Wonky speedo

[worzella]

On 7/10/2018 at 8:50 AM, jimk said:

No, that would be a dumb design.  If the bulb were to burn out the charging system would come down.  Bulbs light up when current flows thru them.  Current flows thru the bulb when one side of the bulb has a greater voltage then the other side.  The bulb lights when the battery voltage is greater than the alternator output voltage and serves no control function.

 

Hi Jim - I am finally getting to my speedo and found the article explaining how a Bosch charging system works. I stand by my statement about the excitation current being dependent upon the existence of the "L" charge bulb.. either present, not burned out and the correct wattage.

 

If you feel like it, read this really good article:

 

https://www.pelicanparts.com/PORSCHE/techarticles/914_alternator_troubleshoot/914_alternator_troubleshoot.htm

 

The key statement is is:

 

 Current thus flows through the lamp, lighting it. This same current, 
     however, also flows through the alternator field winding, producing a magnetic field. 
     This magnetic field is what the alternator needs to start up, and if everything is 
     working correctly, that's exactly what happens.

But agreed... it still seems like a "dumb" design, but from this article, it would seem true. When I have the speed console out, I will start the car and toss a voltmeter on the B+ terminal and if the article is correct, I expect that the alt will not have been excited and will not be charging the battery.

 

Randy

[worzella]

So I pulled the speedo console and direct battery was 13v and after I started car it was 12.96. I believe that means alt is not charging... I need to read the technique to manually excite the alt. Just curious about this topic ?