High Output Alternator and Big 3 Question (Answered)

[mikez71]

thx, good to know! I like your wiring plan.

 

[gmartin1215]

the mega fuse really only protects the wire from the alt to the battery side of the fuse. incase it grounds out and dead shorts. or if the alternator internal shorts or something. if say from the battery postive terminal to basically any ground point anywheres touches, the battery can discharge way more than 300 amps and that fuse won't below. there's no protection provided by that meg fuse to anything else in the car. that's what the fuse block full of small 5 to 30amp fuses are for.

things like modules don't use more power because the source can supply more amps. you'd have to increase voltage to force more energy into something to damage it. like say putting a 24v alt on a 12v system would fry stuff, but never blow a fuse because it's voltage, not current. fuses are basically there to pop if there's a dead short and protect the wiring between the fuse and the short from catching fire.

to be honest, if you out a amp clamp meter on that B+ wire off the alt, with the truck running and read the current and then change the alt out and test again. unless your current alt can't keep up with the load you have on it now, you won't seen any difference in amp load on the meter. alt will only supply what is needed.

ok. Thanks for that explanation.
Then, it sounds like I should be OK just leaving the 175a fuse alone since it only protects the the alt to B+ wire. If I am running the larger 1/0 wire, then the fuse will blow sooner only if something happens to that wire, or should I still consider getting a larger fuse since alt is producing 250+? I wasn't sure how much protection that fuse was providing to stuff downstream and if it would start frying out fuses in the fuse box.

 

[j91z28d1]

I would put the larger fuse in it. not saying you will bit say you someday needed the alt to supply max power, a fuse is basically just a smaller wire. so they can be a restriction and run hot without blowing. like I've seen a 20amp fuse melt the plastic of the fuse and discolor a fuse box from running headlights that pulled around around 20amps. so I'd run that 300 especially since you're upgrading wire size that can easily handle that 300 amps the fuse would blow at.

 

[Doubeleive]

Here is another thing for you to worry about and stress over.

High output alternators don't charge well at idle unless you spin them faster by using a smaller pulley.

Just thought I would throw that into the mix for you to get a hundred more opinions on.... lol

that is the reason I bought the mechan model I have because it is guaranteed to output *** at idle.
the even higher output models are limited to a specific max rpm (due to pulley size), and I drive like batman on steroids so didn't go that route.

 

[gmartin1215]

I finally got someone at MM who was willing to share more detailed explanations for their wiring suggestions/recommendations.

Here is what they said:
Why the extra 0-gauge cable is safe and how it works with your factory wiring.

1. Keep ALL Factory Wiring & the 175 A Megafuse

• Do NOT remove or bypass anything.
• The 175 A megafuse in your underhood fuse block stays exactly where it is.
• This keeps:
  • Factory short-circuit protection
  • ECM current sensing (so your dash gauge stays accurate)

2. The 0-Gauge Cable = Supplemental Path (NOT a Replacement)

You’re adding a second charging path — the new 0-gauge runs in parallel with the factory wire.

Parallel conductors share current inversely proportional to resistance (Kirchhoff’s Law)

How the current splits (real example at full 370 A output):

Path​	Gauge​	Approx. Length​	Current It Carries​
Factory Wire	~4 AWG	30–36"	~70–100 A (20–30%)
New 0-Gauge	0 AWG	18–24"	~270–300 A (70–80%)

Bottom line: The factory wire never gets hot — it only sees a small fraction of the total current.

3. Add a 300 A Fuse on the New 0-Gauge

• Install a 300 A ANL fuse within 12" of the battery positive post.
• Why?
  • Protects against shorts or alternator failure
  • Meets safety standards (NEC/ABYC)

4. Grounds

• Your 2009 Tahoe has three solid factory grounds:
  • Battery → frame
  • Engine → frame
  • Body → frame
• These handle over 400 A when clean and tight.
• No upgrade neededunless you have:
  • A winch over 300 A
  • A stereo pulling >500 A
• Just clean and tighten the stock grounds — you’re set.

5. MM-VC1 Controller – Simple & Plug-and-Play

• Gives you manual voltage control (13.8V – 15.0V)
• Takes over from the ECM — no codes, no issues
• No rewiring or ground changes — just plugs into the alternator’s 2-pin connector, switched ignition source and a ground.
• Works perfectly with your stock setup

Quick Summary – What to Do

Step​	Action​
1	Keep all OEM wires + 175 A megafuse
2	Run 0-gauge from alternator → battery
3	Fuse it with 300 A ANL within 12" of battery
4	Clean/tighten factory grounds

General alternator instructions are hard to write perfectly—every vehicle, engine, idle speed, and country of manufacture is different. Some brands leave just enough overhead for mild upgrades; others don’t. Your Tahoe has solid grounds, but an undersized factory charge wire. PCM behavior varies too; this week alone, two 2016 Malibu 2.5L owners—one idling at 650 RPM, the other near 500—reacted differently to the same alternator. One-size-fits-all rarely works. That’s why your 0-gauge supplemental cable (fused at 300 A) is essential—it shares the load safely. Matching power and ground gauge never hurts, but your clean stock grounds are plenty.

This was from Bryan, one of their service techs.


Given that info, I may try their suggestion of just adding the fused 0-gauge wire from the alt to the B+ and leaving the OEM in place, and see how it goes. It is a more straightforward installation. I'll probably still upgrade the ground wire, even though Bryan says the OEM wires can handle the load. Mine have never been changed, so it's probably a good time to do an upgrade and make sure the contacts are clean and tight.

Anyway, just wanted to share this with you all.

I'll share some pics and info after I finish the install. And report the outcomes of this upgrade.

Thanks for all your help and input.

Sometimes it pays if you keep digging!

 

[Joseph Garcia]

Finally, a decision. All around the world and back to the original destination. Well, if you're happy and can put your shovel down now, then that's good for you.

You obviously feel better about your decision after taking your journey, and that's what is important at the end of the day.

 

[fozzi58]

I have a Mechman 400amp in my truck.

Tony is a bit of a jerk - i had multiple emails with him as well.

Our trucks are a little different because of the added electronics on the later generations, but here is what I did.

I DID REPLACE Alt to Battery cable with a 0 gauge cable without a fuse from alternator to battery.
I DID NOT replace the OEM power cable that goes to the starter.
I DID REPLACE all the ground cables.
I added a 3 post ground terminal to my battery
0 Gauge from Battery to frame.
8 gauge from battery to body
8 guage from battery to engine
Added 8 guage from frame to engine
Added 8 guage from engine to body (firewall)

All wire was OFC oxygen free copper). I used copper ring connectors and heat shrink tubing over the ends.

End result was that my headlights no longer dim when the 1000watt sub hits and all my electrical gremlins disappeared. You can check my post history here and on reddit (Agitated-Carry7579) but my truck would literally "reboot" while I was driving. That hasn't happened since.

 

[fozzi58]

Here is another thing for you to worry about and stress over.

High output alternators don't charge well at idle unless you spin them faster by using a smaller pulley.

Just thought I would throw that into the mix for you to get a hundred more opinions on.... lol

Mechman specifies that when you order. I told them my truck was modified and then they started asking more questions. They also stated that should I upgrade the motor to turn more revs than factory (e.g. rev the 5.3 to 7k+ rpm) that I would need to ship them back the alternator to put a different size pully on and other mods for it to be able to handle the higher RPMs and slower idle speed.

 

[gmartin1215]

I have a Mechman 400amp in my truck.

Tony is a bit of a jerk - i had multiple emails with him as well.

Our trucks are a little different because of the added electronics on the later generations, but here is what I did.

I DID REPLACE Alt to Battery cable with a 0 gauge cable without a fuse from alternator to battery.
I DID NOT replace the OEM power cable that goes to the starter.
I DID REPLACE all the ground cables.
I added a 3 post ground terminal to my battery
0 Gauge from Battery to frame.
8 gauge from battery to body
8 guage from battery to engine
Added 8 guage from frame to engine
Added 8 guage from engine to body (firewall)

All wire was OFC oxygen free copper). I used copper ring connectors and heat shrink tubing over the ends.

End result was that my headlights no longer dim when the 1000watt sub hits and all my electrical gremlins disappeared. You can check my post history here and on reddit (Agitated-Carry7579) but my truck would literally "reboot" while I was driving. That hasn't happened since.

Thanks!

How many of the ground cables from the B- were you able to fit through the RVC, and which ones? If you got that detailed in your other posts, then I'll check those out, too.

 

[justirv]

For my '02 Z71 (which doesn't have any charge monitoring, RVC), I went with a 245A alt and 0 awg to batt. While the chances of a direct grounding is slim to miniscule, I'm the type that invisions the front end crunching in a catastrophic accident. My original alt to batt connector had the original fusible link. I like to think the GM engineers put that there for a reason. My 245A alt has a max output of 270A. I decided to put a 300A ANL blade fuse (and holder) on a fabricated bracket extension of where the starter cable ties in. As far as Big 3 grounding, I ran a 0 awg from batt(-) to the engine block (alt mount), to the firewall at the oem connection point, and then dropped one straight down to frame rail, drilled and tapped for a M12 with star washers at all contact spots. I left all original grounds, running them in parallel to oem as supplemental ground paths. While others may say you don't need the Big 3, I say that ALL of my electrical devices run much better.
For those experiencing alt whine, typically your sparkplug wires minimize that; the hood ground strap not only provides a ground for the underhood light, it also helps to mitigate that (emi)/RFI. I found my alt at Tucson Alternator Exchange, great selection and reasonable pricing. This one also had a smaller pulley for great low RPM output. Belt size needs to be adjusted.

 

[fozzi58]

Thanks!

How many of the ground cables from the B- were you able to fit through the RVC, and which ones? If you got that detailed in your other posts, then I'll check those out, too.

One correction - i was going from my recall and went out to grab some pics.

The main ground from the batt goes thru the rfc and ground on the alternator not the frame. That was the original location I believe. Apologies for that.

Also since the K2XX has that giant distribution block on top of the battery, its not all fun and games so you have to make do with what you have. There were a couple of youtube videos I dug up showing a Silverado with a similar setup so I followed it.

The OEM power cable is a weird shape, I could not find a replacement to support a single 0 gauge cable to jump from the positive terminal to the distribution block. So I used the OEM connector and added a 2nd 2 gauge wire.

 

[Monz11]

I also did the Big 3, added a distro block near battery, and a 400 Mechman with smaller pulley. Mine is boosted and I drive like Wes! So stay the F away from St Pete, snowbirds!!!

The wiring is just as Fozzi and Wes said, pretty straightforward. Quit overthinking it