Voltage drop

[ES 1800]

'73 CS, stock. My headlights dim badly when the engine rpms drop to idle. Looking at the Autobook from www.cscoupe.org, it says "A simple test on alternator charging can be carried out after dark by switching on the headlamps and starting the engine. If the alternator is charging, the headlamps will brighten considerably as the system voltage rises from the nominal battery voltage to the higher figure produced by the alternator." Seriously???? This is NORMAL??? The ignition warning light does NOT stay on when the engine is running suggesting the regulator is working.

I should note that I have installed Daniel Stern relays in the headlamp circuit some years ago.

 

[HB Chris]

You need he check the output with a voltage meter. At the battery terminals: Engine off, engine idling, engine running faster.

 

[Ohmess]

Also, your post mixes two different conditions that can be observed using the headlights. The initial observation -- lights dimming while the engine is running -- indicates a voltage drop while the car is running that is problematic. The alternator should be pushing out approximately 13.8 volts while the car is running, which should be sufficient to run the lights.

The second observation -- an increase in brightness when the car starts -- indicates the increase in voltage because the battery will be running the lights while the car is not running (the battery only puts out 12 volts) whereas when the car starts the alternator takes over, putting 13.8 volts into the various circuits.

 

[ES 1800]

Also, your post mixes two different conditions that can be observed using the headlights. The initial observation -- lights dimming while the engine is running -- indicates a voltage drop while the car is running that is problematic. The alternator should be pushing out approximately 13.8 volts while the car is running, which should be sufficient to run the lights.

The second observation -- an increase in brightness when the car starts -- indicates the increase in voltage because the battery will be running the lights while the car is not running (the battery only puts out 12 volts) whereas when the car starts the alternator takes over, putting 13.8 volts into the various circuits.

Thanks, but I have not observed what you refer to as my "second observation." The part that I quoted from Autobook still seems a mystery to me.

 

[jmackro]

If the alternator is charging, the headlamps will brighten considerably as the system voltage rises from the nominal battery voltage to the higher figure produced by the alternator." Seriously???? This is NORMAL???

I agree with Ohmess: yes, that statement is correct.

The ignition warning light does NOT stay on when the engine is running suggesting the regulator is working.

Well, not necessarily. All the light tells you is that the alternator output is > the battery voltage; it doesn't verify that every aspect of the charging system is working perfectly. For example, if your battery is at 12.0 volts, and your alternator only puts out 12.1 volts at idle, then yes, the warning light will be out. But if it takes 13+ volts to make the headlights bright, then they will be dim at idle, even though the warning light is not glowing.

You need he check the output with a voltage meter. At the battery terminals: Engine off, engine idling, engine running faster.

I like Chris' idea. Please measure the voltage at those three conditions and report back.

 

[ES 1800]

I agree with Ohmess: yes, that statement is correct.



Well, not necessarily. All the light tells you is that the alternator output is > the battery voltage; it doesn't tell you that the charging system is working perfectly. For example, if your battery is at 12.0 volts, and your alternator is putting out 12.1 volts at idle, then yes, the light will be out. But if it takes 13+ volts to make the headlights bright, then they will be dim at idle, even though the warning light is not glowing.

What should the alternator put out at idle?

 

[HB Chris]

Just over 13 volts, maybe 13.4

 

[Ohmess]

This is one of the common misconceptions of folks jumping into electrical stuff. Even though our cars are 12 volt systems, our alternators put out more than 12 volts while our car is running.

And as jmakro noted, the warning light is a very crude device. You need a multimeter to determine the operating voltage of your alternator.

I have not done this, but I think you can have an Autozone type place check the operation of your alternator for free.

 

[ES 1800]

I appreciate the comments. I maintain and drive a [non BMW] race car so I know that alternators put out a lot more than 12V. But even with somewhat sufficient knowledge I was looking in the Autobook to see what it might have to say about the regulator and encountered the quote that perplexed me. Why would a document put out by what appears to be a group that knows about the E9 suggest that dim headlights are normal?

 

[jmackro]

Why would a document put out by what appears to be a group that knows about the E9 suggest that dim headlights are normal?

Is it possible that what you read in Autobook was written back in the early 1970's? As I recall, headlights did dim at idle back then, because the voltage regulators weren't as good (they were electro-mechanical). Modern alternators have internal, electronic regulation which enables them to put out a higher voltage at idle.

I'll bet the anti-lock brakes on your stock '73 CS are pretty disappointing too. And don't get me started about how badly the adaptive cruise control works on our cars.

 

[ES 1800]

Is it possible that what you read in Autobook was written back in the early 1970's? As I recall, headlights did dim at idle back then, because the voltage regulators weren't as good (they were electro-mechanical). Today's alternators have internal, electronic regulation and can put out higher voltage at idle.

Yes. The copyright date is 1977. So you are suggesting a new alternator?

 

[mulberryworks]

Before alternators, generators were what we had. They definitely don't put out much voltage at idle so you'd get dim lights, and no battery charging. Alternators have a much better output at idle as well as more power overall.

Ian

 

[Stevehose]

And some alternators need to get up >1000 rpm or so before the exciter circuit kicks in the charging s a few revs are in order.

 

[ES 1800]

Perhaps I should ask a simple question: For those of you who have a CS, a '73 CS, do you lights go dim at idle? If not, have you done anything like change to a more modern alternator? My car has a stock alternator and stock regulator, to the best of my knowledge, the same ones that came on the car.

 

[HB Chris]

Instead of just buying a new alt, check the voltage and see if it's putting out 13.4v at idle. If you do need one, buy a single wire alt with internal regulator. You can get rid of the regulator or leave it in place and remove the three wire harness. But, you still need to run a blue wire from the old regulator to the new alt to trigger the red warning light.

 

[Stevehose]

I have an 85 amp alternator and relayed low beams. Lights do not dim at idle.

 

[Arde]

My headlights do not dim badly at idle nor when the engine is off because they are powered by a healthy battery. They become a bit brighter with revs but do not dim badly in any case. Maybe the battery is not that healthy and that is why they appear to dim more?

 

[Mike Goble]

I solved my alternator problems years ago with a 105 amp AC Delco CS-130.