Start the car. Pull off the negative battery cable, dont let it touch anything, just hold it in your hand. If the car dies go get an altenator. also check wire from alternator to battery for a problem.
Wow, some writeup-the voltage should be checked at the back of the alternator and the battery at the same time-if the pos post on the alternator reads 12.4 resting, that's what it should read at the battery too-and if it reads 14.4 volts running, at the battery, the alternator should read that too-if it does not (alternator reads 11.8, battery reads 12.40 it is the alternator. If the alternator reads 14.4 and the battery reads 12.4, the issue is wiring or the ecm is not allowing current to flow to the battery-there is an internal regulator in the alternator, but the ECM, if defective, can cause your issue. Check for relays or fuses that might be bad first, but it sounds like your ECM may be the problem and now you probably have another alternator issue which will have to be corrected first-then check the resting and running voltage at both places, and go from there. Testing an alternator off the car only proves it is good (or bad) off the car-therefore you need to determine what changes when mounted, and it can only be wiring (harness) , or control (ECM)
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i have this same problem my car was only reading 12.3 on both the battery and the alternator the wiring harness read fine so it really comes down to the ECM computer in the car it has to be replace you can go to auto zone and purshace a new one which will cost you 594 and change thats including the tax and they give u back 60 bucks when u give them your old computer a lil pricey but if you want a no problem car thats is what you will need hope this helps
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