I'm running a truck 105 alt in my Mustang/Lq9 combo and I have an issue that persists - alternator stops charging at high rpms, and comes back on as the rpms dips. I have a reman Autozone alternator. Got another one, same exact thing. Got a larger alt. pulley as reasearch shows that these things can be over-spun, same thing. I have a resistance wire to the "L" terminal, and I ran another wire with a light to the same terminal just to make sure the alternator is excited. Only thing left is to get an even bigger alternator pulley to slow it down further but nobody else seems to have an issue so it must be something I'm doing. Maybe I need a NEW, as in brand-new alternator???

Anything under ~6000 and it charges rock-solid at 14.4+ volts

 

I have a Delco 145amp alt that is brand new from GM that did much the same thing. Ended up putting the stock one back in the car.

 

What a bummer...I figured a new one might be needed but after hearing your experience now I dont know what to do.

 

My Alt just crapped out on me too! I am following this thread...

https://ls1tech.com/forums/generatio...failure-s.html

 

None of mine crapped-out...not totally anyway. Just shut-off at high RPMS.

 

Well still a good read

 

The problem is the cheap *** rectifier they use in the remans even the expensive remans are crap. Order a 4200hd rectifier with 70amp diodes and swap it into a autozone lifetime warranty alt. That's what I'm running.

 

I had that problem with multiple alternators; didn't matter if it was one from the dealer, Pep Boys, parts store, truck alternator, locally rebuilt, etc. Didn't go away until I installed a 2.75" underdrive pulley, but now I need to spin the motor a little faster to match the factory charging rate at idle. I never get 14.4V even on a 'new' alternator with 0/1 gauge wiring, typically it's upper 12's or low 13's. Thought about buying a Powermaster which is supposedly guaranteed to a alternator shaft RPM of 18K, but I'm not crazy about the inconvenience of potential mail order warranty service. Where do you order the 4200HD 70A rectifiers?

 

Look up DR4200hd-s .

 

what kinda rpms are you turning? So the cheap rectifiers stop charging at high rpms?

 

I took it apart out of curiosity and put an indicator on the slip rings. There's .002 of run-out. The brushes are spring-loaded which should follow the rings ok at low speed, but I could see this causing an issue at high rpms.

 

The rectifiers can overheat at higher rpms the nicer ones have a better heat sink.

But other issues could be brush run out and belt slip. Sounds like you might have a brush issue.

 

Wire the S terminal on the plug to your main power distribution block and see what happens.

Andrew

 

I actually did that and nothing at all changed...not even the amount of volts read at any given time.


So the question is, what's the best approach? New alternator say from Powermaster, get an even bigger alt pulley (currently 2.75", have to go to say 3.5") but worried about charging at idle.

 

Not sure why this would be a big issue,how much time are you really spending over 6K rpm?Is the battery voltage dropping off enough to cause a misfire or reduced power?

 

I noticed at some NASCAR events the battery would die and got me to thinking why. Because of the constant high RPM maybe the alternator isn't putting out for some reason. Finally came to the conclusion the brushes were being blown or bounced away thereby causing a no charge situation. What about more spring pressure? Either add shims or stretch the existing ones.

 

When your in boost and you have 2 fuel pumps, perhaps a meth pump and driving the ignition coils hard you need all the voltage you can get. My biggest concern is a misfire or out-of-time discharge of the coils at high rpms and wrecking my motor.

My last LQ9 blew a hole through the block at ~130mph for no obvious reason. One theory is that b/c the alternator was not charging, battery votage dipped down to 12.4v. Coulda had some kind of misfire that blew the engine apart (AFR, fuel, oil pressures were all good when she let go).

 

I was think about more tension on the brushes but not sure how to go about it w/o causing other issues. I could stretch the springs but these springs I'm sure deal with immense heat and not sure if stretching them would damage or weaken them after a couple of heat cycles.

 

I called Powemaster and spoke to a real nice fellow over there, and he feels I just may plain be running outta amperage at high rpms between the injectors(low imp), 2 fuel pumps, EFI, elec fan and 2 fuel pumps. He suggested a 165a alt that will drop-in.

Do you guys think 105a that I currently run insufficient? The fellow doesn't think this alternator is even putting that out!

 

Do you guys think 105a that I currently run insufficient?
Add up the load, and see where you are w/ the 105A unit.