Skip the auto shop - Our mechanics make house calls

Q: Alternator dead short when power applied.

asked by on

I have just installed a new 200 amp alternator with a Painless 10120 wire harness. When I check for continuity from the positive battery post terminal to the casing of the alternator or any area of the grounded engine block, I get continuity. So when I hook up the 12v. DC and try to install a 60/70 amp maxi-fuse of course it keeps blowing for I think I am creating a dead short. Is not the isolation post on the alternator where you hook up your 12v to keep the 12v from dead shorting. Why the continuity from positive post terminal to ground areas of motor and body of alternator. There is a specific way the wiring must be done when you install a larger than 65 amp alternator and that wiring I am sure is right. Please, WHAT AM i MISSING. I am supposed to be leaving for holidays in two hours and I need my car. HELP>..... DUNCAN Matheson
My car has a manual transmission.

A: Firstly, with the battery disconnected you ...

Firstly, with the battery disconnected you will have continuity between the positive cable to the ground of the body or engine if you have any components hooked up or have fuses in. You are getting a complete circuit through some of these components. You will need to check to make sure all of the wiring on your power side is not touching ground before going through the components.

Disconnect the big main cable to the alternator and see if the short goes away. If it does, then use your meter to measure the ohms between the alternator post and the alternator case. You should have continuity in one direction and if you switch leads you’ll have no continuity. This will tell you the diodes are good or shorted in the alternator. If you have continuity in both directions then the large post is grounded or the internal diodes are bad.

If you can’t get it to work and you decide to get this fixed, consider contacting a certified mechanic who can diagnose your electrical issues firsthand and fix it accordingly.

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: Rear driverside door problem

The door ajar switch on the latch may be not working correctly and should be checked and latch replaced if it has wear on the latch close fingers on latch. The body computer should be monitored to see what door...

Q: Car will start but won't idle

This may be a sign of a failing idle air control valve. The idle air control valve is located on or near your intake manifold that monitors the air intake as it is mixed with fuel prior to being injected...

Q: Car not starting up

Hi, the first thing to check out is the battery. Batteries tend to fail at the 5 year mark so if the battery is around that age it may not be able to hold a charge. You can try jumpstarting...

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...
What are the Car Pool Rules in Hawaii?
Hawaii is widely regarded as a land of vacation and relaxation, and as such, its scenic roads and routes are far better known than the state’s freeways. But, as with all...
P2428 OBD-II Trouble Code: Exhaust Gas Temperature Too High Bank 1
P2428 code definition A P2428 trouble code signifies that the PCM has detected a problem in the exhaust gas temperature sensor circuit in bank 1, which subsequently contains the number one...