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Voltmeter dropping? Check gauges?

DanMan2k06

NAXJA Forum User
Location
Damascus, MD
So this just started happening yesterday. I start up the Jeep, and the voltmeter reads about 11.5-12 volts. Not the normal 13/14 that it should. I thought weird, maybe it will pop back up once I start driving. About 5 minutes down the road, the needle plunges to zero, and my amber "check gauges" light comes on. Has done this every time I get into the car now, but it drive fine. No changes whatsoever. Could it be a voltage regulator? Alternator? I have a new battery, and all new heavy gauge cables everywhere. Checked the connections, they're all fine. Any ideas?
 
I would have the alternator tested. Most auto part stores can perform a load test in the parking lot.

You can perform a simple output voltage check with a voltmeter yourself.
 
Year make model?
 
If the voltmeter says zero, and your truck still starts and drives, the issue is with the meter, not the alternator or battery. You need to test directly on the battery posts with a multimeter set on volts. With the key off, you should read about 12.5 volts. WHen you start it up, it should read 13.5 to 14. The voltmeters in the dash are notorious for problems. Make sure you don't have big problems by testing directly on the battery.
 
It's a '97, Jeep (duh) Cherokee (duh). I'll look at the battery again. I think my alternator is toast. The battery is brand new, so I'm sure the car could run on that for a while without anything happening.
 
check the wire that goes from your alt. to your pdc (fuse block). My connection was all corroded and it was doing the same thing. The wire actually snapped, that's how bad it was. I replaced it with some 4 gauge wire and now it's perfect.
 
If the voltmeter says zero, and your truck still starts and drives, the issue is with the meter, not the alternator or battery.

Actually, its probably a function of the PCM doing this. He didn't state what year but I'm betting its a 97+. When the PCM sees under a certain voltage (IIRC its 9v) it will put the check gauges light on and peg the needle at 0.
 
If your alternator is bad or failing, Replace it today, or you will be getting a new "new" battery at the same time. The battery battery cannot handle the constant load of running the rig.
 
Actually, its probably a function of the PCM doing this. He didn't state what year but I'm betting its a 97+. When the PCM sees under a certain voltage (IIRC its 9v) it will put the check gauges light on and peg the needle at 0.

Didn't know that! Think it would crank with 9V? Do you know if the temp gage works the same way? A lot of posts on here of the temp needle swinging wildly on mild overheat. I know that on my Crown Vic with the digital temp gage, a mild overheat pegs that sucker.
 
Actually, its probably a function of the PCM doing this. He didn't state what year but I'm betting its a 97+. When the PCM sees under a certain voltage (IIRC its 9v) it will put the check gauges light on and peg the needle at 0.

Don't know for sure about the +97, but the 85-90 models do not have a 0 on the gauge, they peg the left side at about 8-9 volts. IIRC 9 volts is the lowest label on the gauge. If the 97 is digital then it may be 0.

It should crank and start at 10 volts. 9 volts will crank IIRC, but there is a 9-10 volt threshold area where the computers see the low volts and will not initiate spark, or something like that. I forget if that applies to HO OBD-I, OBD-II, Renix or all. 5-90 knows that voltage threshold IIRC.
 
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Yea sorry I'm saying zero, but I think it's like 8 or 9. But once it drops, it goes all the way to the bottom. I just don't get how my PCM could be acting up, and the car still drives perfectly?

He didn't state what year but I'm betting its a 97+
It's a '97, Jeep (duh) Cherokee (duh)

Hehe...
 
There is nothing wrong with your PCM. The gauge dropping all the way to the left accompanied by a check gauges light is completely normal operation for a 97+.

Replace your alternator.
 
i know you said you've checked your wiring, but check it again. 99% of the time it's a bad connection somewhere. Could be at the alternator, a ground, or it could even be the gauge cluster. They are known to corrode on the backs of the terminals.

Before you go buying another alternator, clean all your grounds and make sure they are tight as well as your battery and alternator cables. My .02.
 
I think it is the alternator, but it didn't do it today when I drove. I'm so confused. You'd think if it was the alternator, it wouldn't charge at all. The Jeep has over 200k on it, and I have lots and lots and lots of lights.
 
I would suggest taking it to Advance or Autozone.. however I have had varying results. Took two alternators to one Advance.. they both tested bad. Took the same alts to two Autozones and they both tested good. Took they to another advance and one tested good the other tested bad... so I don't know how good they are. Same thing with a battery I had. Varying results from varying places.
 
I would suggest taking it to Advance or Autozone.. however I have had varying results. Took two alternators to one Advance.. they both tested bad. Took the same alts to two Autozones and they both tested good. Took they to another advance and one tested good the other tested bad... so I don't know how good they are. Same thing with a battery I had. Varying results from varying places.

The difference depends on whether they ran a full load test, or a partial load test. There are two halves of the diode sets, if one goes, it still works until everything is powered up including head lights, then it drains the battery as the load exceeds the output of the alternator.

I had your problem once before, the O2 sensor wires had come in contact with the exhaust and it was shorting the 12 volts for the O2 sensor heater to ground through carbon charged insulation.
 
I see. Is that why it still runs? It's got another half to power all of the accessories and such? I'm tempted to go out, start it, turn on all 10 of my driving lights, and see what happens.
 
The difference depends on whether they ran a full load test, or a partial load test. There are two halves of the diode sets, if one goes, it still works until everything is powered up including head lights, then it drains the battery as the load exceeds the output of the alternator.

I had your problem once before, the O2 sensor wires had come in contact with the exhaust and it was shorting the 12 volts for the O2 sensor heater to ground through carbon charged insulation.

So does that just mean that the alternator output is half of the normal amperage?
 
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