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      11-23-2019, 03:03 PM   #62
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Drives: 2013 E92 M3 ZCP factory order
Join Date: Sep 2015
Location: NC

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Originally Posted by WingZeroX5 View Post
I'm going to clarify a bit. When I plugged in ISTA, it was to read out codes and calculate a test plan at the end. I don't remember ever having to go into ECU selection and select the DME so that the program can determine whether the alternator was functioning properly or not. What I do remember doing is taking a multimeter and reading out the voltages and the car held certain RPM's. Measures at the end always end with replace alternator if such and such conditions failed.

You stress an alternator by turning all the electronics, like headlights and media. Then you see if the voltages maintain within range. ISTA will tell you, depending on what diagnostics is being run, to either turn on or off electronics, and take readings.

This isn't a Cyberdyne Systems Model 101 T-800 series machine where it can route additional power to certain places. If you do that, it'll toast the electronics. So no, ISTA does not stress the alternator. Besides, there's voltage regulator behind it for a reason and believe it or not, this does's not uncommon.

I don't even know how to respond to the battery comment.
Knowing the tools is less important than understanding the fundamentals. You said your alternator was "pumping out 15.7-16V intermittently" and you decided to replace the alternator. In that situation the regulator had probably gone bad, not the alternator. The regulator sets how much AC peak voltage the alternator produces by adjusting field currents in the rotor windings. Usually when the alternator fails it produces lower voltages (< 14.3V DC). If you saw over-voltages, the regulator alone should have been replaced first at cost of ~$100.

Second, the total load on the alternator is the voltage difference between the rectified AC voltage and the battery terminals voltage divided by the B+ cable resistance, the battery internal resistance and the alternator windings resistance (load from charging the battery) plus the load from all the electrical equipment.

One way to stress the alternator is to create excessive currents in the windings by disconnecting the battery while the engine is running. All the electrical equipment in the car have supply bypassing caps but these together cannot sufficiently stabilize the voltage ripples left by the alternator rectifier bridge without the battery connected. As a result, the alternator windings will see higher fluctuating currents and overheat which could lead to premature failure. The closed loop control of the regulator is not meant to operate without battery.

I don't quite see why they would do this in the process of splicing the B+ cable unless blindly following steps from a diagnosing and repair tool like ISTA trying to evaluate ohmic resistance of the corroded cable for the recall. Since there are multiple complains from failed alternator after doing the recall, their work is questionable.

Last edited by Rajmun340; 11-23-2019 at 03:10 PM..
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