$20 Upgrade! GM Diode Mod With In Line Fuse for AGM Charging

Did the GM diode mod to bump up the alternator voltage for my AGM battery as described here: I used two fuse taps to retain the 7.5A fuse in line with the diode at the ALT-S location. Parts list:

• Diode GM P/N 12135037.
  
• Mini blade fuse tap Titan P/N 85764, Littelfuse P/N FHM0200ZP, or equiv. x2. The low-profile type mini fuse tap didn't fit in my fusebox, due to surrounding items in the way.
  
• 7.5A mini fuse. The Littelfuse kit above includes one. The OEM low-profile fuse doesn't fit in the mini blade fuse tap.

Installed and tested:

 

What AGM are you running? Been thinking about it myself and got a voltage booster from Arclight but haven't decided if I need AGM.

 

I bought this little guy. Blends right in and ups the voltage while still keeping the 7.5A protection

 

^Yes more elegant but spendy solution. I didn't want to pay a portion of the cost of a new battery for the fused diode lol.

Local store brand 640CCA group 35 battery that I got on short notice when my alternator died earlier this year. Go with EverStart Platinum AGM if your local Walmarts stock it.

 

What are the advantages of going with an AGM as opposed to a lead acid in the 4runner?

 

^More cranking amps for a given size, or lets you downsize (reduce weight) while maintaining the same performance. For folks that go off-road, at steep angles the flooded battery can spill sulfuric acid out of the vents onto the inner fender. AGM batteries are sealed, so they don't have this problem.

 

That is what I have. I like that most anywhere you might have an issue, there is likely a Walmart nearby-ish.

 

I have read that you have to trickle charge every now and then. True?

 

I don't know it, but I think maybe yes.
Mine crapped out at 2 1/2 years on a 4 year warranty. I also have the arclight booster which I just spot checked, and for me it puts out 14.25 on high setting. So I will see how the replacement holds up. I did not have the booster for the entire life of the battery, but most of it. I'm intrigued that the diode guys are showing better than 14.5.

 

The point of the GM diode mod is to raise the alternator charging voltage to a level AGM batteries are happy with, so "top up" with a battery tender isn't needed.

I plug in the tender when the vehicle sits for a week or more, regardless of battery type, just so the vehicle's functions like evap leak tests don't drain the battery. My CTek tenders have a selectable AGM setting.

 

@3JOH22A do you know the voltage before and after?

 

^When I installed the alternator this spring, I measured 14.2V at idle shortly after start. Ambient ~70F. According to kmeeg, the voltage drops once you start driving.

With the diode added, I measured 14.7 V at idle shortly after start, as shown in the picture. Ambient ~45F.

If you have halogen lights, the 0.5V increase will make them measurably brighter (and shorten bulb life).

 

How does a diode increase voltage?

 

Anybody put a second sealed type battery in the rear hatch area instead of normal second battery under the hood? I got zero room left under the hood but wouldnt mine a second one for boost or storage. Unless anybody recommends anything else? I have a compressor on passenger side rear underhood area and a SPOD tray and breather KN stuff and air tank on drivers side so no...room. Thanks

 

ALT-S fuse name denotes the alternator sensing circuit. Adding the diode creates a voltage drop in the circuit. This tricks the alternator into thinking the voltage is lower than it is, and therefore increasing its output to compensate.

 

That makes sense, thank you for explaining.

 

with the increased duty cycle time of the alternator how much life off the alternator do you lose?

 

^What do you mean? The alternator is always running any time the engine is running. There's no clutch or swash plate like with fluid pumps.

The only components that will have their life measurably shortened are filament light bulbs, by about -20% using the power-of-seven rule.

 

That’s right. The forward biased silicon diode has a typical voltage drop of 0.5 to 0.7 volts. The alternator compensates by boosting its output by that amount.

 

So I decided on a Duracel AGM so I get it and finally bust out that Arclight voltage booster I bought 3 years ago and it’s a dud. Any selection other than off causes a charge light and the voltage doesn’t change.

 

This is what I have. Works great on high. Starts at 15v drops into the 14s after a little bit
https://voltageboosterpro.com/product/og/

 

Ordering it now. Just sucks that I had this thing for so long so any warranty is gone. My own fault though.

 

Yeah thays real lame dude

 

All my ArcLight products have failed shortly after warranty. If you go to their site now, they don’t have anything listed other than NSX/S2000 products.

 

Ah that sucks. I have their full interior lighting and they work flawlessly.

 

I'll just leave this little gem here:
https://www.youtube.com/watch?v=Fl8GxeGCppg

 

Thing is I had issues keeping an Diehard Platinum AGM charged in my Tacoma. It failed prematurely so I went back to lead acid. At the time, no one considered the voltage output. When you look at AGM info on the manufacturer's site, they list them as needing higher voltage than lead acid to charge. I don't think adding a voltage booster get's rid of the 3 tiers of charging and only increases voltage at each tier. Also I'd only set it at low which only increases voltage by 0.5.

 

Is there a TLDR?

After installing my yellowtop I had charge issues.

Ive even had issues after installing a voltage booster but putting it on the low setting, afraid of too many volts.

Then checked the manufacturer's specs and realized that high is within specs. Put her on high and its been good so far.

 

TLDR - voltage boosters not needed, most of the information for higher charging voltages comes from mixed information in off grid or other non standard vehicle use cases.

The Optima Yellow top states a charging voltage well within the Toyota 4runner standard specs: https://www.optimabatteries.com/charging
"Recommended charging information:

• • Alternator:
    • 13.65 to 15.0 volts, no amperage limit."

A bit of Google Gemini generative AI stated:
"
The standard charging voltage for a 5th Gen 4Runner is typically between (13.2)V and (14.8)V, though it often starts around (13.8)V when cold and may drop to (13.2)V or lower once warm. With the engine off and everything powered down, the voltage should be between (13.2)V and (14.5)V, while with the engine running and accessories on, the voltage should be between (13.8)V and (14.5)V.
"


I would guess that most people adding a voltage booster have another issue being masked by the addition of the voltage booster. Or there events like full discharges that lessened the life of the AGM battery.

Here is the logic - AGM batteries are sold on the shelves of the nations largest retailers. The products do not come with large orange / red warning labels "STOP - this battery requires additional vehicle modifications." or similar. And we all know consumer protections are in place (see hot coffee warnings) because of the fear of litigation.
Johnson Controls (now Clarios), Exide, and East Penn are basically the 3 companies making batteries, and they make a wide range of products for a wide range of applications. AGM batteries sold for use in a vehicle as a starting battery are designed and sold for the lowest common denominator for function and for consumer knowledge. Thus when you go to your local or online seller and buy an AGM battery designed for a passenger vehicle it will "just work".

Now where people get themselves into a bit of a hassle is when they go searching for a solution that may not be so common. Perhaps marketed as race, heavy duty, extreme, or similar. These types of solutions are NOT typical and thusly come with certain additional requirements to work to peak performance and their documentation states it. Now that same person will go online and say "I bought the FANCY 1000 battery and it wouldn't charge -- I had to add the up the charge voltage, I had to add an external charger, .... " see AGM requires a voltage booster.

You can see how a bit of mixed information soon leads to a stated fact that simply doesn't hold true.

One small anecdote:
Recently when at Costco looking for a battery, the person typed in the vehicle info, and it spit out both the standard lead acid and the AGM variants as options. When I asked the person for any special vehicle notes or cautions on the AGM product recommendation there were none. This is a major retailer selling thousands of batteries a day, and if the manufacture had special requirements it would be listed. The same will hold true at any auto parts store.

Manufactures don't want the hassle, they want the sale and happy customers (and no litigation like a class action lawsuit stating battery ABC works in all vehicles when it doesn't).

In summary - there could be a use case for a device to voltage boost alternator output - however those use cases are not universal and surely not required for every AGM installation in a 5th Generation 4Runner.

 

^"Required"? No. You can run the battery at 80% capacity at stock charging voltages for years with no issues. I did that with Optima Red Top in two Tacomas for 6 years. I wouldn't pay $100 for a voltage booster. But for less than $15 like I did, it's an easy choice.