how do i check charging system?

My 94 Geo Prism LSI would not start this very cold morning (zero degrees F, wind chill). 60,000 miles. No crank, when I turned the key the solenoid just buzzed. Yesterday when driving I noticed the battery icon idiot alert light (ok I am an idiot!) flashed, but went off when I turned off the rear window defroster. So this morning I took the batter out to warm it up, to later take out to start car. Will go buy new battery. But how can I check if belts and alternator are OK? I have a digital voltmeter to check charging, but I forget how to do it, have done it once many years ago. What about belts-- how much give should they have, in case maybe a loose belt is not charging the battery? Any help appreciated. I hate car trouble!

Reply to
Beowulf
Loading thread data ...

I checked the voltage on the (cold) battery that I brought in the house, and it is reading 12 volts. Maybe I do not need a new battery? Perhaps my problem is with the alternator?

Reply to
Beowulf

Reply to
atec77

Once i start the car, (i know this sounds stupid but i have not worked on engines much-- though at age 16 i rebuilt my 67 Ford Mustang-- but cars are so complex these days I do not touch them), do i then just check the voltage across the batter terminals again (to look for 13.8v reading)? It is so freaking cold outside i just can not do much on my own; i would hopefully like some idea what is wrong before going in blind to a mechanic.

Reply to
Beowulf

First if the battery is more than three to four years old replace it. Most batteries fail at about five years. Most auto parts stores can load test them for you. To test the alternator with a charged battery place the voltmeter across the battery, run the engine at about 1500-2000 rpm with a load on, lights and heater on full. The voltage should be about 14 to 14.5 volts.

Measuring 12 volts on an unloaded battery means nothing as that is the static voltage of the cells. Have it charged and load tested. As I said most auto parts stores can do that...

Reply to
Qwack

Hmmm, I could indeed have a bad battery. I put the warmed and charged battery back in the car, and it started great of course. Idling, voltmeter showed about 13 volts across batter. I reved it up a bit as you indicated and the voltmeter showed about 14 volts, but when I turned on the lights, heater fan, and radio, and ran engine at about 2000 rpm all i could get was about 13 to 13.5 volts across the batter -- and if I interpret this correctly it sounds like that is not adequate voltage? So does this mean the battery is likely in need of replacement, the alternator in need of replacement, or both?

I am going to take it in for a check at a TiresPlus store SUnday morning, to discover what is wrong (if I can trust the mechanics), but i am still interested in what anybody here thinks. I will be sure to report back.

At 10 years old, at the least I guess I will be shopping for a new battery tomorrow. ~Beowulf

Reply to
Beowulf

10 years ?. yup new battery time.
Reply to
atec77

I don't get it. My aftermarket repair manual says one thing and most folks says another. The manual says the alternator is fine when it reads above 12V under load at fast idle. Most folks says it should read 14.2 -

14.4V under load or light load at fast idle.

Recently I posted the same problem. You might want to take a look at some 30 replies about this subject.

formatting link
Tibur

Reply to
Tibur Waltson

Reply to
atec77

Update: Got car fixed. Alternator diodes were bad, needed new alternator (ouch$250). Battery was ok per mechanic. Car has now been starting just fine even in subzero weather.

Reply to
Beowulf

Beowulf

Just a couple of pointers. You now have the car fixed and it was the alternator. With bad diodes the alternator frequently can not keep up with the load and cold temperatures do not help. When you can get the car inside you might do a charging resistance test. Basically a test of the charging circuit from the alternator positive terminal to the battery. The voltage reading from the alternator should be less than .5 volts. Usually less than .3 volts. Have the lights and AC on for this test. If the charging resistance and the battery voltage is good then the charging sysytem is keeping up. If there still is a problem then the battery is bad if the battery cables are good and clean. Bob AZ

Reply to
RWatson767

So now you have 14-14.4V with load at fast idle? I have the same problem and thinking of taking your approach. But I got a new battery because most people suggested that I do. After a week, the new battery is appearing to go weak again.

Tibur, my problem

formatting link

Reply to
Tibur Waltson

MotorsForum website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.