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Mechman Alternators

Voltage Drop To Battery Voltage At High RPM


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Sent the following to Mechman via their online form, but I figured I would post here to see if anyone else has any insight on this for me in the mean time while waiting for their response.

"Hello, I purchased one of your alternators, via Sonic Electronix, for my 2008 Chevy Silverado. It has the 5.3L (VIN-J) and the alternator model is the 270A 8302270. I wired it with all 1/0 OFC cable, ensured the ground from the alternator to the battery runs through the Hall Effect sensor, used a 3/8” shorter belt (Gates) and replaced the tensioner for a new one (Gates). All works great except one thing, when the engine gets to about 3000-3500 RPM the voltage drops to battery voltage, then once back to idle the voltage comes back up. I'm not sure if this is by design or if there is some belt slippage at high RPM. If it is the latter, I cannot tell since it is a new install and I see no indication of belt dust yet. I wanted to get your input before I start going out buying anything. I eventually am going to order Gates Green Strip belts since I cannot get them locally, but I want to be sure of the length before I do. Right now, I have brand new Gate Micro-V belts installed since I can get those locally. The stock belt is 94" (K060935). Right now I have the 93-5/8" (K060930) installed, but the 93" (K060923) will fit, however with the 93" installed the tensioner is almost maxed, unsure if that is a factor or not. If you think I need a shorter belt, I will go out and purchase the 93" and if that solves my problem, I will be ordering the 93" Green Strip version (K060923HD). Thanks in advance and I hope to hear from you soon so I can resolve this."

I should also mention I did try the 93" but never ran it with it due to the tensioner being almost maxed, so I returned it for the currently installed 93-5/8". I'm doing a new build to, so other than stock components, there are no aftermarket devices (amps, etc.) putting any additional load on the alternator other than a second battery.

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It's probably shutting off the alternator for high speed. My truck is a gmc envoy and doesnt have the stupid hall sensor on it like newer vehicles have and if I rev the engine above 4200rpm the alternator stops charging and I go down to resting voltage at the batteries. for my truck's size of the crank pulley and the alternator pulley I would be spinning the alternator at some insane speed and that is way too fast for the alternator and will make it break at that speed.

The way around that is either not to rev the engine that high or to put a larger pulley on the alternator that will slow down the rotation speed of the alternator and as a result also decrease the power output of the alternator at idle.

2004 GMC Envoy1 XS power S3400 batt under the hood and 4 XS Power D3100's battery installed in the rear by the amps0 gauge power wire from front to backAlpine iva-w205 touchscreen dvd/cd/mp3/ipod/am/fm/gps headunitSundown Audio SAX-200.4 amp for my mids and highs8 gauge speaker wire from amp to woofer270 amp Mechman AltRockford Fosgate T1652-s component speakersRockford Fosgate 3Sixty.2 sound processorRca's from Rockford Fosgate and Monster Cable14 gauge speaker wire for the mids and highs1 18" Ascendant Audio SMD Dual 1 ohm with custom Black & Blue carbon fiber and hand signed dustcapBox: 5.66 cubic feet net volume box tuned to 30.13Hz with 1.5" wide wooden dowels and 1.5" thick baffle1 DC Audio 5.0k amp wired to .5 ohms nominal with an imp rise of 1.35 ohms for the single AA SMD 18"Future Vision 8000k 50w bi-xenon projector HID's with 4300k 35w fog lightsLink to my build: Buildupdates/progress

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Just spoke on the phone with Richard from Mechman. He recommends going with the shorter belt and said the tensioner being almost maxed out shouldn't be an issue. I should note that the batteries are wired together and then a single ground cable goes through the hall effect load sensor to the alternator negative. I also have a ground cable going from the alternator negative to chassis ground. I did this because it was how things were wired up stock, only difference was in the stock wiring the chassis ground was from engine block to the chassis and the battery ground was from engine block through the load sensor to battery; which for my wiring I used the negative alternator post instead of going through the engine block. Anyway, Richard also mentioned that the additional ground from the alternator to the chassis might be causing the problem. I will try that next after the shorter belt, however after getting off the phone with him, It dawned on me that the ground from the alternator to the chassis should not be a problem as my understanding of the hall effect load sensor was to detect current flow to and from the alternator to the battery. If enough current is flowing from the battery to/through the alternator and the voltage is going down, then the computer should excite the alternator and start charging. If it is the other way around, then the alternator should be charging until the voltage is high again. So my negative wiring goes as follows...

Battery 1 -to- Battery 2

Battery 2 -to- Alternator (Through load sensor.)

Alternator -to- Engine Block

Engine Block -to- Chassis

I don't see how that would cause a problem at high RPM. Anyway, I will report back my findings.

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It's probably shutting off the alternator for high speed. My truck is a gmc envoy and doesnt have the stupid hall sensor on it like newer vehicles have and if I rev the engine above 4200rpm the alternator stops charging and I go down to resting voltage at the batteries. for my truck's size of the crank pulley and the alternator pulley I would be spinning the alternator at some insane speed and that is way too fast for the alternator and will make it break at that speed.

The way around that is either not to rev the engine that high or to put a larger pulley on the alternator that will slow down the rotation speed of the alternator and as a result also decrease the power output of the alternator at idle.

This has some sense to it. I only noticed this when accelerating to merge into traffic. The alternator performs fine at idle speed of 500 RPM and cruzing speed of 1500-2500 RPM. So it really is not that big of a deal for me. I just wanted to be sure that it was supposed to behave like this and that I did not have a bad alternator or did something wrong in my setup. So to make sure I understand, this would be my truck's computer telling the alternator to "cutout" to: 1) protect the alternator, 2) make more HP available during acceleration? That being said, I guess I would need to keep an eye on the RPM's when I pull anything with my truck. I think I am going to drive it for a while and see if I accumulate any belt dust before buying another shorter belt. Or wait to put some load on it and see if it slips, etc. Richard, from Mechman, also said the Gates Micro-V was fine, however once I nail down the length I need I will be upgrading to the Gates Green Stripe.

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I found this and i think it answers all my questions.

http://acdelcotechconnect.com/pdf/imtn_V12I305.pdf

I guess this new regulated voltage control system (RVC) is pretty elaborate. From what I know and what I learned, it seems the computer does not disable the alternator but rather steps the charge voltage down in increments (PWM signal) based on various input data it gathers (like an intellicharger). After reading it, I believe with the Mechman alternator spinning at a higher RPM than stock, the computer is kicking down the voltage earlier at around 3000 RPM because, well, I don't need that additional output yet! So it's reading my batteries state and since they are new and not really loaded yet, it is keeping them from being overcharged. I can only assume that once I put some amp load on it, and some big current is moving, I am willing to bet the computer will keep the alternator voltage up more at 3000 RPM. So I'm fairly certain the symptom I am seeing is by design.

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I ran the RVC thing by Richard, from Mechman, and he said he spoke to Josh, also from Mechman, after our call yesterday about it. Richard said that Josh has an '06 and it also does some of the same stuff. The RVC trying to be he most efficient as possible about it, since I have no real load on it and the batteries are fresh. Richard instructed me to let them know how it performs once I get some load on it. So with that said, I will report back with my findings once I get my stuff installed and wired up.

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