Would a bad diode in the alternator cause..

[John Barbieri]

I have a 79 1500. New battery, guages stopped working after a new starter was installed. Removed alternator, took it to a rebuilder and had it tested. Rebuilder says the alternator has a bad diode which will give fluctuations and sometimes give DC and sometimes ac power. Does anyone know if this bad diode would cause my tach, fuel gauge, other gauges, turn signals and radio to stop working?

 

[dllubin]

I have a 79 1500. New battery, guages stopped working after a new starter was installed. Removed alternator, took it to a rebuilder and had it tested. Rebuilder says the alternator has a bad diode which will give fluctuations and sometimes give DC and sometimes ac power. Does anyone know if this bad diode would cause my tach, fuel gauge, other gauges, turn signals and radio to stop working?

Did the rebuilder define what "Bad" is? It could be an open, a short, a high resistance, etc. The output of the alternator itself is AC. The AC is fed through a bridge rectifier with 4 diodes to convert it to DC (actually pulsating DC but the battery capacity filters it to DC). If the diode was open, you could be losing a half cycle of alternator AC output so the current capability would be substantially reduced. If a short, you might notice some smoke electrical noise on the output.

 

[ng_randolph]

...The AC is fed through a bridge rectifier with 4 diodes to convert it to DC..

6 diodes, actually. The alternator AC output is three phase.

 

[Rod Midkiff]

even with the alt disconnected the gauges would still work. It might have damaged something else, but fixing the alt will not fix the gauges.