I have a 99 been having a issue with voltage drop and replaced alternator with new battery the plug to the alternator is Geting no power. So I spliced into it and put it directly to the hot side of the battery. Light on dash did go away but still low voltage and you drive it 5 minutes up the road and it’s dead. Where does that harness go where am I losing power at

 

Here you go:

George

 

Okay so I looked at the schematic I still have no clue where that single wire from the plug of the alternator runs to its dead even with the ignition on. Still no power so where does it run in the loom? So that I can follow it back to the main source and see if it has power

 

The "L" terminal wire at the alternator plug goes to the VCM on a red wire to connector C3, pin 5 on the VCM.

Schematic attached.

George

 

A few more thoughts on the alternator L terminal.

1) Historically the L terminal circuit was Power> dash lamp> L terminal> field windings. This connection accomplished 2 things, first power to energize the field windings and start the alternator at idle and an indication if the alternator stopped producing power while the truck was running. Most of these alternators will start up if you raise the rpm even if the L terminal circuit is hosed.

2) Next up is the L terminal connected to the engine computer not a light bulb. Usually power is applied at start up to get the alternator energized as before and then the power is shut off so that the computer can monitor the output. The dash bulb is controlled independently. On many alternators this power excitation circuit needs a resistor which substitutes for the bulb resistance and restrict current or damage can occur.

3) The L terminal then started getting pwm digital control from the PCM to more precisely control output for battery life and fuel economy. The alternator does not run b*&&s out all the time and voltage output drops to a trickle charger level at 12,X volts during certain cycles. My 2006 GMC Sierra works like this.

GM is not clear in the service manuals for each year but I believe that your system is #2. You will not see power on that circuit all the time and be careful applying power directly to the terminal to start the alternator. The interwebs has the usual conflicting data and arguing on this point. Resistor, no resistor, a diode, etc. I would be safe and use a resistor, usually around 470 ohms, 1 watt. Your best bet would be first to check the continuity of the wire, unplugged at both ends, then look for a short to ground.


George