If one of the rectifier valves is damaged, the entire rectifier unit must be replaced.
A short circuit in the rectifier unit valves can be checked without removing the generator from the vehicle, by first disconnecting the wires from the battery and the generator and removing the protective cover. Also disconnect the wire from the output «IN» voltage regulator. For a generator with an old voltage regulator, you must also disconnect the output «B» voltage regulator from terminal «30» generator.
You can check with an ohmmeter or with a lamp (1-5W, 12V) and battery as shown in Fig. 7-8.
Pic. 7-8. Schemes for checking rectifier valves:
1 - battery; 2 - control lamp; 3 - generator; I - check at the same time «positive» and «negative» valves; II - check «negative» valves; III - check «positive» valves.
Note. In order to simplify the fastening of parts of the rectifier, three valves (with red mark) create on the body «plus» rectified voltage. These valves «positive», and they are pressed into one plate of the rectifier unit, connected to the output «30» generator. Other three gates («negative» with black mark) have on the body «minus» rectified voltage. They are pressed into another plate of the rectifier unit connected to «weight».
First check if there is a short circuit at the same time in «positive» and «negative» valves. For this «plus» connect the batteries through the lamp to the clamp «30» generator, and «minus» - to the generator housing (pic. 7-8, I). If the lamp is on, then «negative» and «positive» valves are short circuited.
Short circuit «negative» valves can be checked by connecting «plus» batteries through a lamp with one of the rectifier unit mounting bolts, and «minus» - with generator housing (pic. 7-8, II). Burning lamp means a short circuit in one or more «negative» valves. It should be remembered that in this case, the burning of the lamp may also be the result of a short circuit of the turns of the stator winding on the generator housing. However, such a malfunction is less common than a short circuit of the valves.
To check for a short circuit in «positive» valves «plus» connect the batteries through the lamp to the clamp «30» generator, and «minus» - with one of the rectifier block fastening bolts (pic. 7-8, III). Burning lamp will indicate a short circuit of one or more «positive» valves.
An open in the valves without disassembling the generator can be detected either with an oscilloscope or when checking the generator on the stand for a significant reduction (by 20-30%) the magnitude of the output current compared to the nominal. If the windings, additional diodes and the generator voltage regulator are in good order, and there is no short circuit in the valves, then the reason for the decrease in the output current is an open in the valves.