'86 930 no charge until 3,000 rpm

[chuffysplace]

My 86 930 does not charge until 3,000 rpm. This issue was intermittent, now it does it all the time. I cleaned the drivers side rear engine harness but I don't think that was the issue. The ground strap looks secure too. However, The volt meter light does not light when the ignition is on. I have heard this could be root cause. The bulb is good, the socket and its immediate connections look good, however the positive lead for the bulb socket has something else hooked to it, I've read this could be a resistor or a diode. One of those connections to that device, does look burned or corroded (see attached, right side of the picture). My first instinct was to try to separate that device, whatever it is (diode / resistor) and fix the connections to it. Can anyone offer an explanation on any of this and also an idea of how to fix it. I'm not sure what the device is called, and that would be helpful if I need to find a replacement. Thanks everyone.

[rick-l]

its a diode. Do you have a meter to test it?

[rick-l]

Is the wire that looks white in the photo blue with red? All those RT/SW wires are tied together and look like they should have 12 Volts on them.

[john walker's workshop]

The brushes are probably worn real short and barely touch the commutator, which is probably worn also.

[chuffysplace]

Thanks everyone. Ah that is good to know - a diode. Yes, I do have a meter to test it If I remember correctly, it should allow connectivity one way but not the other. Hopefully it's just the connection(s) to the diode that is faulty - it sure looks it. I didn't want to rip into it without knowing at least something about it, this helps. I suppose worst case if I have to replace the diode I can hunt one down too. Thanks again.

[rick-l]

Usually a diode function on a meter shows the forward voltage of the diode one way and open the other.

It could be the diode but John Walker's analysis is a lot more likely.

[DRACO A5OG]

+1, worn brushes on VR

[chuffysplace]

Is the VR part of the alternator on these cars? The reason I ask is, the alternator was replaced. The car had this issue before and after it.

[rick-l]

Originally Posted by chuffysplace

Is the VR part of the alternator on these cars?

Yes it is.

When you rev it to 3000 does it keep charging till you shut it off?

Wherever/whatever this is. The blue wire (below) on pin 61 goes to the 14 (?) pin connector in the engine bay pin 11 and on to D+/61 of the alternator. The blue with red wire on the other pin 61 goes to the light.

[chuffysplace]

ok thanks. Yes, once I rev it to 3K, it charges fine until the car is shut off - you can drive it for hours, with all the accessories on, a/c and all, it charges all the time - no problem (I installed a digital volt meter gauge to verify). When the motor is off, before I start it, at ignition on, the volt meter light does not light, when the engine fires, it doesn't charge. Once you rev the motor north of 3K, it starts charging and keeps charging until you turn it off. So that all said...sounds like a volt meter light issue, correct?

[rick-l]

Yes..... The theory being that there is enough residual magnetism in the rotor that at higher RPM enough current is generated to supply the field through the excitation diodes.

[chuffysplace]

Very good. Thanks again!