How alternator light works, a more detailed description |
|
Porsche, and the Porsche crest are registered trademarks of Dr. Ing. h.c. F. Porsche AG.
This site is not affiliated with Porsche in any way. Its only purpose is to provide an online forum for car enthusiasts. All other trademarks are property of their respective owners. |
|
How alternator light works, a more detailed description |
Tom |
May 3 2014, 01:30 PM
Post
#1
|
Advanced Member Group: Members Posts: 2,139 Joined: 21-August 05 From: Port Orchard, WA 98367 Member No.: 4,626 Region Association: None |
After reading many accounts of how this circuit works, I felt compelled to investigate further as I did not understand how two positives would cause a light to operate. They won't. One must be somewhat negative to complete the circuit. Internet searches turned up the same basic explanation, still was not buying it. I think it was being oversimplified.
This is how I think the alt light works: When the key is on and engine not running, there is 12 volts + at the alt light power side coming from the fused side of fuse #9. The other side goes to a junction on the relay board with D+. With the key to off and a meter connected between D+ and ground at the relay board, the reading is 12 ohms. As soon as the key is turned to on, the reading jumps to 12.5 meg ohms and the light comes on. If the wire for D+ to the alt is removed, the reading stays the same and the light stays on. Removing the VR caused the reading to jump to infinity and the light goes out. For the light to work, there has to be power to one side of the light and some resistance reading to ground for the other. Looking at the wiring diagram, one can follow the blue wire to the junction at the relay board at D+, then up thru the VR to a set of relay contacts, then down thru a ( resistor ?, not sure) and then down to the DF connection and on to the rotor where the current will produce a magnetic field. After the rotor, it goes to ground. When the alt spins enough RPM's, a voltage is produced and fed back to the VR, causing the relay to open and removes the ground path for the alt light. I could be entirely wrong here, but this is what I see and my readings more or less confirm it. If you see an error in my thinking, please post and let me know. Thanks, Tom Attached thumbnail(s) |
type47 |
Aug 26 2014, 10:33 AM
Post
#2
|
Viermeister Group: Members Posts: 4,254 Joined: 7-August 03 From: Vienna, VA Member No.: 994 Region Association: MidAtlantic Region |
The alt I removed this morning and tested good has been reinstalled. Just have to connect the starter connection and reinstall the heating ducts and air guide. Funny, I have a VR sitting out on the tool table so I might as well try that too. Break for lunch ... BLT's!
Rats! All put back together and Alt light still on; changed VR, still on ... no power to oil temp gauge and step on brakes, alt light gets brighter, tach indicates less rpm than it sounds like. Going to check brake light switch wiring circuit ... So, after looking at the schematic, I decided to check out fuse #9 with it's circuit to the tach, brake light switch, fuel gauge, etc. Fuse was blown. Replace fuse, all is good (except to figure out why the fuse blew, but I'm beat and no more today). Moral of the story: If your alt warning light goes on, check #9 fuse before you R & R the alternator. (IMG:style_emoticons/default/ar15.gif) |
Lo-Fi Version | Time is now: 18th September 2024 - 07:15 PM |
All rights reserved 914World.com © since 2002 |
914World.com is the fastest growing online 914 community! We have it all, classifieds, events, forums, vendors, parts, autocross, racing, technical articles, events calendar, newsletter, restoration, gallery, archives, history and more for your Porsche 914 ... |