I am chasing a performance issue. I am not getting the power i should with my setup. I did several datalogs and noticed that at wot the volts reading would drop down to 11.7. The higher the rpm the lower the volt reading. They usually shows around 12.3-12.5 99% of the time and when i let off the gas after the hard acceleration it instantly goes to 12.7-12.8. Is this effecting the cars performance and whats the correct way to fix it right? I assume the reading is what the computers gets after it goes through all the other sensors and such and not the battery or alternator directly. This trouble shooting is wearing me out. Thanks for the help.

 

check the voltage across the battery at idle. It should be around 14 volts. Anything less may be a problem. Causes could be bad battery, bad alternator or wiring issues.

Start with the battery. Check the battery voltage while someone cranks the engine over. It should not drop to less than 11 volts while cranking. If so, you may either have a bad battery or the starter is defective (internal shorting causing it to pull too much current).

Once the battery is proven good, go for wiring issues such as power and ground connections. Remove them, clean them up and retighten them. In some cases like at the battery, dielectric grease is a good idea to help stop corrosion. Don''t forget the clusters on the passenger fender, the ground strap from engine to frame as well as the alternator mounting to the engine.

If all that is good and you still have low voltage, check the possibility of a constant drain on the battery. Remove the ground terminal on the battery and set the volt meter to current (A) 10 amps. Put the black lead on the battery terminal and the red lead on the negative battery cable. It should not read more than 0.01 amps. If so, you have a drain on the battery and thus on the entire charging system. You must find the source of the drain, before you move on. Removing one fuse at time while you watch the current meter is a good start. HOWEVER, if you don't have a problem with the battery going dead over time while the car sits unused, chances are good you DO NOT have this issue so proceed as necessary.

At that point you will then know if the alternator is putting out currectly or not.

Oh yeah, one last thing. Many and I repeat MANY people have been fooled by a defective amp meter. That is, a meter that can read current. If the meter has ever been in a situation that read more than maximum, it would have blown the fuse. When that occurs, the meter will read 0 current when there may actually be current flowing in the ciruit. So if you are looking for 0 current, be sure the meter actually functions first. At least check the fuse if nothing else. Don't assume the fuse is good cause you can read voltage since the fuse is usually only for current readings.

 

Also if your using freescan to log (not sure about other software) the volts appear ~1-2 volts lower than what it reads across the terminals.