Alternator 1 overcharging

[reloader64]

The last time I drove the M1009, the GEN 1 light came on. I assumed the alternator was defective, so I ordered replacement internal parts. Today I rebuilt the alternator and reinstalled it. I started the truck, no warning lights. Then I noticed the voltmeter was reading in the yellow zone, and after about 15 seconds the GEN 1 light came on again. I got my multimeter and checked the battery voltages, about 14 on the rear battery, and 11.4 on the front. Went to the driver side alternator and tested between the main red wire and the ground, and read 70 volts! I shut the truck off, and read the driver side alternator again. Voltage steadily decreased to 0 after about 2 minutes. This is with the key off. What could possibly cause the alternator to put out 70 volts, but the battery is still reading 11.4?

Scott

 

[87cr250r]

Those two things aren't possible. The alternator voltage must be equal to the battery voltage unless they are not connected.

 

[reloader64]

I was thinking after I posted, I wonder if a wire broke somewhere causing to GEN 1 light to come on originally, then I installed a defective voltage regulator. I'll check the associated wiring when I get a chance. I saved the original parts, so I can swap them back in and check the output voltage again.

Scott

 

[Tow4]

Do this.

Testing the alternator on the vehicle

 

[reloader64]

Thanks, Ray. Good step by step instructions. I'll check that first.

Scott

 

[Keith_J]

70 volts? I didn't know that was possible with stock rotor. Something is definitely wrong and you know electron wrangling..
70 volts into a battery would boil it in no time, look at the big wire to battery.

The dash light indicates a regulator defect on case negative connected alternators, on an isolated negative, omitting the negative connection will cause these strange things to happen.

 

[reloader64]

Yeah, I looked surprised, too. Checked the meter 3 times to make sure I was seeing it correctly. It's a Fluke 115, so I consider it reliable. I was busy yesterday after work, but I'll try to check the wires tonight.

Scott

 

[Keith_J]

I had to look over the internal circuit diagram for the regulator to see what could cause this. The spinning electromagnet initially receives current through the dash trouble light. Once the output voltage is above the battery voltage, current doesn't flow through the generator light on the dash. All magnet field current comes from the diode trio, inside the alternator.

Now with a missing negative battery connection, the regulator senses a low charge state so magnetic field current is set to maximum by the regulator. And the source of the field current is the diode trio, from the main output of the alternator. The limit of the snowball effect? I guess 70 volts.

The regulator is probably fried. Classic regulators used old school transistors, probably Darlington, which fail open. Substitute a power MOSFET which fails closed and 70 volts could be possible.

This voltage is capacitive, meaning very little current is possible. Modern voltmeters have very low impedence.

 

[reloader64]

So apparently.... if the alternator positive cable is broken, you can get some funny voltage readings.

I checked voltage at the battery (key off)- 12.4 volts. Checked voltage from the positive terminal to the ground terminal at the alternator- 0. Checked from the positive terminal to the intake manifold- 0. (We are running out of options in the process of elimination. ) Started tugging on the positive wire and found a break near the firewall. Repaired that, now have about 14 volts running everywhere I should have it. A good solid 28 volts across the batteries, and no warning lights in the truck.

Thanks for the help, guys.

Scott