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) |
Dave_Darling |
May 3 2014, 04:24 PM
Post
#2
|
914 Idiot Group: Members Posts: 15,047 Joined: 9-January 03 From: Silicon Valley / Kailua-Kona Member No.: 121 Region Association: Northern California |
A couple of minor misconceptions in the OP:
The light does not need voltage on one side and ground on the other to light. It needs more voltage on one side than on the other--and it doesn't really care which side is higher and which is lower. The thing at the alternator is not a resistor, it is a diode. It keeps the current from flowing "backwards" through it except under some specific circumstances. Toolguy and Spoke both have the functions correct. When you turn the key on with the engine not running, the red/white wire gets +12V. Since the alternator isn't spinning, the wire coming from it has about 0V. Current flows from +12V to 0V through the bulb, and the bulb lights up. When the engine is running, the alternator produces voltage. So the blue wire from the alt will be at about +12V, as will the red/white wire. No current will flow, so no light. Note that you can get resistance on some connections that drops some of the voltage on one side or the other of those, which will make the light glow faintly; and there are other failure modes that will make the light shine in different ways. And yes, the light provides resistance to help "bootstrap" the alternator so that it starts charging. --DD |
type47 |
Aug 26 2014, 06:41 AM
Post
#3
|
Viermeister Group: Members Posts: 4,254 Joined: 7-August 03 From: Vienna, VA Member No.: 994 Region Association: MidAtlantic Region |
... and there are other failure modes that will make the light shine in different ways. Could you expand on this? My alt light just came on (and I changed the alt for a tested good one about 6 weeks ago); fan belt still tight; no obvious observed problems so I removed the alt and took it to the free FLAPS for testing and it tested good. Great (IMG:style_emoticons/default/dry.gif) good alt but the alt light is lit. Need things to check. Now that the alt is out, I'll check the alt harness and the 14 pin harness on the relay board. Want to drive to a local event this Sat so I'd like this resolved. TIA. |
Dave_Darling |
Aug 26 2014, 09:23 AM
Post
#4
|
914 Idiot Group: Members Posts: 15,047 Joined: 9-January 03 From: Silicon Valley / Kailua-Kona Member No.: 121 Region Association: Northern California |
Could you expand on this? If some of your alternator's diodes are failing the light will flicker. If there is resistance in the circuit, the light can come on dimly. There is some other failure mode that makes the light dim at idle, and get brighter as the RPMs increase. (Failed VR? I forget!) And there may be others as well. I used to be able to find a chart with different light behaviors and possible causes, but I can't find it now... --DD |
Lo-Fi Version | Time is now: 18th September 2024 - 06:15 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 ... |