Charge Lamp Glows Under Load

Belt slip is strongly supported. The belt squeals when the electrical load is switched on, because the alternator suddenly demands more torque. The tensioner is at the end of its travel and the ribs are glazed, so the belt cannot grip. At idle with no load, 13.9 V is just enough to keep the lamp dim, but with 40-50 A of load the alternator cannot maintain voltage and the battery falls to 12.3 V. The 0.2 V B+ drop at 40 A is acceptable, so the main cable is not the immediate problem.

Diode failure is also possible. The used alternator may have an open or shorted rectifier diode. A 0.90 V AC ripple under load is higher than the usual acceptable figure of about 0.5 V AC, and a failed diode reduces output by roughly one-third while increasing ripple. However, a DVOM's AC volts reading is not a reliable ripple measurement; it depends on frequency, waveform and meter bandwidth. The 0.35 V at idle is borderline and does not prove a diode fault by itself.

The most likely judgement is both: a worn belt and tensioner causing slip under load, plus a used alternator that may have a weak diode. The belt is the cheaper, more obvious fault and must be corrected first. Condemning the alternator before fixing the belt would risk replacing a good unit, but ignoring the ripple could leave a comeback if the diode is failing.

1. Inspect and repair the belt drive first. Check the tensioner, idler and pulley alignment. A tensioner at the limit of travel and glazed ribs mean replacement, not adjustment. Fit the correct belt, set tension to specification and re-test before condemning the alternator.
2. Verify the battery is charged and in good condition. The load test at 9.8 V is marginal but acceptable; charge it fully so the alternator test is not confused by a low battery.
3. Load the alternator in a controlled way. Run the engine at about 1500-2000 rpm and apply a carbon pile or the vehicle's electrical loads to draw roughly 60-70% of the 90 A rating, for example 55-65 A. Do not exceed the alternator rating or let it overheat. Watch the belt for slip.
4. Measure output current with a clamp meter on the B+ cable. A healthy 90 A alternator should produce a substantial fraction of its rating at test speed. Compare with the manufacturer's curve. Also measure voltage drop on the B+ and ground paths at that current; each should be low, typically under 0.3 V.
5. Measure ripple with an oscilloscope if available. A scope on the B+ terminal shows the rectifier pattern. A missing pulse, one pulse much taller or shorter, or excessive AC ripple after the belt is corrected confirms a diode fault. A DVOM AC reading above about 0.5 V is suggestive, but the scope is the decisive test.
6. Re-test after the belt repair. If the ripple remains high and output is low, replace the alternator. If the ripple falls and output is normal, the belt was the cause.

I would recommend replacing the belt and tensioner first, then re-testing the charging system under load. The squeal, glazed belt and tensioner at the end of its travel are direct evidence of slip, and slip alone can cause the low voltage and dim lamp. Fitting an alternator without correcting the drive would repeat the fault and could damage the new unit.

After the belt repair, I would load-test the alternator and check ripple with a scope. If output is still low or the ripple is still excessive, then the used alternator is faulty and should be replaced with the correct rating and pulley. I would also check the used unit's warranty, because it was fitted only three months ago. The customer should understand that the 13.9 V at idle is not proof of health; the system must hold voltage with loads. This approach avoids throwing parts and gives a defensible diagnosis.