Welcome to CPGNation.com! Log in or Sign up to interact with the CPGNation community.

Voltage differences?

Discussion in 'FAST Support Forum' started by J-440, Dec 4, 2017.

  1. J-440

    J-440 Member

    Joined:
    Jun 16, 2012
    Started a previous thread on here and am getting some great help but I have another concern regarding voltage drop. I stated earlier that I'm getting 14.2 volts from a 140amp alternator at the terminal, getting 13.9 volts at the battery in the trunk and getting a 10.5 volt reading from my ECM on my laptop. This is with all the accessories on and the car idling at 800rpm.
     
  2. A A

    A A Well-Known Member

    Joined:
    Jun 22, 2014
    Without your having the cables correctly sized, and more specific details, it's difficult to know what all is going on. Follow the advice Richard already gave you. You could need a larger amperage alternator or a pulley resized on the alternator, but you'll never know until the cables are correctly sized.
     
  3. J-440

    J-440 Member

    Joined:
    Jun 16, 2012
    Ok, but if everything is reading right with the multimeter, then why would the ECU be reading wrong? Not trying to be a pain, just curious is all.
    Oh and I called Powermaster with my alternator specs and they said I was good to go.
     
  4. Fastmanefi

    Fastmanefi Active Member

    Joined:
    Jun 4, 2011
    Location:
    Sonora, CA
    The ECU reads the voltage at the Switched 12V wire, not the power leads. Measure the voltage there with your DVM.
     
  5. J-440

    J-440 Member

    Joined:
    Jun 16, 2012
    Good deal. Thanks again.
     
  6. J-440

    J-440 Member

    Joined:
    Jun 16, 2012
    Ok, by running a 1 gauge ground and 1 gauge positive from the alternator to the battery, I only picked up 1 volt. The ECM is now registering 11.5 volts whereas my multi-meter is registering 14 volts at the battery. Do I need a bigger alternator, say 180amps? Or am I still overlooking something?
     
  7. Fastmanefi

    Fastmanefi Active Member

    Joined:
    Jun 4, 2011
    Location:
    Sonora, CA
    If the battery is 14V then you should have that everywhere. Did you measure the voltage AT the the ECU with your DVM? It always possible that the ECU is reading it wrong. Verify the 11.5 V that the ECU is reporting....
     
  8. A A

    A A Well-Known Member

    Joined:
    Jun 22, 2014
    The fact you picked up any increased voltage, by increasing the wire gauge, confirms your previous wiring gauge was too small. Richard is giving you excellent advice.
     
  9. A A

    A A Well-Known Member

    Joined:
    Jun 22, 2014
    Fastmanefi,Dec 3, 2017

    "The basic problem comes from using a one-wire alternator and high resistance in the system. 1 and 4 gauge is NOT acceptable. At that current level the minimum cable size should be 2/0. BOTH the cables from the alternator to the battery need to be very large to handle the voltage drop over that length. I advise to NEVER rely on the chassis for the ground return. Run a separate black cable to the battery."
     
  10. J-440

    J-440 Member

    Joined:
    Jun 16, 2012
    Ok new info. With the 1 gauge wire all around (1 gauge wire going straight to battery) with the car idling at 800rpm and all accessories on: 14.2 at the battery, Laptop ECM 10.9, multimeter at the ECM pink wire also 10.9.
    With my original install using 4 gauge wire (alt to starter): 14 at the battery, Laptop ECM 10.9, multimeter at pink wire also 10.9.
    Unfortunately I don't have a 1 gauge ground from the alternator to the battery that is long enough to get another reading. Given the info above, would it make a huge difference?
     
  11. A A

    A A Well-Known Member

    Joined:
    Jun 22, 2014
    High current wire gauge does make a huge difference - especially with the accessories powered on. The smaller the wire - the more resistance. This becomes critical in low volt (12v) systems. It sounds like you are also using the chassis as the negative. No automotive chassis is designed for high current voltage transfer over any distance from the battery. That's also why a direct ground cable from a front mounted battery to the engine is always used. A separate negative cable is still required between the engine and a rear mounted battery. Don't try to get around using smaller than the advised cabling on the long runs.

    The positive cable for the alternator only needs to be sized for the alternator used. This is only a short cable that can be connected to the +12v battery cable connection at the starter solenoid.
     
    Last edited: Dec 17, 2017
  12. J-440

    J-440 Member

    Joined:
    Jun 16, 2012
    Just performed another test. By using the 1 gauge negative cable from the alternator to the battery I'm getting the same results. Like I said, I don't have 2, 20 foot, 1 gauge wires so I'm alternating using the same wire.
     
  13. A A

    A A Well-Known Member

    Joined:
    Jun 22, 2014
    Just for a comparison. A stock C3 (68-82) Corvette uses a 0 gauge positive battery cable - and that cable only travels from the rear seat battery box to the engine. The C3 also uses a separate negative cable to the engine.
     
  14. J-440

    J-440 Member

    Joined:
    Jun 16, 2012
    Preciate the help and patience AA. I found a chart for what alternator a car needs and I'm right at 140amps...which is what my alternator is. Given all the above info with a small difference (.2 volts) between the 1 gauge wires going to the battery, any idea what difference a 180amp alternator would do?
     
  15. A A

    A A Well-Known Member

    Joined:
    Jun 22, 2014
    I have a 1985 TA with a just over 500 horse 406 small block. The car is loaded with all power and a large stereo amplifier system. I installed all LED lighting trying to get my power needs down using a 150 amp 12 Si alternator. Even getting the correct pulley, the 12 Si was too weak to supply good power at 800 RPM with all accessories on. I installed a 200 amp CS 144 and my engine can now supply the power needed at load at 700 RPM. My problem was the 150 amp alternator could not supply enough amperage at idle to the battery under load. The 200 amp CS 144 solved that for me. The CS 144 circuitry allows for higher low RPM output without over supplying at high RPMs.

    Your problem is insufficient cables/size. Your cabling issues must be solved first. Up-sizing the alternator will not correct that. Afterwards,, you might also find other issues - like a bad ignition switch supplying low voltage to the ECU ignition wire.
     
    Last edited: Dec 17, 2017
  16. J-440

    J-440 Member

    Joined:
    Jun 16, 2012
    This is what's confusing me though. With my existing gauge wire/setup, I'm getting 14 volts all the way back to the battery. Since the ECM main power wire is connected to the battery, then why the low 10.9 reading on my laptop. Granted the ECM power wire is hooked up to my ignition switch, if the switch was bad then the car wouldn't start, correct? Or, would the lone terminal on the ignition switch be bad and not the whole switch? Man I hate electrical!!
     
  17. A A

    A A Well-Known Member

    Joined:
    Jun 22, 2014
    As long as the ECU voltage threshold is met, the engine will start. Voltage into the switch should be the same out of it. There are multiple terminals inside the switch - any number of which could be bad from corrosion. Insufficient cabling/size also helps add to switch problems - and will also burn or cause a starter solenoid to stick closed. When that happens, you'll be seeing how quick you can pop the trunk to disconnect the battery.
     
  18. J-440

    J-440 Member

    Joined:
    Jun 16, 2012
    Ok, I'll check the switch then. Hey man like I said, thanks for the electrical lesson.
     
  19. A A

    A A Well-Known Member

    Joined:
    Jun 22, 2014
    Just an FYI: I have a 1970 Ford drag car I built in 1972. I placed the battery in the trunk for better weight transfer. The battery cables? 2/0 welding cables fabricated to cast brass battery terminals and sealed copper terminals at the opposite ends. The entire car still looks factory new and could be raced today. Why welding cables? It's what everyone used then. 2/0 was the proper size, and both cables travel through the body from the trunk mounted battery to the right inner fender starter solenoid and engine. All the cable insulation is still perfect.

    25' cables:
    https://www.ebay.com/itm/25-Foot-of-Red-2-0-Welding-Battery-Cable-Made-In-USA-/371305252373

    https://www.ebay.com/itm/25-Foot-of-2-0-Welding-Battery-Cable-Made-In-USA/271271158082?_trkparms=aid=222007&algo=SIM.MBE&ao=2&asc=49566&meid=8de92adf3e84493dbc4c6253fdfa81cf&pid=100005&rk=5&rkt=6&sd=371305252373&_trksid=p2047675.c100005.m1851
     
    Last edited: Dec 17, 2017
  20. Fastmanefi

    Fastmanefi Active Member

    Joined:
    Jun 4, 2011
    Location:
    Sonora, CA
    My 64 HEMI drag Plymouth also used 2/0 welding cable. Never trusted the chassis for anything electrical.
    Here's a slide I use in my FAST Training class.
     

    Attached Files:

Share This Page