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Alternator and amp gauge and headlights

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  • Alternator and amp gauge and headlights

    I have what I believe is a 30 amp alt. I also have the higher CP standard bulbs in headlights ( 50/30)? When on - they peg the ampmeter to very near full discharge. Belt is tight. Something wrong?


  • #2
    -is this new behavior?
    -what does the ammeter read when all lights are off and car is just idling
    - do you know the source of the alternator? Is it one-wire or 3-wire? Are there any identifying marks on it? Can you post a pic? It would be good to see if this is a GM 10si
    - if the headlights are off and the taillites are off, and you have the engine at a fast idle, and step on the brake, what change do you see in the ammeter.... from what to what


    • #3
      Umm, and what makes you think the 30 amp alt can put out 30 amps? There is a misconception about alt and their possible outputs. To get 30 amps out you need to couple in a certain amount of HP to the pulley on the shaft of the alt. The problem is the V belt needs to be fairly tight get be able to translate the HP into the alt shaft. How tight, will look up in a shop manual for an older vbelt car. You will find it has to be really tight and if you look at the wrap the belt goes pretty far around the pulley.
      On the A you are not supposed to have the belt very tight. Plus the belt has just a smal contact patch. One place I read leads me to believe the diameter of the pulley is too small to even be able to transmit enough HP. Also my brother noticed people are changing thier belts cause they go bad. My brother has 40 years on his belt with the generator.
      Then there is the actual rating of the alt. Often the rating is cold and you can expect half that output when it warms up.

      The generator will put out over 20 amps all day long for decades.

      So first check the alt belt tension. I have personally used my pinky finger to get a full revolution on the pulley by flicking the fan part of the pulley on a number of A's at car shows. I can remember as a kid being taught the alt belt has to be so tight that you can not turn the alt pulley with your fingers. Clearly loose belts are going to reduce the max power output.

      Then keep in mind mechanics loved alt when they came out. They failed more often and were quick to fix.


      • #4
        Brian, you might want to have a cogged belt that can wrap around the pulley easier. I use a Gates because the cogs are smaller and it seems to flex better plus the width fits better in the grooves of the pulleys.

        And not sure about this but horns draw a good amount of amps, maybe more than lights.


        • #5
          Originally posted by Brian in Wheeling View Post
          I have what I believe is a 30 amp alt. I also have the higher CP standard bulbs in headlights ( 50/30)? When on - they peg the ampmeter to very near full discharge. Belt is tight. Something wrong?

          Yea something is wrong. Those bulbs should not draw anywhere anywhere near that amount.

          I believe you are telling us that you drive along and the ammeter is reading normal and when you turn on the lights the ammeter pegs. Yea this is not normal, the headlamps with no alt/gen pull about 10 15 amps. Put a meter on the battery without the engine running. The make the same reading with the engine running. With the engine running with no lights on should give you a higher reading then with the engine off. Make the measurements and report back.


          • #6
            Thank you all for the info. Kevin, I absolutely agree with everything you've said. Also, no, not new behavior. I'll post a pic of the alt setup later today. It came from one of the major suppliers and when I got it second-hand it didn't work. Was repaired by Buckeye Auto Electric before I installed it and has an outside-the-case diode pack or regulator.

            I do not think the belt is tight enough - being the main problem - and I do not really want to make it so tight, but might just for awhile as an experiment. It behaves normally till the lights put a load on it. I have two good rebuilt generators on the shelf (one with Tom Wesenberg's internal regulator) - which I may switch back to.

            Picture to follow.


            • #7
              Iwill also do the test Mike suggested and report back.
              Brian W.
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              This gallery has 2 photos.


              • #8
                Don't think that is a GM 10si alternator, they are internally regulated.
                The Transpo external regulator shown in the pic is of chinese origin.
                In fact all alternator regulators are now chinese. Read as 'unreliable'. The regulator is usually the weak point of an alternator.

                I am getting my 10si alternators only from O'Reilly which has a lifetime guarantee. They have 2 product lines, the better line is the lifetime guarantee. I can give you the PN if you like. They are only 60 bucks.

                The problem, as you know, is tightening the belt much more will compromise the water pump bearing. With correct tension, you should be able to turn the fan blade under the belt with the engine off with just a little bit of force. The alt pulley has more wrap and won't slip at this setting. The advice to use a cogged belt is VERY good advice. The PN of the cogged belt I use won't help you since I also have AC.
                It seems to me Mitch has a PN that would help you.

                Kevin's points are well taken. However here in Texas we all have AC setups and are running electric evap fans and electric condenser fans and with the right setup as I have described, we have logged many thousands of trouble free miles, even at night with the lights on, and most of us are using Halogen headlights. So, to me, it is only a matter of getting the correct setup.
                I could not tell from the pix, what is the diameter of the alt. pulley? We use a very small diameter which spins up nicely, and with the cogged belt, we get a good grip on that small pulley
                Last edited by tbirdtbird; 10-19-2017, 12:08 PM.


                • #9
                  This is in our chassis tech forum... the belt is listed under MISC .....

                  Battery Sizes Battery sizes 6 Volt = Group1 12 Volt = Group 25 6V Group 1 L=8-7/8 W=6-7/8 H=8-3/4 12V Group 25 L=9 W=6-7/8 H=8-3/4 Electrical: Motorcraft DC-1 Condenser Motorcraft DP104 Point Set Points CS30 Echlin (original style) Points CS751 Echlin (new Ford style) Condenser FA5 Echlin Rotor FA3 Echlin Condenser-Mallory
                  3~ Tudor's & 1~ Coupe
                  Henry Ford said,
                  "It's all nuts and bolts"
                  "Start by doing what's necessary; then do what's possible; and suddenly you are doing the impossible."

                  Mitch's Auto Service ctr


                  • Mitch
                    Mitch commented
                    Editing a comment
                    The Gates cogged is a BX-40

                • #10
                  Simple Alternator Test, Backyard way, i found this on the Nurex site

                  I always use an ext meter to see whats up.. i never trust ANY in dash gauges for an accurate result even Model A ammeters

                  ""On a 3 cell 6 volt or a 6 cell 12 volt battery system connect ONLY one terminal of a voltage test light to the output stud of the alternator and the other test light terminal to a ground such as an engine head bolt. Disconnect all other wires from the alternator output stud.

                  Start the engine and accelerate the RPM up to turn on the altrernator then reduce the RPM to a modest speed. The test light should glow on a 6 volt system and full brilliance on a 12 volt system. This shows proper output voltage and regulation of the alternator. There is no connection to the battery so there is no indication on the ammeter.

                  Turn the engine off, remove the test light and reconnect the usual car wires to the output stud to complete the test.""
                  3~ Tudor's & 1~ Coupe
                  Henry Ford said,
                  "It's all nuts and bolts"
                  "Start by doing what's necessary; then do what's possible; and suddenly you are doing the impossible."

                  Mitch's Auto Service ctr


                  • #11
                    I sure like idea of a voltmeter, tho
                    On a 12V system the alternator output should be approx 14.4 volts with only the ignition as a load. I usually do not disconnect anything, I just put the voltmeter on there. Under load, with lights and such, the voltage should NOT drop below 14 volts. That voltage reading tells the whole story

                    Usually if the voltage is correct the amps will follow as they should. I have never needed to test amperage output. A typical GM 10si alternator is rated at 65 amps and no matter what accessories you glue onto a Model A you will come no where near needing all that
                    Many years ago I worked in a carb-starter-gen-alternator shop, a type of shop that no longer exists

                    Alternator regulators are not adjustable. If the output is low, then the regulator (which has diodes and triodes inside) is usually compromised, and you are looking at replacing the alt. 60 bucks for what I described above is a very good deal.

                    I have absolutely no trouble with the concept of the stock generator. BUT since there is no such thing as a 6V AC compressor, those of us running AC have no choice but to go with an alt. And with the proper setup and proper brands you will have many many trouble free miles

                    The other parts stores such as NAPA etc may also have a lifetime guarantee alternator. O'Reilly's is just around the corner from me, so that is where I go. And I can get that warranty honored anywhere I go in the country
                    Last edited by tbirdtbird; 10-19-2017, 01:40 PM.


                    • Mitch
                      Mitch commented
                      Editing a comment
                      I always use a voltmeter when checking a generator or an alternator for proper output

                    • Dennis
                      Dennis commented
                      Editing a comment
                      Same here on checking for output. Does anybody know of a source of volt meters that will fit a Model A panel without modification to the panel?

                  • #12
                    All good info and advice. So below are my numbers. I think my battery was a bit low to begin with. Anyway:

                    Everything off - bat voltage- 6.0 v
                    Engine fast idle- no lights - 6.8 v on meter and 10 amp on gauge.
                    Engine fast idle- HL low beam - 6.0v and -25 amp on gauge
                    Engine fast idle- HL high beam - 6.0v and -25 amp on gauge
                    All lights off, fast idle, brake on - 6.6 v on meter
                    same as above for brake off
                    As above but with horn - 6.5 v

                    maybe my belt is slipping?
                    Brian W.


                    • #13
                      Oh. I used a wide scale analog meter ( with needle) just FYI.
                      Brian W.


                      • #14
                        TBird- what is the O'Reileys part number for the 6v pos ground alt (GM10si) for $60?
                        Brian W.


                        • #15
                          Brian, sorry I thought you were a 12V system. And all my advice was based on that assumption.

                          At fast idle no lights you are getting 6.8 volts and 10 amps
                          To me, this indicates what your trouble is....the output of the alt. is low.
                          For a 6V system, the output voltage should be exactly half of a 12V system, so the voltage reading should be 7.2 under those conditions, and should never drop below 7 volts under loads such as lights. The alternator is weak and has been chronically undercharging your battery, thus the 10 amp reading, the battery is trying hard to catch up.

                          The 7.2 and 14.4 are voltages that were worked out many many years ago, and represent a compromise between over- and under- charging the battery, and running the accessories properly.
                          The 4 tenths of a volt difference you are getting may not seem significant to some people, but it is VERY significant. The battery itself has internal resistance and these numbers are required to push the juice into the battery, and properly run accessories. And, as the voltage comes up to the proper levels, the amps required to run lights and such will drop off, because the true goal of operating lights is to meet the wattage requirement, which is the product of amps and volts.

                          As mentioned earlier, if you get the voltage right, the amps will follow correctly every time

                          The PN I have for an O'Reilly alternator is for 12V neg ground, sorry
                          You might be better off to revert back to a stock generator.

                          Ken Davis has sold thousands of 12V and 6V alternators, here is a link to his web page, and his contact numbers and email are there as well. He is quite reliable and accessible. This is the guy who developed the AC kit for a Model A

                          Last edited by tbirdtbird; 10-19-2017, 10:36 PM.


                          • #16
                            Here is a Voltage to Amps chart your actual readings may vary:
                            Taken from the Les red book

                            Generator Output Voltage Ammeter Reading
                            7.8 volts ==15 amps
                            7.5 volts ==12 amps
                            7.2 volts ==8 amps
                            6.5 volts ==5 amps
                            3~ Tudor's & 1~ Coupe
                            Henry Ford said,
                            "It's all nuts and bolts"
                            "Start by doing what's necessary; then do what's possible; and suddenly you are doing the impossible."

                            Mitch's Auto Service ctr


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