No power

SOUTHERNBELL85

MEMBER
1999 JEEP GRAND CHEROKEE
V8
4WD
AUTOMATIC
147,000 MILES

Was a 2000 Cherokee larado and I love um. That why I decided to buy this 99 grand jeep Cherokee last monday. Well after I paid the guy and signed papers he goes on to tell me he changed the computers n it. Now this was a small dealership. Seemed OK but after he said that about the computer my heart sunk. Well drove off and all was fine.I drove it for the very max 3hours probably 2.I was headed home at 12am and my check gage light came on. Radio died. Head lights went to flickering everything went bonkers. Lights and dash lights flickering and the lost the speedometer and seen my volts were going all the way down. Didn't even get a mile from my house almost home and off it went! Well I have a trailblazer to and no even 3 months ago it did this to and was the altanator. Upset I was but thought OK 200$ it will b fixed ok. Well got it checked after taking it off and they said it was bad. So got a new one. Didn't work. So with my experience with auto zone giving me 5 bad starters before I took it the new one to advance to get it checked. And they said it was bad. So bk to auto zone still with high hopes it's just the altanator. Got another one. Nothing. It did they same thing stayed on till batt died.I also got it checked and it's firing at 650cca so it's fine. Well so now on 2nd brand new altanator still nothing so I decided to take it bk and go get one from o'Reilly's since they seem to have better stuff. Also took my original altanator in there for them to test. Well it comes bk at putting out 14 perfect volts. This being same one I took to auto zone and they said it was good. So before I had tried getting a code with the key thingy before and it would never read one off would just say done. Well tried again and it read off p0622. So I'm wondering what it is you think it might be. Or is there a starting point I should go to next or what.I'm so stressed knowing I gave this dealership my mone paid for the truck and it's been sitting in the yard not moved since day I bought it! And beings this guy said he put a new computer on could it b that he didn't take it to dealership to be reprogrammed and it ran good for a while and now it's stopped.I'm scared to death to know I may have to take it to dealership beings idk if I can afford it. But I know it will get done right. So I'm just mind boggled with what it could be. Any information AT ALL WILL BE VERY APPRECIATED PLEASEEEEE! Thank you in advance for your time!Was a 2000 Cherokee larado and I love um. That why I decided to buy this 99 grand jeep Cherokee last monday. Well after I paid the guy and signed papers he goes on to tell me he changed the computers n it. Now this was a small dealership. Seemed OK but after he said that about the computer my heart sunk. Well drove off and all was fine.I drove it for the very max 3hours probably 2.I was headed home at 12am and my check gage light came on. Radio died. Head lights went to flickering everything went bonkers. Lights and dash lights flickering and the lost the speedometer and seen my volts were going all the way down. Didn't even get a mile from my house almost home and off it went! Well I have a trailblazer to and no even 3 months ago it did this to and was the altanator. Upset I was but thought OK 200$ it will b fixed ok. Well got it checked after taking it off and they said it was bad. So got a new one. Didn't work. So with my experience with auto zone giving me 5 bad starters before I took it the new one to advance to get it checked. And they said it was bad. So bk to auto zone still with high hopes it's just the altanator. Got another one. Nothing. It did they same thing stayed on till batt died.I also got it checked and it's firing at 650cca so it's fine. Well so now on 2nd brand new altanator still nothing so I decided to take it bk and go get one from o'Reilly's since they seem to have better stuff. Also took my original altanator in there for them to test. Well it comes bk at putting out 14 perfect volts. This being same one I took to auto zone and they said it was good. So before I had tried getting a code with the key thingy before and it would never read one off would just say done. Well tried again and it read off p0622. So I'm wondering what it is you think it might be. Or is there a starting point I should go to next or what.I'm so stressed knowing I gave this dealership my mone paid for the truck and it's been sitting in the yard not moved since day I bought it! And beings this guy said he put a new computer on could it b that he didn't take it to dealership to be reprogrammed and it ran good for a while and now it's stopped.I'm scared to death to know I may have to take it to dealership beings idk if I can afford it. But I know it will get done right. So I'm just mind boggled with what it could be. Any information AT ALL WILL BE VERY APPRECIATED PLEASEEEEE! Thank you in advance for your time!

1 Reply

CARADIODOC

MECHANIC
33,873 POSTS

Testing of alternators off the car is the least desirable way to do it. All bench testers can do is test for proper charging voltage, (13.75 to 14.75 volts), but that is a product of the tester. On your vehicle that is a product of the voltage regulator which is in the Engine Computer. You also must test for full-load output current which requires about five horsepower to run the alternator wide open. Most bench testers only have a one horsepower drive motor. You also must test for a failed diode which will reduce the alternator's output to exactly one third of its maximum rating. 30 amps from the common 90 amp unit is not sufficient to meet the demands of the entire electrical system under all conditions. The battery will have to make up the difference until it slowly runs down over days or weeks.

To start the troubleshooting, measure the two voltages on the two small terminals on the back of the alternator. This has to be done while the engine is running. We can skip one step in the description since we can assume the brushes are okay on a new alternator. On one terminal you must find full battery voltage. The key is what's on the second terminal. It must be less than battery voltage, but not 0.0 volts. Typically you'll find between 4 and 11 volts.

On that second terminal you're going to find 0.0 volts, the same as on the first terminal, or something in the 4 - 11 volt range. That finding will tell us where to go next. If both have exactly the same voltage, there is a break in the circuit going to the voltage regulator, or the regulator is defective. That would be very uncommon but could be why someone replaced the engine computer.

If you find 0.0 volts on both terminals, there is a break in the feed wire. That one won't be very long because it's spliced in with the wires that feed the ignition coil(s) and injectors.

If you find between 4 - 11 volts on the second terminal, the entire input circuit / voltage regulator is working. The problem then is in the output circuit. Measure the voltage on the large output terminal on the back of the alternator. That must always be exactly the same as battery voltage. If you find it's higher with the engine running and 0.0 with the engine stopped, there's a large bolted-in fuse that's blown. That is the first thing that should have been checked.