in total shame it occurred to me that all this time I had been checking circuits by wire color instead of terminal position. This whole time, the connector has been wired wrong. I found the terminal position and wire colors in service data and swapped the light circuit and the computer controlled circuit and everything worked as designed. Wow! Did I ever learn a valuable lesson about the importance of carefully studying the service data and paying attention to details! Especially when other hands had already been in there "fixing" things!
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