Mechanics

BATTERY LIGHT OCCASIONALLY FLICKERS RAP

2000 Mercury Cougar

Electrical problem
2000 Mercury Cougar 6 cyl Front Wheel Drive Automatic 86,000 miles

My car has ALWAYS had an electrical problem of some type. When I drive or idle my cars lights are constantly dimming and getting brighter rapidly and not in any particular rythem. My dashboard lights do the same thing. A few weeks ago my battery light starting flickering the same way but not all the time. Then I noticed it staying on at higher speeds (70 mph.) This ultimately lead to my car dieing and me assuming it was my alternator. After bringing it to the shop they confirmed the alternator had gone bad and after they replaced it I went ahead and installed a new battery for good measure. Now my lights still do the same thing with the random dimming and my battery light still randomly flickers but usually after I have been driving for a while then it usually stops. I have only had the new battery and alternator in my car for over 2 weeks. What do you think is causing the battery light to randomly flicker and my lights to still be constantly dimming rapidly? Also are they related? Could it be the reason my alternator died in the first place? I'm stumped and im in the military I need my car to be reliable. Thanks.
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Nonx
October 3, 2010.




Hi nonx. Welcome to the forum. It sounds like there's an intermittent connection in one of the wires for the generator. That will cause it to turn on and off while driving. That could potentially lead to stressing the built-in voltage regulator and failure of the unit to charge. The lights flickering are the clue. Normal charging system voltage must be between 13.75 and 14.75 volts. Full battery voltage when the generator is not working is near 12.6 volts. The difference in the two voltages is what you see as the flickering lights.

The tests have to be done when the problem is occurring. That might mean connecting a wire to a test point and running it into the car where you can monitor it with an inexpensive digital voltmeter. The yellow wire must have full battery voltage all the time. If it disappears momentarily, the generator will stop working. That voltage comes straight from the battery positive post but it goes through a fuse or fuse link wire. The fuse could have corrosion on its terminals causing the intermittent connection. Sometimes fuse link wires develop corroded splices.

The green wire with red stripe should have near full battery voltage when the engine is running. When the ignition switch is turned on, that voltage will be around 2.0 volts. That's the circuit that turns on the dash warning light. Once the engine is running, that voltage will go up to full system voltage. If the voltage drops to 0 volts, the generator will also stop working but the warning light will not turn on. A break in that wire will cause those symptoms. The most likely place to find an intermittent break would be where it comes through the bulkhead connector on the firewall.

If you can monitor those voltages, one at a time, while driving, holler back with your findings if one of them changes.

Caradiodoc

Caradiodoc
Oct 3, 2010.
I must drive abount 1.5 hours to base a couple times a week. I want to get it looked at/test it myself by the end of the week due to lack of time off/money. How risky is it to drive assuming your diagnosis is correct? Also could I be causing further damage to my car by waiting untill
the end of the week.

Tiny
Nonx
Oct 3, 2010.
I suspect you had two different but possibly related problems. The flickering lights is evidence of the charging system cutting out intermittently. In normal operation, the generator is actually being switched on and off hundreds of times per second. It's designed to operate that way. (It's the ratio of on-time to off-time that varies to adjust the charge rate). The voltage regulator, however, is not designed to turn on and off rapidly. Just like in a tv or radio, there are circuits that handle a surge of current much larger than they could handle for a sustained period of time, but they have no trouble doing that for the fraction of a second when they are first powered up. Think of shooting an automatic weapon. The barrel would melt down from the heat from shooting a thousand rounds one right after the other, but if you shoot just one round, the barrel has a chance to cool down and will live again to fire another round later.

When you have an intermittent connection, the item(s) it feeds may be turning on and off many times. Each time a little heat is generated. Normally those circuits have WAY plenty of time to cool down before you turn the ignition switch from " off" to " on" again, but when this happens rapidly, the heat builds up. Heat is the deadly enemy of electronic components. The voltage regulator, which is bolted to the back of the generator, is going to fail, ... Some day. That might happen 50 years from now, but stressing the transistors by rapidly turning circuits on and off that weren't designed for those kinds of repeated surges will bring about the failure of the unit sooner. No one can say how much sooner. The best you can hope for is to have solid electrical connections so the system will operate as designed. Consider how much time passed from the time you first noticed the flickering lights until the generator failed. That will tell you the new generator probably won't give you trouble right away. You could potentially drive for months or years that way, but do you want to take that chance?

A second way to look at this is the two problems could be entirely unrelated. Your generator might have just decided it was time to fail for some other reason, but the intermittent connection causing the flickering lights is still there. There's no way to know for sure without autopsying the old generator / voltage regulator to see what failed. If the brushes were worn out, that's just due to mileage. If an electrical component failed, that COULD be due to the repeated current surges caused by an intermittent connection.

Caradiodoc
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Caradiodoc
Oct 3, 2010.

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