Skip the auto shop - Our mechanics make house calls

Q: Battery light comes on after starting and stays on running the battery down

asked by on

I changed alternator and PCM and the problem still exists. I checked the wires and continuity from the PCM to the alternator and they are good. alternator will not charge even though it is new. I have been told there is a bad wire or a shorted wire. All fuses are good. ???????

My car has 216000 miles.
My car has an automatic transmission.

You need to have the alternator’s large red terminal from alternator to the starter tested for proper battery voltage. The orange wire coming out of the alternator should have battery voltage at all times. This is the volt sense wire and is fused by a 10amp fuse in the engine fuse box. The last alternator wire , a small red one, goes to the PCM and this is the wire that the PCM uses to control the alternator output. The PCM should be scanned as well as the data from the instrument cluster to see if the instrument cluster is causing the problem by not giving feedback voltage to the PCM.

You may have a defective cluster or you’ll need to disconnect the connections at cluster and PCM. Then add dielectric lube to connector pins and seals. This will help stop fretting corrosion in connectors. This can potentially cause high resistance in connections and cause the problem you’re having.

If you need some help with these checks, consider YourMechanic, as a certified technician can tests these components and diagnose the issue with your Battery Light.

Was this answer helpful?

Need advice from certified mechanic? Get help now!

Over 1000 mechanics are ready to answer your question.
The statements expressed above are only for informational purposes and should be independently verified. Please see our terms of service for more details

Ask a Mechanic
(100% Free)

Have a car question? Get free advice from our top-rated mechanics.

Ask A Mechanic
Over 10,000 questions answered!
  1. Home
  2. Questions
  3. Battery light comes on after starting and stays on running the battery down

Get an instant quote for your car

Our certified mechanics come to you ・Backed by 12-month, 12,000-mile guarantee・Fair and transparent pricing

Get a quote

What others are asking

Q: Service engine light only in warm weather

Hello. Typically the service light is a vague warning and can indicate something as simple as too many miles between oil changes to potentially serious faults in the engine. The problem could be totally benign or very serious, and a...

Q: How to install electric body control model

Hello. The first thing to do is make sure the battery is disconnected. Next, follow a repair manual for instructions on properly removing the old module. After that, you can install the new one in reverse order. You will then...

Q: Will crank won't start

Hi there. Based on your feedback, it appears that your ignition system components are functioning good. I would be concerned about the reduced amps on 1,2,3 and 6; but if I had to guess, I would start with inspecting fuel...

Related articles

How Do Power Car Windows Increase Passenger Safety?
Power windows are responsible for approximately 2,000 emergency room visits every year. When a power window closes, it exerts enough force to bruise or break bones, crush fingers, or restrict an airway. Though...
P0240 OBD-II Trouble Code: Turbocharger Boost Sensor B Circuit Range/Performance
P0240 code definition Turbocharger Boost Sensor B Circuit Range/Performance What the P0240 code means P0240 is an OBD-II generic code triggered when the Engine Control Module (ECM) detects the intake boost...
How to Avoid Back Pain in a Car
If you have back problems, sitting in a car for an extended period of time can be excruciating. Even without back problems, you could experience discomfort and soreness from...