Got a 99 XJ Sport and the voltage gauge pegs out at 19 volts. The check engine light is on but I can not get a code reader to hook up to it. Tried 2 different readers and neither would connect. Alternator was just put on about a month ago and the battery tests fine. Could this be a problem with my computer or something else?

 

Check the grounds around the engine bay. i had this problem and cleaning the grounds helped it out. There are one over by the computer, on the block next to the alternator and coil, on the back of the block by the trans dipstick, the main ground next to the battery on the fender, and the ground strap on the back of the bock up to the fire wall. clean all of those and see if that helps.

 

Dan has excellent advice. Also, just because the dash gauge reads 19 volts doesn't mean the charging system is actually putting out 19 volts, always verify. Check the voltage at the battery with a meter with the engine running. It may be perfectly normal and the gauge is lying to you.

 

So if the voltage at the battery is normal I could just have a bad gauge? What could cause me not to be able to pull the codes from the ECM? I thought maybe the two problems were related

 

If the voltage at the battery is normal then it would be the gauge. When this was happening with me it was running at 19 volts because the windows would go up and down faster and the lights would get brighter. Don't just assume that it is a gauge without testing it first. There are a lot of things in there that can handle 19+ volts and there are a lot of things that can't.
As far as the scanner not working the engine has to off and the key in the on position for it to read the computer. don't plug the scanner in when it is running at 19+ volts you might fry the scanner.

 

While your advice about sending 19v to a OBDII scanner is good advice, the scanner will work just fine with the engine running - and in many cases is necessary to troubleshoot running problems.

 

What could be some causes of the scanner not connecting? I tried 2 different scanners, and cleaned the pins. Never had any issues connecting a scanner to it before. Really want to know what code it has stored

 

There's a thread over at Cherokeetalk discussing this. It may be a corroded or bent pin. There are charts and diagrams to look at.

http://www.cherokeetalk.com/forum/f2...ot-read-16508/

 

I know that in 98 the key on and off trick won't work. But you might want to try it and see. maybe Chrysler put that feature back in on the 99 models.

 

Electrical problems in 96+ XJs can lead to No-Bus problems which can make it impossible for a OBD2 device to successfully read messages off of the data bus that carries data between various sensors and control modules.

Sometimes, a seemingly unrelated failure (like a crankshaft position sensor failure) can lead to a No-Bus condition because it messes up the circuit it is on, which in some cases is a circuit in common with the proper functioning of the data bus.

Chrysler calls their bus the Chrysler Collision Detection Data Bus and you can read more about it here:

https://mopar1973man.com/cummins/art...data-bus-r329/

This bus has a lot to do with why dashboards can act funny when there is a data bus problem, because something as simple as a temperature gauge might not actually be depicting measurements of temperature, but is -actually- responding to messages sent via the ECM on the data bus.

The following is one of my favorite South Main Auto youtube videos about fixing what starts as a "No Data Bus" type problem, but ends up being a loose bolt connecting the dash to the vehicle, which happened to be an important ground for the dash, which was allowing the data bus to get stray voltage and screwing up its ability to communicate.

South Main Auto - Jeep From Hell

 

I have had this happen twice and both times it was due to the battery. Replaced the battery and all was well.

 

Checked all the grounds last night and found nothing loose or corroded. Started the jeep and keep the meter on the battery. Let it run for 30 minutes and not once did the voltage spike stayed around 13.75 with a/c on and 14.05 without. Still couldn't get a code reader to pull the code.

 

next you may want to look at pulling the instrument panel and see if there are any loose wires contributing to a possible Data bus problem like Jordan stated. There is a problem with some of the 98+ Cherokees with the instrument panel connection with get corrosion on it or come loose.

 

For any vehicle with an airbag, i always disconnect the battery and wait a couple minutes before messing with the instrument cluster, or nearby wiring. I have seen others not do this without issue but i would rather be safe, than sorry.

CCken posted a couple ground pics here and there, and what they are for. See post 20.
https://www.cherokeeforum.com/f2/eng...130071/index2/

All grounds must be clean. Remove and clean them until they are shiny. I took G101 one step further on my 97 and removed the studs, replacing them with fresh ones. The originals were not in the best of shape. Yours may be fine, its just something i decided to look into. The ground strap on the firewall can be a stubborn sob. I pulled the nut on the backside thru the firewall as the bolt was that badly seized. What fun that was. Had to remove the wiper linkage to make it right again.

When multiple scanners fail to link, check the wiring to the DLC itself to rule that out. You can test the wiring if you like. See post 6 and 8.
https://www.cherokeeforum.com/showthread.php?p=2320806+

 

This happened to me a few days ago, I checked and cleaned the grounds next to the battery and cleaned the terminals on my battery. So far so good!