voltage dropping with rpm

Dirty Harry · #1 (**permalink**) 06-18-2012, 10:59 PM

Took my car on its first night time drive of the summer and the voltmeter stays at around 12.8 when driving down the road, but the voltage creeps down to about 10.5 and 11 while I am at a stop light. Then, it recovers as I go through the gears. This is the first time it has acted like this. My idle rpm is about 750. The battery is 2 years old.

Any advice on what to suspect?

Tommy · #2 (**permalink**) 06-19-2012, 04:16 AM

I'm sure people more knowledgeable than me will chime in. But those numbers sound to me like it's not charging at all. I see 14v when my engine is running, 12.8v when it's off Without charging it would drop further at a stop light because of the brake lights. .. Check your voltage before and after starting the engine and see if it is charging at all.

Ron61 · #3 (**permalink**) 06-19-2012, 04:52 AM

Tommy is correct. I see 14 volts on mine when running and about 12.7 when it is shut off. Also check for a loose ground wire. If you have a place you can have your battery load tested drive the car down there and have them put their tester on it. They can test the battery itself, whether the alternator is putting out what is should or if there is another problem. This should be a simple problem to find.

Ron

Jaydee · #4 (**permalink**) 06-19-2012, 06:27 AM

If in doubt, check to see if the light go brighter as you rev it. Sounds like voltage regulator, or bad diodes.

Skuzzy · #5 (**permalink**) 06-19-2012, 02:59 PM

Possibly a dead cell in the battery?

takomaned · #6 (**permalink**) 06-19-2012, 03:45 PM

Most voltage regulators have a thermister built in to reduce charge as the temp increases. I would expect a min.of 13.2 volts. Do you have a V belt drive or serpentine belt? Test the battery with a volt meter 1st.with the engine off. If you've been running the engine,turn the headlights on for 10 sec.to remove any surface charge. You'll want to see 12.6V. min for a good battery(new is 12.9V). Next look for a voltage drop on both positive and neg.sides of the battery. Engine running,(volt meter on DC volts) take the red lead and touch the output side of the alternatos,place the black lead on the POSITIVE post of the battery. You should see less than .3 volts (perfect is 0.0v) Also check the neg.cable the same way.Touch the lead to the case of the alternator and the other to the post of the batterry (neg).If you have voltage drop,feel the battery terminals. If they're more than warm,thats where the problem is. Also,make sure that the cars volt meter matches the hand held one.

Dirty Harry · #7 (**permalink**) 06-19-2012, 04:45 PM

Great advice. I'll get on it tomorrow evening and report back. Thanks, everyone.

brandx · #8 (**permalink**) 06-19-2012, 04:53 PM

One other thing is to make sure your belt is tight as some times people forget the simple things

Dirty Harry · #9 (**permalink**) 06-20-2012, 11:53 AM

It seems to be better now. I tightened the belt a bit, tightened all the connections on the back of the alternator, and had the battery load-tested. I took apart the ground connections from the battery to the frame, frame to engine, etc, and cleaned those connections and re-assembled those connections. When I fired it up, the voltmeter jumped up to about 14 right away. It may be OK now. I'll keep an eye on it at London this weekend. Thanks, everyone, for your advice.