 Alternator warning light actually functions as a brake pedal application reminder |
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| Tdskip |
 Apr 30 2020, 08:26 AM
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#1
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Advanced Member  Group: Members Posts: 3,686 Joined: 1-December 17 From: soCal Member No.: 21,666 Region Association: None 
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Good morning gentlemen, hope it’s been a good week so far.
My alternator warning light goes crazy anytime I step on the brakes or use a turn signal. I confirmed that I’ve got 13.6 V at the battery with the car running, so it appears that I have a wiring issue somewhere, most likely behind the dash I assume. I was hoping to see if anyone’s had a similar issue and what they did to resolve it to help narrow my focus before I start digging in. Thanks! |
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Replies(1 - 7)
| 914Toy |
Apr 30 2020, 10:01 AM
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#2
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Senior Member Group: Members Posts: 718 Joined: 12-November 17 From: Laguna beach Member No.: 21,596 Region Association: Southern California
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QUOTE(Tdskip @ Apr 30 2020, 07:26 AM)  Good morning gentlemen, hope it’s been a good week so far. My alternator warning light goes crazy anytime I step on the brakes or use a turn signal. I confirmed that I’ve got 13.6 V at the battery with the car running, so it appears that I have a wiring issue somewhere, most likely behind the dash I assume. I was hoping to see if anyone’s had a similar issue and what they did to resolve it to help narrow my focus before I start digging in. Thanks! Tom, I am no expert, but on my '72, the alternator warning light wire is blue and connects into the back of the left hand gauge. Perhaps you can trace it to and from the gauge to see of it is loose or damaged somewhere behind the dash (?). |
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| Tdskip |
Apr 30 2020, 10:05 AM
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#3
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Advanced Member Group: Members Posts: 3,686 Joined: 1-December 17 From: soCal Member No.: 21,666 Region Association: None
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Good morning - let me start there, thinking there are other wires hanging off this....
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| cn2800 |
Apr 30 2020, 10:19 AM
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#4
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Member Group: Members Posts: 119 Joined: 11-August 14 From: Amarillo, Texas Member No.: 17,766 Region Association: None |
QUOTE(Tdskip @ Apr 30 2020, 10:05 AM) Good morning - let me start there, thinking there are other wires hanging off this.... Check your grounds, too. I’ve seen a lot of strange things happen with bad grounding. |
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| Tdskip |
Apr 30 2020, 10:33 AM
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#5
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Advanced Member Group: Members Posts: 3,686 Joined: 1-December 17 From: soCal Member No.: 21,666 Region Association: None
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Excellent point, thank you
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| GregAmy |
Apr 30 2020, 10:42 AM
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#6
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Advanced Member Group: Members Posts: 2,308 Joined: 22-February 13 From: Middletown CT Member No.: 15,565 Region Association: North East States
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The alternator circuit is a very simple one: one side of the bulb connect to the alternators D+ terminal, the other side connects to system voltage at fuse S9. S9 runs most of your gauges, too.
If there's a differential voltage across the bulb - for example, the alternator isn't putting out (0 volts) but the battery still has voltage (12V) - then the bulb lights. If there's no differential voltage - alternator is putting out 13.5V and alternator is putting out 13.5V - then the bulb stays extinguished. Your goal is to find out where that differential is happening when you engage the brake lights and fix it. My guess would be it's related to dropping voltage at fuse S9. |
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| Tdskip |
Apr 30 2020, 05:52 PM
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#7
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Advanced Member Group: Members Posts: 3,686 Joined: 1-December 17 From: soCal Member No.: 21,666 Region Association: None
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Thank you @GreyAmy
Very helpful. |
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| Spoke |
Apr 30 2020, 08:58 PM
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#8
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Jerry Group: Members Posts: 6,986 Joined: 29-October 04 From: Allentown, PA Member No.: 3,031 Region Association: None
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Here's the simplified circuit for the alternator, battery, and GEN light. When you turn the key to ON before cranking the engine, the GEN light sees 12V from fuse 9 and 0V from the VR as the alternator isn't turning.
Once the engine is cranked and started the GEN light current bootstraps the VR and alternator into operation. If when you press on the brake the GEN light comes on, then either the voltage at the fuse drops or the voltage at the VR drops. One of them has to drop for there to be a difference in voltage and the GEN light comes on. One way to check this is to hook your voltmeter up to either side of the GEN light and drive the car. Obviously the wires would have to be securely fastened so they don't come loose or you holding the voltmeter. Attached thumbnail(s) 
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