Skip the auto shop - Our mechanics make house calls

Q: Overcharging 18volts +

asked by on

My truck overcharges the batteries. I changed out my alternator,I even took it to a shop that deals with electrical issues.I was told the ECU in my truck has a internal voltage regulator they tried bypassing it with a external one still no dice! Please help

My car has 350000 miles.
My car has a manual transmission.

A: The shop was correct about how the ECU cont...

The shop was correct about how the ECU controls the field current of the alternator which will cause the alternator to charge the batteries. If the wire on pin 57, the dark green with orange stripe wire at the ECU, is getting battery voltage then it is normal. If not, then it is broken and needs to be repaired.

You should then be able to disconnect ECU pin 20, the dark green wire, and the alternator should stop charging. If the alternator does not stop charging, this wire should be checked between the ECU and the alternator field for grounding in between. If the alternator charges when you ground the dark green wire, and doesn’t charge when you don’t connect it to anything, then the wire is not grounded. Check to make sure the alternator is grounded. The ground is near or below the a/c compressor.

If the wires listed all check out to be okay, and the alternator will turn the charge with the dark green wire on and off, you will need to have the ECU replaced. If the shop put in an external voltage regulator then it should of worked if it was installed correctly. If the alternator overcharges the batteries even after the field circuit is disconnected, then the alternator is shorted and needs replaced.

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!

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: OBll CODE : P1899

Hi there. As the service manual reads, the resistance between pin 2 on the range switch and pin 6 black PCM connector must be checked for less than 5 ohms of resistance. Any more than 5 ohms of resistance and...

Q: Van experiencing electrical system problems

The turn signal lighting is all supplied power from the same turn signal relay. Since LED lighting consumes less power it should be heavy duty enough to use but you may want to check with LED lighting manufacturer through their...

Q: Excessive white smoke out of crankcase tube. Fuel in oil

That is definitely the result of a loss of oil pressure. A very bad oil leak, failed pump or lack of maintenance can be big trouble for an engine especially with such high miles. As long as there is no...

Related articles

Rules of the Road For Iowa Drivers
Driving on the roads requires knowledge of the rules, many of which are based on common sense and courtesy. However, even though you know the rules in...
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...
P2103 OBD-II Trouble Code: Throttle Actuator Control Motor Circuit High
P2103 means there is a fault with the throttle actuator control motor circuit, likely due to a defective electrical component or part.