Charging problems

[msclcrmn]

I have an intermittant charging issue with my 97 cherokee. Upon starting the engine the gauge reads 12 volts. As I drive it, dash gauge suddenly drops to 9 and check gauges light comes on. I have a digital volt meter plugged into cigarette lighter that tells a different story. It reads 12.?? when I start engine, but slowly drops as I drive. It seems that when it gets to 11.70, the dash gauge immediately drops to 9 and the volt meter then jumps to 13.63-13.78. After it shows as charging for several minutes, I find that if I turn engine off and restart within an hour or so, the dash gauge will read correctly again.

Has anyone had this? All the other charging problems I searched out did not show anyone checking with DVOM. According to meter and bench test, my alternator is good.

Does dash gauge get voltage from PCM, ALT, or BATT?

Thanks for helping!

 

[Tim_MN]

The PCM tells the dashboard voltage gauge what to say. If you have low volts, the PCM swings the needle low and lights the check gauges light.

You have an alternator, battery, or wiring issue. The dashboard gauge is a pretty good indicator of your charging system. If it reads low, you have an issue. Have the alternator and the battery Load Tested in machine that applies a simulated work load. Worn alternator brushes will work, and then get stuck not working, then get unstuck on a bump, so passing a bench test is not 100% accurate.

Perform routine maintenance of the start and charge systems. Remove, clean, and firmly reconnect all the wires and cables to the battery, starter, and alternator. Look for corroded or damaged cables or connectors and replace as needed. Do the same for the grounding wires from the starter to engine block, and from the battery and engine to the Jeep's frame/body. You must remove, scrape, and clean until shiny, the cable/wire ends, and whatever they bolt to. Jeeps do not tolerate low voltage, bad connections, or poor grounds.

 

[8Mud]

Sounds like what my 96 did when the red top was failing. I knew it was getting weak and it was time for a change. My check engine light would come on, then go away. It seemed pretty sporadic. One day i stopped and when I went to start again the battery was as dead as I've ever seen one get. I mean totally dead, absolutely nothing.

May be related to your issue, may not, but worth a look see. Maybe take the battery down to the local parts place and have a load test done.
One thing I did notice was my batteries usually hold more than 13 volts over night, this one fell to around 11.5 or lower. It wasn't holding a charge well.

In my experience you can't really rely on what the dash gauge says. Mine always reads low, but at the battery perfect.

I don't really know if this effects the battery gauge or not, but does affect other gauges, the contact strip for the instrument cluster gets oxidation on it and needs to be cleaned periodically. Any time I'm back there changing instrument bulbs or whatever, I clean the contact strips.

 

[Ecomike]

Also, the voltage regulator is in the PCM on a 97 IIRC!!!

 

[msclcrmn]

This has been an on going problem for about 2 months. Battery is fairly new. I had already cleaned the battery terminal ends on the cables. I have been watching that voltage as a means to monitor the intermittant problem. With the consistancy of it turning on at 11.70v just about everytime I watched it, I guess I dismissed the idea of anything like sticky brushes. I also found it interesting that the gauge dropped when charging voltage went up.

Today I had some time and decided to follow up what Tim_MN had said. I grabbed a 2 foot long piece of scrap wood and used it to simulate a bump.

Dash gauge showed about 12v, Cigarette lighter DVOM read 12.35 after I started engine.

I confirmed with second voltmeter connected to battery.

I then tapped on the back edge of alternator, straight down with scrap wood.

2 taps immediately brought battery voltage to 13.75

I then turned engine off for a few minutes and repeated. Similar numbers with same results, as soon as I thumped it the voltage came up.

So I have another question. Once the check gauges light (NOT check engine) comes on, it does not go off and dash gauge does not go up, until key is turned off then on again. Is this normal then? It does not reset when problem is resolved, only by restarting?

 

[Tim_MN]

The gauge drops either because you have a wire connection issue, or an alternator issue. Fix the problem and you fix the symptoms. IDK what the ECU charging/voltage regulator parameters are.

13.5-14.2 volts output is what you want to see. 13.8 is what all my Jeep have shown.

I don't have time or patience for messing around with charging stuff. Any vehicle that is not performing right gets a new battery, new battery wire clamps, or a new (not rebuilt) alternator as needed.

 

[O-Gauge Steamer]

This has been an on going problem for about 2 months. Battery is fairly new. I had already cleaned the battery terminal ends on the cables. I have been watching that voltage as a means to monitor the intermittant problem. With the consistancy of it turning on at 11.70v just about everytime I watched it, I guess I dismissed the idea of anything like sticky brushes. I also found it interesting that the gauge dropped when charging voltage went up.

Today I had some time and decided to follow up what Tim_MN had said. I grabbed a 2 foot long piece of scrap wood and used it to simulate a bump.

Dash gauge showed about 12v, Cigarette lighter DVOM read 12.35 after I started engine.

I confirmed with second voltmeter connected to battery.

I then tapped on the back edge of alternator, straight down with scrap wood.

2 taps immediately brought battery voltage to 13.75

I then turned engine off for a few minutes and repeated. Similar numbers with same results, as soon as I thumped it the voltage came up.

So I have another question. Once the check gauges light (NOT check engine) comes on, it does not go off and dash gauge does not go up, until key is turned off then on again. Is this normal then? It does not reset when problem is resolved, only by restarting?

This is a classic case of a brush not making full contact with the slip ring. I am in process of replacing my alternator for the very same reson. Voltage drops, lightly tap the case and the voltage goes back to the normal 14.2VDc, or at least closer to the norm.

The charging system CEL is a "latching" function that requires a restart to clear. Some of the CELs latch and some do not. An example of a non-latching CEL is for mixture faults. They will come and go based on what the PCM is doing to correct the AFR.

A fully charged Lead/Acid battery has a standing voltage of 12.6VDc. This is a quote from the 98 FSM. In order to charge to that voltage, the system needs to generate a 1.2VDc differential. Each cell (there are 6) in the battery generates 2.1VDc.

12 volt systems are not and have never been 12VDc... The vehicle is designed to operate on the 14 volt level.

Back in the 6 volt era, it was relatively common to install an 8 volt battery and then adjust the regulator to charge the battery to 7 volts which was about all the over-volt the electrics could stand.

 

[msclcrmn]

Monitoring the voltage was making me lean toward the PCM, but after thumping the back of alt, I decided it was in fact the alternator.

I was only able to load test the old one using my accesories. After tapping it to make sure it came on, charging voltage was 13.65v. I then began to turn on accesories to put a load on it. A/C, fan hi, headlights, int lights, stereo/amp, flashers, and by the time I keyed my cb, it had dropped to 11.45v. I then turned it all off and voltage went back to 13.65v

New alternator spec sheet shows 157 amps max output. With nothing on, charging voltage went to 14.0v. Using the same method to load test, it only dropped to 12.9v. Throttling it to 1700 rpm brought voltage back to 13.7v.

So, I guess all is well again. Thanks for the help and comments.