Stock alternator up to the task? If not, how many Amps will I need?

Electrical and Ignition

  1. MRGTX

    MRGTX Well-Known Member

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    I'm running an OEM/remanufactured alternator in my car currently (pun intended).

    I am about to set up a pair of shrouded electric fans in place of the stamped OEM fan. Unfortunately, I don't have the specs for the fans. I bought them along with my Champion radiator (matched by the rep who seemed to know what he was doing) a few years back and never hooked them up. I also have a set of 55W Hella fog/running lights that I would like to run.

    The fog lights will need less than 5A when on. Not sure about the rest.

    Will a stock alternator do the job? If not, can someone recommend an affordable alternative? It doesn't have to look pretty.

    Thanks for any advice.
     
  2. yeah

    yeah Well-Known Member

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    From what I know stock mopar alternator is only a 60amp unit . I would think you will need something bigger to run two fans . But if you do decide to go the bigger alternator route you got to be sure that your wireing will handle the upped Amps , because fire bad haha .
     
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    • crackedback

      crackedback FABO Gold Member FABO Gold Member

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      The problem with stock alternators isn't the total output, it's the crappy output at idle and just off idle. I'd put an alternator on the car like a Denso that has better utput at idle speeds to keep up with any potential loads.

      2 Fans could pull 10 amps or 50+ depending on what they require. Where you pull power is a HUGE consideration on these cars.

      Not sure how you plan to wire things, I would run headlights and fans on relays using separate circuits from any of the OEM wiring. I can help you there building custom kits if you are so inclined.
       
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      • jos51700

        jos51700 Green Bearing thread connoisseur

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        Fire it up, turn on ALL your accessories, measure your amperage with an amp clamp and see what the usage is.

        Then shut it off, get in another car and start it, put it in drive and drive to the auto parts store, and buy as many relays and connectors and big ass wires as you afford, and relay that shit until your car has more electrically operated switches than ENIAC.

        Then look up bulkhead bypass, do that, and be sure to add the fusible link.

        Then get a bigass alternator (now with extra molecules) and install that, and you're good to go.
         
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        • jos51700

          jos51700 Green Bearing thread connoisseur

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          Dammit crackedback beat me to it
           
        • 67Dart273

          67Dart273 FABO Gold Member FABO Gold Member

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          Well it's all of that and more........

          The poorly designed ammeter circuit and problematic terminals in the bulkhead connector

          Most cars are NOT 60A most are 37 or 45 or so. And of course "that assumes" whatever alternator you have is actually putting out what "it claims."
           
        • MRGTX

          MRGTX Well-Known Member

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          Okaaayy...I’m getting the message that the stock wiring is not going to cut it. :)

          I thought the stock alternator (even if working properly) might not be up to the job but the fire hazard risk from over-amping thin gauge stock wires, isn’t one I hadn’t worried too much about. Thanks to all of you guys.

          Yeah! I can definitely use some help. Can you point me in the direction of a diagram? Better yet, do you make/sell kits?


          Lol...message received here. Bulkhead bypass is next on my search list. :)
           
        • jos51700

          jos51700 Green Bearing thread connoisseur

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          Doing the bulkhead and ammeter bypass solved so many problems in my car, it was unbelievable.

          A solid state regulator for the instrument cluster is also rectamended.
           
        • puregts

          puregts Well-Known Member

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          The factory ammeter wiring is questionable at best. They can barely survive going to the later 65A (square diode cooler back) alternator. In those systems total alternator output is going thru two 1/4" blade wire terminals in the bulkhead connector, twice before finding the battery on the other end. I would recommend any time you jump to a larger alternator to completely bypass the OE wire, leave it connected though, otherwise you loose power inside, and fab up a larger gauge wire directly to the starter relay then to the battery also. Instead of using the, or any Ammeter, use a voltage gauge. Anything above 13.1v engine running means it's charging. 14.0v is better but 15v or higher is bad; your boiling the juice out of your battery at that point.
          Case in point, 100A aftermarket one wire alternator feeding:
          Two 16" fans (20A each)
          Electric water pump (8A)
          Electric crankcase evacuation pump (30A)
          MSD (?)
          Fuel pump (?)
          An additional multi-strand wire went directly to the battery (a.k.a. starter since the batt's are in the trunk). This car's favorite pass time was overheating, limiting its range severely. Heaven help me if I turned on the headlights!
          I replaced the alternator with a 250A beasty (I have plans on EFI and a larger fuel system) replaced the add on wires with a 4 gauge cable in addition to the OEM wires. I ran everything I could turn on for a while and the overheating problem was gone. Voltage never drops below 14.0v. Besides everything else it has to keep two Optimas topped off.
          Do I need all 250A?? Probably not all of it, but I'd rather have more than enough than come up short.

          Hope this helps
           
        • 72bluNblu

          72bluNblu FABO Gold Member FABO Gold Member

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          I ran a 100 amp alternator in my Challenger for like 60k miles, all stock wiring, no issues. Now, the only additional load on that car was halogen headlights and a stereo with a couple of 6x9’s, so no real big amp draws. So, it’s unlikely it ever pushed serious amps through the system.

          I also run a 100 amp alternator in my Duster, again with the stock wiring (currently). Mopar 3 wire set up with an external regulator too. That car has a Ford Contour two speed, dual electric fan set up that can pull a whole crap ton of amps on start up (40+) or on the high speed fan setting. It also has a A/F gauge with a wideband o2 sensor, stereo, halogens, etc. It’s held up fine for a couple years now, although I did clean up the bulkhead connector and inspected the amp gauge. I run a continuous duty relay as part of a cut off switch and battery relocation, as well as separate relays for the fans. But it still has the stock amp gauge and wiring so at some point that draw goes through the ammeter and wiring.

          Now, I would not say that’s the ideal set up, not at all. And I do have plans to convert both cars with an amp gauge bypass and voltmeter. But it’s been awhile and nothing has melted or burst into flames, and I do keep an eye on the bulkhead connector for signs of trouble. The bypass definitely is the better way to go for longevity and safety, but the factory wiring can handle more than a 65 amp alternator if it’s in good condition. And yes, that’s a big “if”. I was fortunate and both cars have stock wiring that’s in good condition, not hacked to death stuff like a lot of these cars have.
           
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          • cdnEHbody

            cdnEHbody Well-Known Member

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            I think the issue really begins when a battery goes flat or gets weak, then that big alt will go full mullet tardation ouput and cook the stock barely good for 30a connectors etc,
             
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            • puregts

              puregts Well-Known Member

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              While you can get away with it, as you said, it is not the optimal set up and can bite you. Case in point: in 2012 I was in the Mopar Muscle "Real Street" competition and one of the other contestant's cars gave up (beautiful car!), shut off and wouldn't restart after getting off the freeway. He thought it was just a bad battery, so he called back to the pits, and we had to wait, while he had a friend bring a good battery out to where we were. He got it in fired up and was good for about hour and it did it again. Turns out the alternator wasn't capable of keeping up with all the electrical goodies he had running and he didn't have a whole bunch.
               
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              • Mattax

                Mattax Just the facts, ma'am FABO Gold Member

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                Yes - in a stock situation. When additional equipment is added to the battery side of the ammeter, that's another scenario when all those connections can get cooked.
                 
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                • BillGrissom

                  BillGrissom Well-Known Member

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                  The alternator's job is to keep the battery charged, say over a week of driving. The battery can meet the peak fan load by itself, and the fans shouldn't be running most of the time. So, answer depends on where you live and how much time you spend at stoplights in the summer so the fans kick on. I would first try your current alternator. Put the battery on a charger after a day, then week. If it takes little time to show "full charge", then your alternator is keeping up. You will power the fans via a relay, so current stays in the engine compartment and won't stress your bulkhead and ammeter any more than currently. Realize that your dash ammeter won't show the current consumed by the fans, just as it doesn't show current used by other under-hood loads (starter, horn, add-ons), so with fans running the ammeter might show "C" when actually the battery is discharging.
                   
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                  • puregts

                    puregts Well-Known Member

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                    Keep in mind this all depends on where you tap in for your power source. If you tap in between the alternator and the bulkhead - yes, the bulkhead connections, as well as the ammeter, won't see the draw. But, if on the other side of the harness, between the bulkhead and the battery it will see it and the load through the OE wiring will be there.
                    Most folks tap in at the starter relay, or the battery itself, as it provides an accessible and easy point to add a hot source lead (just like the factory did with a fusible link). Either of these will draw through the ammeter circuit bulkhead connections. The added draw translates to heat through small gauge wires and weak, at best, connections. That's why I add a heavier wire directly from the alternator to the battery and switch over to a voltmeter.
                     
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                    • crackedback

                      crackedback FABO Gold Member FABO Gold Member

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                      The alternator provides power for the vehicle while the engine is running. Battery is a storage device. Why do cars run at 13+ volts and you don't see static batteries with more than about 12.7 volts?

                      The welded factory power splice is between alternator and ammeter, that is the side the power is coming from. The residual is supposed to flow through and keep the battery charged, ammeter intended use.

                      I see a lot of people, including sellers of electrical items, suggest pulling power from the starter relay. That is a HORRIBLE spot to pull power on most every mopar with OEM wiring. Where you pull power depends on a few factors. In some cases there is no perfect point to pull power.
                       
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                      • BillGrissom

                        BillGrissom Well-Known Member

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                        If you tap power from BATT+, the ammeter will not measure the draw. The dash ammeter measures current on the BATT+ wire under-dash, so if you tap it anywhere after the ammeter, the current draw will not be measured. In the factory design, currents by the starter, horn, or optional spotlight (police option) are not indicated on the ammeter. Mine seems to be a minority opinion. Others seem to insist that the alternator meet all max instantaneous loads when idling. If you do install a mega-alternator, understand that you may fry your factory wiring and dash ammeter, unless you perform the "MAD Bypass" (many posts).
                         
                      • Mattax

                        Mattax Just the facts, ma'am FABO Gold Member

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                        Not sure why there is disagreement or confusion on this.
                        Maybe its this - something I struggled to understand until is was pointed out to me.
                        Current will come from whichever source has higher potential.
                        When the engine is off, the battery is the highest voltage source.
                        When the engine is running, the alternator is the highest voltage source.

                        So for example when the engine is running and the alternator working, and the lights are turned on, the current for them does not go through the ammeter. Same for the horns (at least on my 67, diagram).
                        The exception to this occurs when the alternator isn't producing enough power to meet the demand.

                        Maybe it will help the discussion to add a diagram.

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                        Last edited: Dec 15, 2017
                      • Mattax

                        Mattax Just the facts, ma'am FABO Gold Member

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                        To the OP, MRGTX's, query. The fans will be running the most at idle. Taking Rob's (Crackedback) point, the only way safe way to add the fans to the stock system is from the alternator output. For that to work OK will need an alternator that can supply enough power at idle speed. Otherwise the battery will have to send power - and it will have to go through the long circuit with ammeter. Therefore, it probably makes sense to rewire the connection direct and not use an ammeter.
                         
                        Last edited: Dec 15, 2017
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                        • crackedback

                          crackedback FABO Gold Member FABO Gold Member

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                          Power from Batt +, the ammeter will absolutely see fan draw as the alternator is going to be throwing charge both in and out of the bulkhead and through the ammeter when engine is running. Engine off, it won't see a sniff.

                          Fans are the toughest to pick a point as they can run in both engine on and off situations. Pick your poison. Do a wire around and the ammeter/bulkhead doesn't really matter much in either engine situation.

                          A good Denso 65-85 amp alternator off a toyota PU will usually provide enough output at idle for our cars without a bunch of additional accessories. No need for any 150 amp units. In proper operating function, the alternator will only put out as much as is required by the system. A bunch of overhead power is a waste IMO.
                           
                          Last edited: Dec 15, 2017
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