A Stupid electrical question

Mike - Is this issue one you notice when engine is idling? It might be normal at idle, but not when above 1500 RPM.

The ampere meter reads 40-amps max-0-min. Fifty watt headlights at 12 volts will draw 4.2 amps (rounding) each. Multiply by 2. Add tail lights, (5 watts each x 2, or total of 1 amp). Add dash lights (approximately 2 amps), add front parking lights (approximately 2 amps total), add capacitive ignition, add coil current, add fans (if on - 8 amps), and you get approximately 20 amps total. These are, as we say, engineering approximations.

I would expect a 20-amp, or half-scale deflection of your amp meter. The meters are quite inaccurate, and a full scale deflection would not be out of the question. The important observation is whether the needle centers with slight increase of RPM.

If, indeed, the meter is accurate, then one must entertain a low resistance path to ground (not a short circuit, which would blow a fuse, but low resistance to ground). If one of the light conductors is touching ground through a rusted surface, this could create such a finding. However, another way of adding up the numbers is simply to follow the circuit diagram and see which fuses are uses to light the lights. Add up the capacity of all of these fuses and see if they approach 40 amps. Better yet, remove one fuse at a time and see which one results in the largest drop. If it is NOT the headlights, then there is, perhaps, a low resistance path to ground.

Hope this helps.

Jim S.

 

Jim.

Could also be his charging system, right?

If he's running wit a 35 year old alternator that's seen better days, he could easily be discharging the system even at 1500 rpms or more. And they have a separate regulator too?

The charging system really isn't made to run everything in the car at once. And, you forgot to include A/C in the calculation, which would drag him even farther into the red.

Additionally, inefficient motors (cooling fans that need service) could add significantly to the load.

When I replaced my cooling fans with new, and replaced my alternator with a modern one from an Alfa, all my charge/discharge problems went away. MY car actually will run at 0 at idle, even with lights on. OF course the new alternator makes significant additional amps.

Also, remember the amp gauge is looped into a fairly main circuit, is it not? Mine just got rebuilt and had just about burned through one of the terminals. That could also cause a big problem it not attended to.

Ahh, the joy of old cars.

Disclaimer. I'm not an EE, nor do I profess to be an expert on this stuff. I'm just relaying personal observations. And your mileage may vary. But I did sleep in a Holiday Inn last night..

DM

 

Dave - your observations are right on. No question that alternator health and fan health make a great contribution to good/bad things.

However, the fundemental question is the number of amps flowing from the battery to the lights. As you correctly observe, the amp meter is in series with the main battery conduit. Hence, all current to and from the battery flows through it.

Mike's initial question could have been stated, "...when the engine is off, and I turn on my lights, the needle drops to 40 amps discharge..." The status of the alternator does not come into play with his concern (if I understand correctly).

No doubt that if the radiator fans, air condition, windshield wipers, lights, etc., etc., are all energized that the 40 amp discharge is appropriate. Mike did not provide information as to what other loads (devices) were active during this observation. This information would help.

For those interested, the amp meter simply measures the balance of current being supplied by the battery to the car, and by the alternator contributing to the battery charge. Hence, if the car requires 10 amps to function, and the alternator is generating 10 amps, then the amp meter will read zero. This assumes a full charge to the battery. When the load requirements exceed alternator output, then there is negative flow (from the battery to the car), which lowers battery voltage. The regulator senses this, increase field current to the alternator, which then generates more current to supply the car and recharge the battery.

To understand alternator output requirements, simply turn the car off and turn on all loads. This includes air conditioning, lights, wipers, and radiator fans (by shorting the switch on the radiator), radio, etc. You will see the amp meter discharge, perhaps, 30 amps. Now add charging current of 10 amps, and one arrives at a minimum of 40 amps output by the alternator. Stated differently, when driving down the road on a raining night, with all fans going, wipers on, lights on, A/C on, etc., and the amp meter reads plus 5 amps charging, the alternator must supply the car's requirement (e.g., 40 amps) plus the charging current to the battery (arbitrarily 10 amps), or 50 amps.

Jim S.

 

This is interesting!

electrical detective work. Haven't done that since I tried to figure out why the three way hall switch didn't work right, only to discover that there was a wire color change from the box at the top of the stair to the box at the bottom of the stairs.

So, if I understand this, we need to know if this drain is with the car running or not. If the car is running, probably an alternator PLUS wiring issue, if car is off, might not be a problem at all.

MIKE, is the reading with the car at idle? With the car off, or at running speed?

Dave

 

Sounds like you're discharging, even when you're running with good RPMS.

For sure, not the best situation, but, exactly the same situation I was in.

Does you car have a cutoff switch under the front hood? If so, bypass it. That may help. Mine had a bad ground as was systematically draining my battery.

The rest needs some tech help. Measure the output of your alternator. IF it sucks, time to rebuild/replace. Again, I went with a more modern 80 amp (might have been 65) alternator, which caused some issues on its own, but we fixed pretty quickly, and now, even at idle I don't discharge the battery.

Happy hunting!!!

DAve

 

the car has so many original items, only replace what you have to.

every time i drive my dino at night, i leave the high-beams on. it doesn't help the electrical drawl, but the "older" high beams are much less intense than the current low beams (zenon, etc....)

i have yet to have someone flash at me......

during the day or night, i typically see a needle go negative at idle. with high rpm's it doesn't seem to be a problem.

that being said, i live in houston. the a/c on dino's is comical at best. it won't due crap here, so i rarely run them. clearly, this helps on my power drag.

 

I'm not sure about US versions, but on our "Euro", a three-position stalk controls parking lights only (up) low-beams (middle) and high-beams (down). It's all to easy to go to parking lights when you want to dip the high beams, which can be pretty exciting! I think I'll try the high beams all the time thing, though I rarely drive it after dark.