Hello everyone,

I am making this post for one of our employees who is not very computer savy. Last night he stayed at the shop to change his alternator but things didnt go very well. He is now not able to shift into any gear. The shift indicator light on the dash does not change either. I have seen some posts about this here and there but couldnt find a clear solution. Any help would be appreciated. Thanks in advance.

 

Did he disconnect the battery before beginning the replacement?

Is the service engine soon light on?

If so, please get the codes read and post here as Pxxxx w x being a number

 

I'm wondering how the hell replacing an alternator can effect the transmission not shifting. ONLY GM could screw that up.

 

One of the control modules is probably much more sensitive to voltage and current spikes than the others. If the voltage regulator was compromised, you'd have to use an oscilloscope to see what was really happening to the voltage and thus the current on very short time scales. I suspect the original defective alternator and voltage regulator took out a module with a spike.

Another possibility is that the battery may have been hooked up and the vehicle started without the alternator connected. If it was then connected with the vehicle running, there would likely be a sizable Spike in voltage and current which would go straight to the input voltage connection of every control module in the vehicle.

But I'm not going to speculate anymore because I made a promise to myself to stop doing that. I added the above only because I have seen it happen many more times than once.