The Stovebolt Site Forums The Shop Area The Electrical Bay Do I need a shunt for a 60A alternator

Hi again. Figure I would start a new post instead of hijacking my other one. My truck is a 1959 Apache: in the middle of installing a new complete wiring harness, 60A one wire alternator. While at a local SoCal speed shop, the guy behind the counter said it would be a good idea to install a 12ga shunt between the Ammeter Posts to tame the potential current accross the Ammeter. I'm not going to run a ton of accessories other than a heater and electric wipers. Do I need this shunt? I was told the alternator will only put out what the battery requires to charge and what is required to run the accessories. If I'm not running a bunch of accessories am I in jeopardy of pegging or ultimately damaging the Ammeter?

Do I need this shunt? I was told the alternator will only put out what the battery requires to charge and what is required to run the accessories. If I'm not running a bunch of accessories am I in jeopardy of pegging or ultimately damaging the Ammeter? [/quote]


Yes!

Wow, you are very animated about this. Is this something you have done to your truck because something happend to your ammeter? If you have done this, how did you tackle loosing two posts on the ammeter for the shunt, leaving only two posts to hook everything else to?

As BC59 says you do need a shunt if you aren't going to peg the ammeter needle when you start your truck with a slightly flat battery.

Just connect the two wires from the shunt to the ammeter. then connect the other ammeter wiring to either side of the shunt. Look at this photo, the two wires go off to the ammeter, the connector block takes the other wiring which would normally go to each side of the ammeter itself. BTW, the shunt in the photo was made for a 20A ammeter on an old Austin car. you need 3 100W 1 ohm resistors in parallel to shunt the ammeter on a chevy truck, I run this setup on my truck with a 68A alternator.

Wow, that's way different than the 12ga wire I was going to use as a shunt. Why would that guy tell me to use a plain old 12ga wire instead of the 3 resistors you are advising to use??

A simple, and very effective shunt can be made from a piece of steel sheet metal about 3/8" wide, 1/32" thick, and long enough to bolt to both ammeter posts. Enough current will bypass the ammeter to keep it from pegging out on a heavy charge, and there will still be enough current flowing through the ammeter to make the needle deflect a little, indicating charge or discharge. There's no need for a terribly sophisticated electronic device when the ammeter is going to be turned into an inaccurate charge/discharge indicator by whatever shunt is installed.
Jerry

huh ... my 58 with a 63 amp alternator hasn't fried the ammeter, or even pegged it, and I've run the battery down good using the starter to move the truck across an intersection when it ran outta gas, then had to crank it long enough to get fuel flowing again .... maybe you guys are talking about the older AD gauges - or the wimpy gauge Stuart cites on the Austin

I've always converted my old trucks to alternators and never shunted the ammeter - no problems

Bill

How's this, this is what I initially wanted to do. It's a 12ga wire with two spade connectors. If it needs to be a bigger gauge, I can go to a 10ga
http://i1236.photobucket.com/albums/ff441/AZ59apacheguy/IMG_1144.jpg

The trick is to make the shunt steel, not copper. It has enough higher resistance that a small amount of the current still takes the path of least resistance- - -through the ammeter. This causes the needle to deflect. The shunt simply allows enough of the current to bypass the ammeter that the needle doesn't peg out in high-current situations.
Jerry

Thanks for that, here's a picture of the non-wimpy stock Chevrolet 40A ammeter pegged on my '50 3100, it was like this when bought the truck, the PO obviously didn't think that an ammeter shunt was necessary when he fitted a 68A alternator.

Any device which shunts the ammeter with a low resistance will stop it pegging, but the lower the resistance you use for the shunt, the less the needle will move. If you use a short length of large diameter copper wire, the needle may not move at all, so not really worth having the ammeter in circuit IMO.

I used a shunt with known resistors rather than a length of wire/piece of sheet metal because it allows you to accurately calculate the FSD (Full scale Deflection) of the ammeter, I used 3 100W 1 ohm resisors in parallel which gave an FSD of 70A on my truck; and yes, the ammeter needle goes right the way over when I start the truck after leaving it for a few weeks.

so maybe we're talking the difference between gauges of the different year series, I don't find a rating shown for the 55-59 gauge anywhere [series in question here], but I know there was a 55amp HD generator available, so I'd expect the gauge should be able to handle that with a margin of safety .... and I've never seen a TF gauge peg under any conditions, although I'm sure a defective regulator could do it

but good tip on the shunt sizing for those that think it's needed, although 'upgrading' to a volt meter might be preferable

Bill

Where can I get these resistors, I just went to Radio Shack and the greatest wattage resistor they had was 10watt.

[quote=AZ59apacheguyWow, you are very animated about this. Is this something you have done to your truck because something happend to your ammeter?


No, not on my rig. I've been a G.M. dealership mechanic for 30 years & counting.

US Ebay link
These should do the job, but I would use branded USA/Brit made resistors for choice though.

I meant no offense, I was actually being serious.

I got a 30 amp panel meter with shunt from

Centennial Electronics, Inc
2324 E Bijou st
Colorado spings, Co 80909

800 320 4666
719 633 4666
www.centennialelectronics.com

The # is AIM 6030A. The shunt for this meter would bypass enough current to protect your ampmeter. I think it cost me about $20.
Install the shunt in parallel with the posts on your ammeterand it should work fine.

I have a spare if you cannot get one from Centennial.

I meant no offense, I was actually being serious. [/quote]
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None taken. I saw you post w/ no reply at that point, too sleepy for good answer, just wanted to make the point. Many others have given you the info needed to keep from having a wiring malfunction, a.k.a. letting the smoke out.