Car charges fine, amp light stays on

Tiny
RYKAELVIK
  • MEMBER
  • 1985 LINCOLN MARK VII
  • V8
  • 2WD
  • AUTOMATIC
  • 150,000 MILES
Grandma's car wasn't charging; changed alternator and then it was charging fine, but the AMP light on the dash won't go out.
Saturday, March 12th, 2022 AT 8:43 AM

3 Replies

Tiny
JACOBANDNICKOLAS
  • MECHANIC
  • 109,724 POSTS
Hi,

What is the voltage being produced by the alternator? If you haven't confirmed it, here is a link explaining how it's done:

https://www.2carpros.com/articles/how-to-check-a-car-alternator

I need to know what the voltage is at the battery, so that test will let us know.

Next, I attached the wiring schematic below for the charging system below.

This vehicle has an external voltage regulator. It is what signals the light to be on or off. If the alternator is charging the battery and the light stays on, the regulator is my first suspect.

If you look at pic 2, it shows the location of the regulator. It is under the hood on the right side (passenger side) inner fender (fender apron). You may want to locate it and inspect the wiring to it and the connector. Disconnect the battery if you plan to disconnect the regulator.

Let me know what you find. I will watch for your reply.

Take care,

Joe

See pics below.
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Saturday, March 12th, 2022 AT 10:37 PM
Tiny
RYKAELVIK
  • MEMBER
  • 2 POSTS
Alternator is charging the battery at 14.5 vdc at idle. Battery is at 12.7vdc (ish)at rest. Yeah, I was wondering if it was the regulator that throws the volts (or grd) for the AMP lamp to illuminate or extinguish. I'll take a look at that diagram. Thanks a bunch!
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Monday, March 14th, 2022 AT 4:05 PM
Tiny
JACOBANDNICKOLAS
  • MECHANIC
  • 109,724 POSTS
Hi,

I suspect that is where the problem is. The alternator is working perfectly based on the voltage readings you mentioned.

Let me know what you find or if you have questions.

Take care,

Joe
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Monday, March 14th, 2022 AT 6:32 PM

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