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Dreaded "low voltage/dim lights at idle"

Discussion in 'Repairs, Maintenance, Help' started by kzooman83, Oct 7, 2016.

  1. kzooman83

    kzooman83 Active Member

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    Hey everyone,

    I have been trying to banish the typical "low voltage/dim lights at idle" issue as of late and I have not had a lot of luck; wondering if anyone can offer any advice. This is on my '75 big block duster btw. I only have the problem when the car is at idle in gear with both the headlights and the heater on. Under these conditions, voltage is between 11-12 volts. With the lights/heater off voltage is about 13-13.5 at idle in gear and 14-14.5 volts at idle in neutral. Idle speed is 800 rpm in park/neutral, 600 rpm in gear. Also should mention the battery is in the trunk.

    What I have done:

    -New 60 amp alternator
    -Junkyard 90 amp 1990 dodge truck alternator (and 6 gauge jumper wire from alternator output to battery)
    -new voltage regulator
    -checked/cleaned grounds on voltage regulator and from battery to frame/body
    -checked voltage drop from alternator output to starter relay; voltage drop is 0 volts
    -checked resistance between alternator case and body of car; resistance is 600 ohms
    -checked resistance between negative battery terminal and car body, resistance is 300 ohms

    Not sure where to go from here; maybe install a relay for the headlights and the blower fan?
     
  2. ImperialCrown

    Level III Supporter

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    The alternator case should be less than 1Ω to body and battery grounds. The negative battery terminal should also be less than 1Ω to body and engine grounds. You have poor grounds.
    You mention a battery-to-frame-to-body ground. Are there engine-to-body and engine-to-battery grounds? It is important not to skimp on return paths between all components. Relying on current to pass through one component to get to another component will be a high-resistance path back to the battery and may create 'ground loops'.
    The voltage regulator case also needs a clean and tight ground. I have also pinched the v/reg terminals closed slightly with pliers to make better field circuit contact.
    Headlamp switch and socket terminals may be 'tarnished', making a high-resistance circuit. Even a 1 volt drop in the circuit will noticeably dim lighting. Sometimes unplugging and re-plugging in connectors a few times will 'scratch' in a better contact.
    Is the radiator core support and front fenders securely bolted to each other and the car body? Are the headlamp ground terminals using clean screws and sheetmetal?
    Sometimes corrosion can hide under a screw head or battery terminal and look fine from the outside. Scrape the paint away under the terminal so you have a metal-to-metal junction.
    Idle speed and alternator pulley size can also affect alternator output.
     
    Dave Z and Bob Lincoln like this.
  3. Bob Lincoln

    Bob Lincoln "CHECK FAULT CODES"
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    Here's another issue:

    You are using 6 AWG cable from the alternator to the battery.

    You need 0 AWG with that long a run of cable. You are creating too much of a voltage drop due to the resistance of the 6 AWG cable.
     
    chuzz likes this.
  4. kzooman83

    kzooman83 Active Member

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    Sorry, I was mistaken on the resistance readings regarding the grounds. I thought I had checked them with the multimeter on the 20k ohm setting, but I actually used the 200 ohm setting. Those values are not 300 and 600 ohms, they are actually 0.3 and 0.6 ohms.

    Also, the wire from the battery (in the trunk) to the starter relay (under the hood) is 0 AWG wire, the 6 AWG jumper goes from the alternator output to the starter relay where the 0 AWG wire from the battery connects. I installed the 6 AWG jumper along with the 90 amp alternator because I did not want all that current going through the bulkhead connector.

    I am running a ground strap from the engine to the body; I do not have a ground going directly from the engine to the battery since the battery is in the trunk. When I checked the alternator ground and voltage regulator ground (also 0.3 ohms), I had one lead of the multimeter touching the component in question and used one of my hood pins as the contact point for the other lead.

    I notice that the reading on my voltmeter (it is an aftermarket unit I installed) seems to fluctuate some at idle in gear; Could my somewhat lopey idle (from the aftermarket cam) be causing this?
     
  5. KOG

    KOG KOG
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    If you're still running a stock ammeter on that Duster then all charging current runs through it. That means that all current runs through the firewall connector twice, in and out. That connector is a notorious trouble spot. Check it first. And then run a 10 gauge wire from the alternator to the ammeter and back to the battery cable. Better yet, replace the ammeter with a voltmeter and run the 10 gauge alternator output straight to the battery cable.
     
    Dave Z likes this.
  6. AC TC

    AC TC Well-Known Member

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    Simple, its nothing thats wrong...
    the alternator doesent charge enough at low idle, its spinning to sloowly.
    Either live with it or get pulleys that spins the alternator faster.
    Lareger wires wont hurt either but it wont completely solve the problem. They probably did this even when new.
     
  7. ImperialCrown

    Level III Supporter

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    Bulkhead (firewall) connectors were problematic on many of these. Melted plastic from high-resistance heat on current carrying terminals was an issue. Brass or tinned-copper terminals were the best conductors, but the brass oxidized after awhile and became a poor connection.
    A smaller alternator or larger crankshaft pulley will spin the alternator faster and increase output at idle.
     
  8. kzooman83

    kzooman83 Active Member

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    KOG, I do have an aftermarket voltmeter connected in parallel to the stock ammeter. The 6 AWG wire I mentioned above connects the alternator output directly to the positive battery cable at the starter relay.
     
  9. kzooman83

    kzooman83 Active Member

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    Where would I go about getting a larger crank pulley or smaller alternator pulley? I didn't think the 90 amp nippondenso truck alternator would have this problem......
     
  10. John Wood

    John Wood Well-Known Member

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    You don't get necessarily get full voltage charging at idle. A fully charged 'good' battery should produce 12.2 to 12.5 volts for 15 to 20 minutes depending on the load. So... dim lights at idle suggests either a weak battery, dirty connections, or a bad ground. I have had batteries that start an engine with no problem, but had one weak cell that you could only identify by dim lights at idle when the alternator can't turn fast enough to produce adequate charging voltage.
     
    Rick Anderson likes this.
  11. KOG

    KOG KOG
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    Well, you've already dodged the biggest bullet on A body charging systems in that case.
     
  12. pt006

    pt006 Active Member

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    The 70's mopars I've owned all acted similar to yours when at a stoplight. They showed a negative 10 amps [headlights on- fan on high]. As soon as I took off, the ammeter would read positive ~ 10 amps and drop down to ~ 0 amps after a few minutes. This is with a 35 amp alternator. Normal stuff.

    Truck pulleys seem larger than car pulleys? Also, if one diode failed, the result would be lowered output.
     
  13. AC TC

    AC TC Well-Known Member

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    I repeat ...this is nothing wrong.
    They can build it so the alternator charge well at idle in drive but then they would have greater parasitic losses and more wear and tear at Cruise.
    It really doesent depend of the size of the alternator, its the speed of the rotor through the magnetic field in the alternator that have to be high enough
    to reach the needed voltage.( this speed is a design choice when manufacturing the alternator.)
    - its all about optimisation and not charging at idle in gear is an easy trade off.
     
  14. Dave Z

    Dave Z It's me, Dave
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    Vintage Chrysler electrical repairs and updates

    With an older design that doesn't work well there. Dan Stern told me, if I don't like that, I need a more modern style alternator. I can't recall his specific advice on which one to get but can look it up.

    Yes.... I have that issue on my car as well and have had it on every classic Mopar I've ever owned, from the base 35 amp units up to the 1976 taxi/police V8-A/C unit.
     
  15. kzooman83

    kzooman83 Active Member

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    So I think I found the "problem." It looks like AC TC is right on with the pulley ratios being the culprit. The crank pulley on my big block measures ~6 1/2" in diameter; I compared this against a '89 318 crank pulley I have and it measures 7 1/4. So it looks like later engines with fuel injection and more electronics used a larger diameter crank pulley to spin the alternator faster to meet the greater current demand. I am not sure if the larger crank pulley will even clear my water pump pulley, and even if it will I don't necessarily want to deal with the added parasitic loss. I think for now I will maybe just bump my idle speed up slightly to compensate.
     
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  16. schelled

    schelled Well-Known Member

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    It sounds like you have found your problem. When you are looking for bad grounds an ohmmeter is not the most reliable method, they usually have a little nine volt battery that sends current through the connection to get an ohm reading, in larger connections like you are measuring that is not reliable. Use voltage drop on both the positive and negative side of the circuit, that way you have a live circuit and it will tell you exactly what is going on. You want as close to zero on both sides of the circuit.
     
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  17. Rick Anderson

    Rick Anderson Well-Known Member

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    I've been told that cheap ammeter's in the 70's and earlier vehicles can often go bad and draw a lot of current off the charging circuit. (remember, in this era they were only designing cars to last 3 years) I don't know how true that is, but you might note most vehicles, if they come with some sort of gauge for the electrical system anymore, they come with a voltmeter.

    Headlights dimming a bit while at idle is pretty common in 70's and earlier vehicles, like mentioned. Its a matter of how severe it is, as to if something is wrong. Again a voltmeter is probably more helpful than ammeter in this case. When at idle, the alternator is clearly not spinning fast enough to meet the current draw of the vehicle, this will cause the voltage to drop, but as soon as the voltage drops to the same as the battery, the battery will start helping the alternator by providing current along with the alternator. In these charging systems the battery is needed for far more than just starting the vehicle, they are also an electrical reservoir to aid the alternator when needed. So if you're battery is NOT in good shape or the cables and connections to the battery are NOT good either, then your headlight dimming will be worse.

    With a voltmeter, you can tell what the battery voltage is, it will be the voltage that the voltmeter drops to when you first go to idle, and if the system can hold 12 Volts or better while you're idling for up to a few minutes, that seems like a normal system to me. If the system is dropping below 11 Volts right away or after a minute or two, then off hand I'd say its weak, and it could be wiring, alternator or the battery.

    Don't forget battery cables can go bad, especially 35 year old battery cables. Their condition can be deceiving, since the battery cable has to flow an incredible amount of current when starting, I've seen battery cables pass most typical checks with multi-meters that flow pico-amps through it to check, but can't flow the 350 amps for starting. Stuff like corrosion, the lead terminals to copper wire connection loosening or corrosion forming in between, stands on the copper cable breaking. Cleaning the terminals on the cables and battery never hurts and often helps a lot more than you would expect.

    Oh, a bigger diameter pulley on the "Engine Crankshaft" should spin the alternator faster. A bigger diameter pulley on the "Alternator Shaft" should spin the alternator slower. Not sure if you're were talking about a bigger pulley on the engine crank or the alternator shaft, it seems like we are sending confusing messages on the pulley diameters.
     
  18. Dave Z

    Dave Z It's me, Dave
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    Good points raised there. I found the problem much less annoying with a new battery. The cables do go bad and good luck finding them for a vintage car - some are around and some are not.
     
  19. kzooman83

    kzooman83 Active Member

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    The diameters I mentioned are indeed for the crankshaft pulley. The crank pulley I have on the engine now is a single groove and measures 6 1/2." I have some parts from an '89 318 TBI engine in my garage, including the crank pulley. That crank pulley is a 4 groove pulley and it is 7 1/4." The water pump pulley appears to be a smaller diameter on the 318 than on my 400 also. My guess is that this prevents voltage drop at idle for the TBI engine and also improves cooling (bigger crank pulley, smaller water pump pulley).

    In all fairness, dropping down to battery voltage in my situation is annoying, but the only things really drawing current are the heater, headlights and electronic ignition. The battery alone can power those things for quite a while. I had the field windings lose continuity on an alternator a while back and I limped it nearly 10 miles back home with the alternator not functioning at all.
     
  20. Rick Anderson

    Rick Anderson Well-Known Member

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    Heater? You have an electric heater? Or are you saying the blower fan for the heater core?

    The headlamps draw a lot of current, electric heaters (which window defrost often is) and blower fans also draw quite a bit of current. The electronic ignition, not so much.

    Again, you have to look at the actual voltage the whole system drops too when the engine is at idle. A weak battery (and other things wrong in the electric system could make the battery appear weak or actually be weak) will let the voltage drop much lower than 12 Volts and that would make for a bad diming of the headlights.

    Dimming headlights in this era vehicle is normal, its subjective to how annoying that is. Checking and tracking the voltage when the engine goes to idle and is using the battery to supplement the current draw, will tell you if its normal or something is wrong.

    If the electric system is working like it is designed, than you either have to live with the annoyance or change the design. Like someone suggested before, retro-fit a modern alternator to your vehicle, won't be cheap or easy, it may NOT be reliable either, but it likely will solve the annoyance of dimming headlamps.

    Another alternative, if the electric system is working properly, look into some new types of LED headlamps. LED likely will draw a fraction of the current of the current incandescent bulbs used in the headlamps and might avoid the dimming at idle.
     

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