9-10 volts with ignition on gradually decreases from 14 after start up
This may be a long one... As the title states when I turn the ignition on my voltage gauge reads anywhere from 9-10 volts but it fires right up. Never skips a beat. Upon start up in the morning it reads 14. As I start it more throughout the day it decreases to a low point of about 11 with nothing but the radio on. If I turn on heat it drops a little lower and continues with more with lights, wipers etc. So yesterday I broke out the multimeter. Battery read 12.6 on multimeter iirc, around 10 on gauge. Started it up...read 13.4 on meter, gauge read around 12. Turned fans on highest setting, high beams, radio turned up (also have amp/sub on) wipers on high, rear defroster and the gauge dropped below 9. No cel, no stalling or sputtering. Only thing I noticed was the wipers slowed a bit (happens when real cold too). Reading at the multimeter stayed steady. Shut the engine down left the lights on a few minutes then turned them off and waited a few minutes. Checked reading at the battery cable on the power distribution center and the body body ground, 12.6. Checked it at the starter, 12.5. So the questions I have are... what other test can I perform to check impedance of the cables, does what sends info to gauge also send it to pcm or is it separate which would explain why I have no cel but gauge reads bad. Could something wrong between gauge and switch or could the gauge just be a liar. This has been happening since I bought the jeep in July 2012. I have replaced alternator with 136a and put in a new battery in Dec 12. It is a 93 5.2. If the gauge is reading right and I am loosing that many volts between where I've tested and it, I would like to see it fixed so it doesn't die on me at some very inconvenient point. One more thing is I had all those accessories on for over a minute with the gauge reading below 9v. Feel like the engine would have had trouble with that low of a current for that long. Ok I lied one more thing... no battery symbol light on the dash bit I've never seen it on this vehicle so the bulb could just be out for it (I have a couple others that are out). Thank you for reading this very long post.
A couple of things come to mind, but I know nothing about the 5.2, so bear that in mind...
1. On the 4.0, the voltage regulator is in the PCM. It could be heating up and going bad.
2. You might have a slight short to ground somewhere. It's heating up, making better contact as it does so, and pulls your voltage to ground. ( just a theory)
3. You got a bad alternator. It happens. But I'm guessing maybe a bad connection at the alternator or engine ground.
That's all I have before my second cup of coffee this morning. Hope it helps !
My first thought was the alternator. Replaced it. Also replaced the pcm as my second guess. Both were very old and the pcm was throwing random codes anyway. Should have put that in the first post but when I wrote it I had been up for 22 hours. I will have to look for the small ground that theory makes a lot of sense. And thanks for that link. This is my first attempt at tracking down electrical issues. I'm going to get back out the meter and test everywhere I can.
Yea I still haven't been able to figure this out. If I hook up cables to jump it my gauge reads right under 14. Take them off and back to 9.5ish with the battery on. Driving this morning I noticed it was at 12 at a stop light, then I turn on my rear defroster boom down to right above the red line on the gauge and my headlights dimmed. Where else should I do a voltage drop test besides to the starter and fuse box under hood? Where does the voltage reading come from?
Dont trust the voltgauge in the instrument. Its not that accurate to tell you exactly what the alt is charging.
Mine has a life on its own sometimes some days it will read over 14V all day, the next it will only go to the line that is before 14V and stay there all day...
If your voltmeter says its charging over 13V (but not over 14V) when you measure across the battery the alternator is ok.