I had to put in a new alternator in my 99 Isuzu Amigo. The old one burnt up. Everything seemed fine...output was around 14.4 volts, however, the plug that goes to the "S" & "L" terminals on the alternator had partially melted so I ordered a new one. Well, I put on the new plug by twisting the wires real good and covered them with heat shrink tubing. After doing that my battery light came on, my alarm no longer works and my OBD-II connector no longer works. The output voltage was reading around 15.6 volts. Then I read that the connections had to be soldered (not just twisted). So...I soldered the connections. Now, my battery light is no longer on but I still have no alarm or OBD-II connection and the voltage output is only 10.8 to 11.1 volts. Also, the battery is good and I can't find any blown fuses. HELLLLLLP!!

  • 1
    No blown fuses, how are the relays... When you put the wires together was the battery disconnected, have you checked all your fuses for an open ground and when you put the new wires onto the old plug, did you color code them or go off a wiring schematic ?
    – user38183
    Commented Jun 27, 2019 at 9:27

1 Answer 1


15.6 V will probably not do it, but ECUs and parts of them have failed from over voltage.

I believe that on your alternator there is a voltage regulator which can be separately replaced. Probably $15 to $20.

My suggestions are as follows:

  1. Pull the alternator, and take it to the parts store. Most of them have a tester to bench test an alternator.

  2. Once you establish that the alternator is OK, reinstall it.

  3. You should not have to run the engine to get the ODB-II port to work. Simply having the ignition ON should give you access to the ODB-II interface. If it doesn't work just on battery, you will have to hunt that down.

  4. You say your old alternator burned up. Do you know why? Is there some electrical issue in the car which is drawing a high current, so that the alternator ran at full output for an extended period of time?

  5. Are other electronics in the car, such as the radio, still working properly?

It is possible that your ECU has been damaged, but that cannot be determined just by the information you provided.

You will also need to find the root cause of the melting wires, the blown alternator (if not simply old age), and any other car electrical problems.

If you have access to a DC clamp on ampmeter, it would be nice to know the current coming out of the alternator while the voltage is sitting at 11 v or so.

  • Thanks for your reply. The alternator was still under warranty so I just replaced it. The output voltage is ok so far but now the battery light is on> The battery is being charged though. (output is around 14.2 average). My OBD-II is still not working. My alarm appears to be working , however, the horn for the alarm as well as the main horn are not. (It seems I now have a silent alarm). I checked the fuses and they all seem to be ok. I'm really hoping it's not a damaged ECU but at this point I'm starting to wonder. Commented Jun 30, 2019 at 23:10
  • . So far, everything else seems to be working ok (all lghts, turn signals, hazard lights etc.) I'll check the current draw with my voltmeter.As to why the alternator burnt up in the first place...I'm really not sure. I do have a small oil leak close to where the alternator is as was wondering if oil getting into the alternator could cause it to burn up (?). I have to add a quart of oil about every tow to three weeks. As far as testing the current draw, is it correct that I disconnect the main lead to the alternator and put the meter between the post on the alternator and the main lead? Commented Jun 30, 2019 at 23:13
  • First, I have not known an alternator to die because of too much engine oil. I am sure it's possible, but I have seen many very dirty alternators, where engine oil and grime did not overtly cause them to burn up. Secondly, you might look at a wiring diagram for your car. Is the alarm factory, or aftermarket? What do they, and the horns and the ECU have in common? Thirdly, the vehicle horn is a fairly simple system electrically...essentially a momentary contact switch, a relay and a device which is a buzzer or vibrator connected to a sort of acoustic horn. Actually, usually two. Hi/lo.
    – mongo
    Commented Jul 1, 2019 at 14:39
  • Where are you at in diagnosing this?
    – mongo
    Commented Jul 9, 2019 at 15:45

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