My car is a Mitsubishi Magna TH 1999 3.5L Auto.

The alternator gives no voltage at idle with headlights & fan on. The battery gets drained. The alternator warning light pops up. If revved a little bit, I get 13.4 volts and no warning light. Without load, 13.4 v at idle and 14.3 v at 1000 rpm. When the engine is off the battery shows 12.4 v when connected and disconnected.

When bench testing, the diodes read 475mv one way. The alternator reads 670mv one way when the black probe is to the alternator positive and the red probe to the body.

Is the regulator faulty? The brushes still look good.

Please advise me. Thanks in advance.

P.S. I checked the voltages again this morning when the engine was cold. I fully charged the battery with an external charger and it read 13 v. I measured around 14.3 v without load and 13.3 v with load. But it was 11 v after the engine had reached the normal temp.

  • Welcome to Motor Vehicle Maintenance & Repair! Nov 29, 2023 at 11:32
  • Is there any oil leakage from the engine which might be getting onto the drive belt? If the drive belt has oil on it, it could be slipping. Just a thought. Nov 29, 2023 at 11:36
  • If the alternator drive belt isn't slipping, zero slack from belt tensioner, your 24 yr old alternator is most likely worn out. There are six rectifiers for generating three phase ac voltage going thru a regulator to output no more than 15vdc. Even without a load at idle rpm, voltage should be around 14.5vdc. While brushes may look fine and not worn, the solid state voltage regulator cannot be repaired. Minimum rebuild; new bearings, regulator, diodes (if faulty), machining the comutator, new brushes. Unless you have requisite skills, it's wiser to replace it with a rebuilt or new one.
    – F Dryer
    Nov 29, 2023 at 16:33
  • Thanks friends for your replies.
    – Raj
    Nov 30, 2023 at 13:26
  • I solved this problem. Dryer's suggestion was very helpful. The diodes looked good initially but failed after running for a while. When the current flows through them one or more lose their function due to their old age. I bought a second-hand alternator same as the old one but unfortunately, the pulley was small. It looked easy to change the stator part rather than changing the pulley. Now I see that the battery is getting charged and no battery light is coming up. Thanks for everyone for their suggestions.
    – Raj
    Dec 5, 2023 at 23:44


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