Chasing down electric problem

[LeatherNarwhal]

My 2002 ranger 4.0 lost electric power last week. I’m trying to find the issue. No movement from starter when cranking, even with charged battery and jump from a running car. When I turn the key to the on position the battery drops about 3.5 volts. No additional draw when I turn it to start. Have removed every fuse and the biggest draw seems to be coming from the fuel pump. When I put the key into the run position there is a humming coming from the top of the engine, but haven’t identified the culprit yet. The humming stops when the fuel pump relay is removed. When the key is moved to the on position the tachometer moves wildly. Speedo moves sometime too. I have tried running ground to frame and firewall, but no success. Does anyone have an idea of what could be happening here?

 

[RonD]

Welcome to TRS

"Hum from top of engine"
You could have an internal short in the alternator, rare but possible
Disconnect battery positive cable
Unplug the 3 wire connector on the back of alternator
Remove the nut and larger wire from B+(the stud/nut terminal on the back of the alternator), this wire is connected directly to battery positive so cover the end of this wire or MAKE SURE it can't come into contact with ANY METAL(ground)

Hook battery positive cable back up
Turn on key and see if hum has stopped and battery voltage doesn't drop


With fuel pump relay removed do you get the same key on voltage drop?

 

[LeatherNarwhal]

With the fuel pump relay removed the noise stops and the drop goes from 3.5 to about 2.5 volts. Disconnected the alternator didn’t seem to change anything, same drops, same noises.

 

[Dirtman]

What is the battery voltage before the "drop"? I.e not hooked up to the truck.

 

[LeatherNarwhal]

11.8 right now. Have been putting a charger on and disconnecting when I’m not tinkering with it.

 

[RonD]

Pull out the PCM relay or fuse, it powers all the engine solenoids along with the PCM(computer)
i.e. EGR solenoid, EVAP solenoid and IAC Valve solenoid