engine and drivetrain Headlights getting brighter/dimmer

[momanace2002]

Hi everyone,

I have a 1976 02

There is something funny going on electrically in my car (daily driver)

I noticed that when I have my headlights on and I'm idling, that they get "brighter" when when I step on the gas and rev the engine up. (this happens either in regular or bright mode)

I'm also noticing that sometimes my little (fasten seat belts) indicator on the dash board will light up as I'm accelerating out of intersection or from a stop sign. It will light up for about 2-3 seconds and then turn off again. (my seatbelt is fastened)

What's going on? Am I losing electrical charge some place in the engine (alternator, coil ? )

Any ideas?

thanks for your help

[laser1918]

Have you replaced any of the relays, wiring or battery in the last couple of weeks? Be careful here, I had a similar issue with my fog lamps and I ended up having an electrical fire under my hood. Not fun. Noticed I had my fog lamps getting too much juice from my battery, and bam, instant fire.

[Mike87]

Maybe the voltage regulator is going bad. I have had a similar thing happen. Connect a volt meter to your wiring and drive around. Watch what happens when the lights get bright/dim. That should give you a clue to what is going on.

Could also be poor grounds.

[c.d.iesel]

load test yer battery - you need a VOLT meter

1. key off - battery voltage ________V_?

2. voltage while cranking to start______V__?

3. motor running @ idle ___________V_?

4. while holding revs at 3,500 rpm _______V__?

# 1 and # 2 are most important because i

think you have a stinko batt.

and this all assumes that your wiring connections

to the battery, grounding points, alternator, voltage

regulator, and starter motor are all perfect?

[roadhog0]

In my 76 the dimming at idle is normal I believe. Your motor isn't spinning your alternator fast enough to keep the battery charging. The seat thing may mean you have a bad seatbelt relay. Mine after 5 minutes of run time freaks out real fast then is fine. I just accept it.

[Napes]

With a stock output alternator, although fairly new, I will get a slight, nearly imperceptable, dimming of the headlights. Maybe the original engineers on the car figured that if you're not moving your headlights don't need to be as bright as when you're movin' on down the road.

Cleaver dudes.

Bob Napier

[Emyers]

What is your idle set at? Spec is 850 - 900 if my memory serves me right. If below that might be your issue. Just a thought.

Earl

74 02Lux

02 M Roadster

72 Volvo 1800ES

[Chris_B]

Dunno about the seatbelt indicator, but the "headlights get brighter" when you move revs above idle scenario is not uncommon.

This can be exacerbated if your battery is not fully charged or is not holding a full charge. As CD said you can eliminate the battery as a cause by load testing the battery, and making sure it is fully charged. The two are not the same. You may have a good battery, but your charging system may not be fully charging it.

Generally, your charging system will not put out full power at idle. If your battery is not fully charged, there will be noticeable increase in brightness as you rev. This is in part because there is a non-linear relationship between light output and voltage increases at the lamp. For example, if the voltage at the light increases +1 volt as you rev your motor, light output may increase 25%, which is quite noticeable.

Is this a new problem? You should get a multimeter and check the charge level of the battery (with motor not running) and output of the charging system (at idle and higher revs).

[TobyB]

To add to the above,

check the battery post to cable connections, and then the high current

connections at the frame, engine and alternator.

I've had this happen when the battery posts build up a layer

of oxide between them and the clamp.

Also had it happen when I used battery grease on the connection before assembly.

hth

t

[momanace2002]

Hi everyone,

Thanks for the great ideas.

I did take a look at my battery this morning and noticed that it was purchased (based on little stickers on the top) in May of 2006.

So, would I be right in thinking that's a long time for a battery? It might not hurt to replace it this weekend and see where I am from there. I'll be sure to clean the battery cables / connectors too.

I assumed the battery was fine cause it seems to start the car just fine so I never thought to check when it was replaced.

RE: idle speed

Yea, under ideal conditions my car idles at or just under 900.

But it does vary a bit depending on how it feels. It never idles so low that it wants to stall but its not like it always sits right at 900 exactly.

As I was looking this morning at the battery I also looked at the alternator. The alternator seems like its been on the car for a while too.

What about the "tension" of the belt that goes from the alternator to the fan right behind the radiator?

I wonder if that belt might be getting too loose. I read my owners manual and it said that when you push on the top of that belt it should move between 5-10mm. My belt seems to move about twice that much.

Hummmm

I have the day off so I'm going to get out there and see what I can do. Thanks again for the help

Morris

[Chris_B]

That is an old battery, but they can last longer. Have your battery load tested and determine how many volts at puts out when charged. Most places that sell batteries can do that for you for free. It is either good or not.

Belt slippage is most likely to occur on acceleration or when there is a load on the alternator, not at idle, but you should adjust the tension if it is too loose.

[Chris_B]

P.S.: you may want to read this.

http://www.bmw2002faq.com/images/stories/faqs/charging_system/02_charging.pdf

If you don't test your charging system, you are just shooting in the dark. The shotgun approach you are using may work, but can be expensive.

[momanace2002]

Hey Chris,

Thanks for that info and the instructions on how to do the electrical tests. I need to get a Multimeter. I did a quick search online and they don't seem too pricy.

If it turns out that I need a new alternator I wonder if I could replace it my self? I was looking at how it's mounted and from what I could tell it looks like I could do it. There just seem to be a couple of bolts.

question....is the tension of the belt adjusted by where you slide the alternator back and forth on the mounting bracket? I noticed there seemed to be a place where the alternator's position can be adjusted.

Other than the correct wrenches, do I need any special tools to replace the alternator?

...just wondering.

thanks for your help

[Guest Anonymous]

yes it does sound like a battery with diminished capacity . . . but inspect the ground strap (large Cable) from battery (-) connects with engine block. Clean off grease/dirt then make 13mm bolt tight. This is a major flow and need snuggness and cleanliness