GEN 1 Overcharging

[user65]

A friend and I picked up a M1009 recently. We’re a little lost on GEN 1 overcharging. We can’t seem to find a similar issue through search and after a week or so of testing we feel like we might be chasing our tails and a little unsure where to test next.

Before our ownership it appears the truck suffered a melt down. The fuseable links on the power wires on both alternators have been replaced with inline fuses. A 20 amp on GEN 1’s big red wire and 40 amp on GEN 2’s big red wire. During testing we replaced the fuseable link on the voltage regulators’ constant power wire or #2 on schematic E-9 with an inline 5 amp fuse as a temporary fix because it stretched when testing and lost continuity. We are planning to replace all inline fuses with fuseable link when it arrives. Other then that the engine wiring harness is original.

Currently we have two, new, bench tested DB electrical 100 amp alternators (isolated ground) installed and GEN 1 is outputting 90 V. GEN 2 is outputting 27 V. The ac delco’s (3 total) One came installed as GEN 1 read 64v so initially we ordered the db electrical that’s currently installed it is non isolated. The second one came with the truck in a box so we replaced the voltage regulator, tested, installed and read 27v when installed. it is isolated ground. The third came from GEN 2 and we haven’t tested that one except it is isolated ground as well.

Key on, GEN 1 and GEN 2 light is on.

GEN1 - drivers side
Key OFF
small red wire - 12.87 V
small brown wire - 0 V
Key ON
small red wire - 11.65 V
small brown wire - 11.30 V

GEN2 - passenger side
Key OFF
small red wire - 25.43 V
small brown wire - 0 V
Key ON
small red wire - 23.86 V
small brown wire - 23.32 V

Battery voltage
Truck off
Front - 12.74 V
Rear - 12.81 V
Truck running
Front - 11.73 V
Rear - 14.42 V

• Batteries have been maintained throughout testing.
• 20A Heater fuse in tact.
• GEN 1 ground checked, cleaned, crimps verified.
• Voltage regulator plugs on GEN 1 and GEN 2 are original and tested for continuity.
• Replaced GEN 1 light with new 168 bulb. Removed and installed socket a few times hoping to clean up anything there on the ribbon board.
• Engine wiring harness connector has been cleaned with electrical cleaner and a brush and cleaned of the black goo.
• The GEN 1 exciter wire has been jumped to test continuity per warthog’s advice found on the forum. From 12v engine block to engine wiring harness plug on ignition circuit. Jumped brown exciter wire from engine wiring harness plug and had acceptable voltage with key on.
• Tested GEN 2 diode and starter diode (under air filter with purple wires) and got continuity one way only.

Since we’ve been questioning alternators we built a bench tester, it consists of a mount, alternator bracket, switch, 168 bulb, alligator clips, wire and battery. We hooked up both DB electricals and read 14.8 at roughly 3000 rpms. Since it worked properly on the bench test we robbed the wiring portion from the bench tester and set it up on GEN 1 while it’s in the truck, disconnecting the small red/brown wire and connector from alternator. Running our jumper the alternator outputs 13.87 V at idle. During this test we went ahead and threw the big red wire with a 20 amp fuse back on GEN 1 and it blew with the 13.87 voltage output.

We seem to think that there is a problem with the small red and brown wire connected to the voltage regulator however we’re a bit stumped on where to trace as we feel we’ve gone through the schematics and what we've traced looks/tests good.

Using our bench testing setup with the alternator on the truck we’re further confused why the GEN 1 big red wire inline fuse is blowing even with the correct voltage output.

 

[Skinny]

I would think if the fuse is blowing with the correct voltage you have a short circuit. Or something causing a severe draw to the point of blowing the fuse.

 

[user65]

I would think if the fuse is blowing with the correct voltage you have a short circuit. Or something causing a severe draw to the point of blowing the fuse.

Appreciate the response, thanks!

Would I need to trace it on the 4x4, oil pressure, and heater lines? Do I also need to trace everything on the 12V ENG WRG Harness block as well? I guess I'm wondering where the next place is to start and isolate where this is happening.

It would seem the short would be somewhere on the exciter lines, small brown/red wires since the alternator is overcharging even without the big red wire connected to the 12v wiring harness block (fuse blown)

 

[Tow4]

During this test we went ahead and threw the big red wire with a 20 amp fuse back on GEN 1 and it blew with the 13.87 voltage output.

We seem to think that there is a problem with the small red and brown wire connected to the voltage regulator however we’re a bit stumped on where to trace as we feel we’ve gone through the schematics and what we've traced looks/tests good.

Using our bench testing setup with the alternator on the truck we’re further confused why the GEN 1 big red wire inline fuse is blowing even with the correct voltage output.

The "big red wire" you are referring to and have highlighted in the diagram is the B+ lead for the alternator. I would think a 20 amp fuse is going to be a little small for a 100 amp alternator. 13.87 volts from an operating alternator is low. The regulator set point is 14.8 volts. If the battery is low, it takes time to charge at idle. My truck reads close to 14.8 at idle.

Connect wires directly from the alternator outputs to the battery and see what voltages you get. Be sure to have the battery connected to the alternator if it's operating and don't disconnect the battery while it's operating. Load dumps will eventually kill the regulator.