Car dies when alternator is pluged in.

Tiny
NEMMIEDRAGON
  • MEMBER
  • 1997 PONTIAC GRAND PRIX
  • 3.8L
  • V6
  • TURBO
  • 2WD
  • AUTOMATIC
  • 200,000 MILES
I have a 97 grand prix was driving down the road and it started to sputter, then died. Would not start back battery was dead. Changed the alternator, was bad ( Had checked at autozone ). Now the car will start sputter and die until I unplug the two prong plug going to the alternator is unplug. Then it will run fine till the battery died. Had the alternator checked again tested good. Replaced the ecu and had it reprogrammed and it still does the same thing. What is causing this trouble?
Tuesday, November 24th, 2015 AT 7:11 PM

6 Replies

Tiny
HMAC300
  • MECHANIC
  • 48,601 POSTS
Was battery checked for condition including load test when you replaced alternator? If not get that done. Then have it scanned for codes as the alternator sensor may be bad causing it to maybe give to much juice. If battery is sulfated will do that as well. It may also have a wire shorting out going to sensor or pcm when you pull it off it moves the harness away. This should be checked out by a local pro with this type of problem if no results are found with what I've given.
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Wednesday, November 25th, 2015 AT 6:34 AM
Tiny
NEMMIEDRAGON
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  • 3 POSTS
The battery is new. When the car give the command to start the alternator to charge is when all this happens. Using the tech 2 when you turn of the alternator no trouble, then you give it the command to charge about 30 second after it kicks in the car start to sputter then dies. Now if I remove the battery cable from the alternator and using jumpers cables attach it to another battery and give it the command to charge the car keeps running. But the moment you touch the another jumper cable from the main battery to the alternator it sputters and dies. I have run the wires from the alternator all the way back to the ecu. All intact no cuts or breaks. Could this be a bad ground somewhere? I am at a loss. GM said it was the ecu they put a new one in did not fix the problem. Then they told be since it was not throwing any code faults nothing more they could do. So right now I have a new alternator (not reman) new ecu from chevy parts, new battery from Walmart and I still have a car that will not run.
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Wednesday, November 25th, 2015 AT 3:45 PM
Tiny
HMAC300
  • MECHANIC
  • 48,601 POSTS
I would check grounds then these oddball things are hard to diagnose even by computer.I guess if you check the ground for pcm which ion transaxle stud and other one by ignition module use dielectric compound on it as well and make sure battery has good ground to engine. The only other thing I can think of is the ground under battery has corrosion on it and or the ignition and fuel pump relay may have same so try using dielectric on those two relays it might be the problem because the ground is same between others it may be a broken or frayed wire at c4 causing the problem but hooking up another battery kind of throws me. What kind of voltage does it put on while running on regular battery? Either that or it's some kind of battery cable problem is about the only thing I can think of without actually looking at this.
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Wednesday, November 25th, 2015 AT 4:30 PM
Tiny
HMAC300
  • MECHANIC
  • 48,601 POSTS
Forgot pic again damn it.
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Wednesday, November 25th, 2015 AT 4:31 PM
Tiny
NEMMIEDRAGON
  • MEMBER
  • 3 POSTS
So what I am seeing after setting up a small rig is this. The alternator is being told to keep charging even when its not needed. With the volt meter on the battery watching when the alternator is commanded on it start climbing to 15 volts where the car start to sputter and die. In seeing this I am able to do the same thing with the battery charger now. With the alternator unplug fully from the system and with the car running I can set the battery charger to 100 amps. Watching the volt meter when it reaches 15 volts the car sputter and dies. Does this mean they sold me a dude ECM?
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Wednesday, November 25th, 2015 AT 7:12 PM
Tiny
HMAC300
  • MECHANIC
  • 48,601 POSTS
No it means for some reason your alternator is putting to much power out or the battery is sulfated which will cause alternators to do that. The internal voltage regulator in alternator may be bad and if it's rebuilt it depends on who the rebuilder was. You said when you put a different battery on it that it ran ok. Then try that. I t may be a bad battery even if it's new. Your alternator should put out no more than 14.5-14.7 volts and they do charge most of time even at idle. Just at a lower rate. Like I said for me to try to diagnose this is very hard because I really can't see what is going on. Also is battery the correct size or bigger? If you have a vin K it should be at least 690 A which should be listed on battery top. If you have Vin I then it should be 770 A if you got a 690A for Vin I it will work but won't last as long due to power requirements. All that info should be on top of battery.
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Friday, November 27th, 2015 AT 7:30 AM

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