Bought a new alternator, installed it, pushing 15v to back megafuses but once it gets to megafuse 500amp it drops to 11 volts and isn’t charging.

ideas?

 

Please help 😢

 

If the alternator is producing 15 volts (measured at the 500A mega fuse), you need to check that the voltage on the other side of the mega fuse is 15 volts also.
There should be no discernible voltage drop across a good fuse.

If you have 15 volts on both sides of the 500A fuse, then you need to check the cable terminations from the mega fuse block to the battery for a bad connection or bad cable, to find out where there is high resistance or just track at what point the 4 volts is going missing.

 

I went out there again and confirmed I am getting 14 volts between the alternator positive wire and the alternator body as a ground.

I am getting way less voltage between the alternator positive wire and the ground wire at the back of the vehicle that connects to the vehicle body.

I will get a video tomorrow as well.

Can I run a ground wire from the alternator body to ground the positive at the back of the car?

NOTE** I got 14 volts at the front of the vehicle right at the alternator but 11 or less at the back.

 

You do not want to run a ground wire from the alternator body to the positive anywhere as that will effectively be installing a short circuit !

Check the voltage you are getting from the alternator body (alternator ground) and an under hood grounding point (ground studs behind the headlights or behind shock towers).
Just wondering if you have a bad earth in the engine bay somewhere.
If that is a good voltage still, then check the voltage at the starter motor positive terminal as the alternator connects there rather than going all the way to the battery directly.
You could have a bad connection or wire linking the alternator to starter.

 

I will do more testing tomorrow on more earth points. Thanks!

Will let you know tomorrow

 

SOLVED!!!

Took off the primary ground from engine to chassis, sanded it down and reconnected to body and now it’s pushing 14 volts.

 

Well done!
Does beg the question how the high current for the starter motor was still finding a way to a good earth........but a win is a win

If the problem comes back ......do check the alternator output wire that ties back to the starter motor power terminal.

Don't toss your original alternator as sometimes aftermarket alternators do not fully understand the variable charge command signal from the ECU, so you might need to refit the original alternator if you have different charging issues appear.

 

The old alternator is TOAST! It locked up completely and won’t turn. The alternator I installed is a used one off an X350 so no aftermarket worries!