Battery discharges overnight. Have disconnected alt and have a draw of 8.5 volts off neg post. Found if I disconnect one of the yellow wires off the start solenoid positive side have no draw. Looking for circuit diagram for wiring.

 

Connect your meter again and start pulling fuses, that may help to isolate the fault.
Can`t help on the wiring diagram but it would help those that could if you included year/model/engine of the truck.

 

How old is the battery? Try completely unhooking the battery, giving it a full charge, and then see what it's reading. Anything less than 12 volts and it's dead.

 

yup I'm with this post 100%-that way ya can try to isolate where the drain is coming from-just pull one fuse at a time and check to see which circuit is draining the battery

 

1990 f-150 4.9 L. New battery. Pulled all fuses no change. Meter hooked to neg side battery post to unhooked neg. cable. Shows 8.5 volt draw.

 

Check your battery cables. I replaced my ground and the clamp on my positive. The positive wire looked new. I found that the positive at the starter had the insulation deteriorated from heat (?) back abount 3 inches. The bare wire was green. Apparently moisture created ground path to drain battery. I replaced the cable and battery holds charge and starter cranks like new.

 

This means nothing.

You need to use an ammeter between the negative post and cable to measure for a drain on the battery. You shouldn't have more than a couple hundred MILLIAMPS of draw. You'll need to pull the bulb for your underhood light, or have someone hold in the switch, everything needs to be off.

Also,

1. What is the voltage on the battery after charging it overnight?
2. What is the voltage on the battery with the vehicle running?
3. Pop off the vent caps on the battery (vehicle not running). There should be six holes with acid showing. Working from one side to the other, stick your voltmeter probes into adjacent holes and measure the voltages (i.e., 1-2, 2-3, 3-4, 4-5, 5-6). They should all measure about 2.1V or more.

Jason

 

Check for stuck relays both fuel pump and ecm. does the fuel pump prime when u hook up the terminals with the key off?

 

Found bad ecm relay. Thanks deeznuts822 and others for the input.

 

glad u figured it out