Check engine light flashes and engine stalls at the same time voltage gauge drops to 9 volts

It sounds like a loose cable that is vibration sensitive. I am not exactly following you but if it stops when the car is turned off then get the alternator and or regulator tested. Your battery should hold about 13.8v when running.

Check electrical connects at the battery , power cable to the main fuse box under the hood in the engine bay. Voltage drop testing starting an charging system circuits . Find electrical distribution diagrams here http://www.bbbind.com/free-tsb Enter vehicle info, year , make , model an engine size . Under system click on electrical distribution ,then under subsystem click on power distribution . Click the search button then the blue links one at a time . Check main power feeds . Check for resistance in power feed cables .
Starter Voltage Drop
Voltage Drop Testing the Charging System
Multimeter basics voltage and resistance tests free SD Premium video
Electric Testing Techniques You Need to Know

check alternator,check battery connection,fuel filter,air filter,check coils and module,spark plugs,last thing crankshaft sensor.

Sounds like an electrical issue. You'll want to take it in.

replace the voltage regulator
amp gauges read amps or current flow
volte gauges read volts so if the volt gauge reads 6 with the engine running indicates that the regulator is not coming into service until the battery voltage drops and that triggers the regulator
an accredited auto electrician will be able to test the complete charging system and advise on the fault

Hi,
The Alternator is JUNK, sorry.
Regards,DT

check engine light on ? Do you know how to do electrical testing with a volt meter ? Check power an ground circuits , voltage drop test the charging an main power supply's ,test fuel pump and ignition system circuits .
Testing Electric Fuel Pump Problems with Voltage Drop Test
Voltage Drop Testing the Charging System

Electric Testing Techniques You Need to Know

You want to fix cars ? You need to learn the correct way ! not just replacing parts . A lot of good videos on youtube .

you will haft to take it to the jeep deller and have them scan it to check your pcm wich cuntrols your voltage

Buick dealers have a diagnostic box that checks gauge sweep and range, an 0-100 Ohm variable potentiometer with key on can check this gauge however you must tap into the wire going for the sending unit at the tank. With a voltage meter check the four wire connection (only one)connector going to the top of the tank, find a good ground and check for voltage one will be 12 volts with KEY ON ENGINE OFF look for a 12 volt reading for 5 seconds and then it will stop this is for priming fuel rail and is to fuel pump, you want the ground side of the gauge the other two will be grounds, fuel pump and gauge. The grounds may be near the wiring harness going to the frame. In either case they will be grounds you are looking for the one that is not either ground or power, most likely it will be either a flashing light or very low voltage. Attach the pot to the gauge wire and have someone watch the fuel gauge if it is erratic the gauge is the problem. If the gauge sweeps slow and steady the problem is in the sending until in the tank. AND HAS TO BE DROPPED! Run the tank down on fuel as low as you dare and drop tank. Be carefull working with fuel can be very DANGEROUS if you are not experienced working around fuel DON'T Attempt!

this is eithere a loose positive cable from the altenator to the battery or the regulator in the altenator is about to fail. if you have not had any problems starting the vehicle then most likely its a loose connection between the gauge and the charginging circuit or the gauge is faulty. take a multimeter and test the charging votage at the altenator while the vehicle is running it should measure 13.5 volts and should not change more than .5 volts. if the voltage drops more than 1 volt the regulater is bad and the altenator needs to be replaced. if it is working fine then looke for a bad connection make sure all positive and ground cables are tight. if they are replace amp gauge