2010 DC electrical issues

Hi all, looking for suggestions/info on a problem I'm attempting to help a friend sort out.

The backstory:
Friend goes out to lunch in his truck from work. It drives there fine, and same on the way back to work. 15 min later he hops in the truck to find it won't start. Apparently a dead battery.

They jump (battery to battery) it with another vehicle and he drives to some errands. After the second errand the truck won't start and idle. It starts, runs for a second then dies. There's also a clicking from the engine and trans that sounds like a sensor/solenoid repeatedly going through power up then shutting down.

Long and short he had it towed to the local dealer who spent 7 hours diagnosing with no answer, and no real direction to take things in. He's since brought the truck home and asked me to take a look.

I spent the better part of a weekend and have found the following:

The truck runs fine with the alternator sense plug disconnected, but obviously doesn't charge. The second I plug the sense plug in, it starts charging but dies within a second or two. Charging voltage is also high (like 14-18v). We've replaced the alternator with no change in issue.

There is also an intermittent short to ground on the GREEN wire on the sense plug. I spent the entire weekend tracing the circuit back and isolating it, and found that the ground short appears to be coming through AM1/AM2 fuses from the starter. As it's intermittent and only seems to show up when we've attempted to run the truck with the sense plug connected I'm guessing it's only a symptom existing from another issue. It seems to clear with pulling the battery leads for 10 minutes.

There is one thing I'm hoping for an answer on though. Checking the sense plug wires with a multimeter yeilds battery voltage on the white wire, battery voltage on the green wire (when it isn't shorted to ground), but about 10.1v on the grey wire. Looking at wiring diagrams indicates the grey wire runs to both the combination meter (gauges) and also the body ECU (behind the drivers' side fuse pillar). If I unplug the body ECU the voltage goes to 0.

Does anyone know if the grey wire is supposed to have more than 5 Volts, and whether the body ECU actually controls alternator output? I know most alternator systems use 5v reference to enable/disable charging. Just hoping for more info on that.

Any input or suggestions would be appreciated! We're both frustrated and I want to be absolutely sure of my diagnosis before sending him to get a body ECU.

 

The grey wire is connected to the alternator voltage regulator and shorts to ground when the voltage regular senses a malfunction in the charging circuit. Normally I'd expect battery voltage ~12V at the grey wire that will keep the charging light off with respect to ground.

http://wilbo666.pbworks.com/w/page/39441708/Toyota Alternators#AlternatorRegulatorSTerminal

Edit: This may help as well
https://www.customtacos.com/tech.ol...f/06toyrm/06toypdf/06rmsrc/rm2006ta/02400.pdf

 

Ive seen the connector to the alternator corrode.

 

Thanks for the quick reply. I really appreciate your input.

Reading through the first link you posted and comparing it to my harness colors I think I see an issue. With the alternator sense plug disconnected I'm still getting 10v to the grey wire, and it doesn't seem to change state plugged in. It's almost as if something is feeding power down to the alternator via the grey wire. Is it possible that the bcm or gauge cluster has a diode that failed and is sending voltage back to the alternator via the grey lamp wire?

The short to ground on the green IG wire still puzzles me. I've isolated every bit of the circuit it ties to and the last place I got before the short disappeared and didn't reproduce was by the battery cable. Still very confused.



It's definitely not a corroded connector.