charging problems

[cjannelli]

My 87 carrera started occasionally showing high voltage(voltmeter plugged in the lighter
socket). At times from 15-17 volts. It would suddenly jump up, then back to 13.9 and stay there consistently only to have it happen off and on. I purchased another voltmeter and verified the overcharging. I just replaced the VR. Now the red light comes on and stay on until 4K rpm. The voltmeter registers 12.3
Help!!
Chris

[pete3799]

Did you have the alt. tested while it was out?

[cjannelli]

No I didn't, but I think I should have. I've charged the battery, checked the batter connections.
The indicator light is bright at Idle, dims by 2K rpm and out at around 3K rpm. Fan belt is tight and adjusted. I was hoping charging surges were due to the VR but the alternator may be the problem.
I guess now that I've taught myself how to disassemble the fan housing etc, doing it a second time should be easier.
Chris

[HawgRyder]

An altenator is realy a 3 phase AC generator.
Each phase has 2 diodes stacked in a series circuit and fed in the middle of each pair by the windings (AC).
So...if any diode goes bad (open, rather than shorted) you will get all sorts of problems....over charging...under charge...etc.
The main problem is: when a diode fails...it creates a situation of intermittant charge.
The regulator sees the varying voltage but has trouble keeping it stable....so you get a spike or a valley in the voltage to the battery.
All connections inside the alt itself and any wires coming out can also cause this problem.
When they test a unit...they should do it under load and for several minutes...to get the alt to heat up a bit.
This proves out the internal connections and diodes.
Bob

[cjannelli]

Before I installed the new VR, the red indicator lamp never lit up w/the engine running. Why
would it occur now w/ a new VR? Did it unmask a damaged alternator?
Chris

[cjannelli]

Replaced the alternator. All is good, Thanks for the help!
Chris