Headlights dimming

[varyat]

Hello,
I took my car, 87 930, for an evening drive and the headlights and instrument lights kept dimming up and down. The dimming was not related to any bumps in the road so I don't think its a loose connection. Any explanations for this phenomenon? The car has a new battery. Thanks in advance.

[ben parrish]

voltage regulator going bad in the alternator.

[BRLuecke]

I am sure others will chime in, but that sounds like a bad ground. I'd start with the ground strap at the battery since you just changed the battery. Remove it, non battery end, clean it, and put it back.
Did the car run well other than that issue?

[Joe Bob]

While futzing with the ground.....check voltage at rest, idle and 2500 rpms.

[varyat]

Yes, car ran fine other than light issue. I really think the battery is well grounded and secure, but I will check it out. Voltage regulator sounds like it may be the culprit- it is 24 years old now.Anyone else have this issue?

[rick-l]

What is probably happening is the brushes in the alternator on the slip rings are worn down to where they make intermittent contact. When the alternator is putting out current the battery voltage is about 13.5, when it is not about 12.5 and you see a difference in the light output. That is what was wrong with mine. You can replace the brushes by buying a regulator since they come with them or if you have a motor shop try just the brushes. Look at the grooves in the slip ring to see they are not worn to the point the brushes won't last long (almost worn through).

The other regulator failure I've never seen but must be true since some members report a zenner blown in their DMEs is that the regulator sticks hard on and the system sees 17-30 volts from time to time. You don't have a DME but your radio might not like it.

[djcdmv]

That was the exact sign of my voltage regulator in my 85. Lights/gauges dimmed and seatbelt warning light came on during acceleration. Check with volt meter as Joe Bob states and post results. Mine was 17 volts at 2500 rpm. Way too high.

[HarryD]

Problems like this is why I added an aftermarket voltage gauge to my car:

Originally Posted by HarryD

The Cigarette lighter solution is ok, but in my mind, unsightly. I went to my local FLAPS and bought a small gauge and installed it. It is connected to one of the switched wires that powers my radio.

My install looks like this:

[Joe Bob]

[wwest]

A simple VM will not suffice.

You not only need to read DC volts with some accurracy you also need to read the ripple, saw tooth waveform, P-P applitude. The P-P, peak-to-peak, ripple amplitude is only applicable to readings taken in the engine compartment. Battery acts as a HUGE capacitor, filtering out most of the ripple content "up front", but cannot provide the same level of "filtering" all the way back in the engine compartment due to the milliohms of resistance inbetween.

[Joe Bob]

Originally Posted by wwest

A simple VM will not suffice.

You not only need to read DC volts with some accurracy you also need to read the ripple, saw tooth waveform, P-P applitude. The P-P, peak-to-peak, ripple amplitude is only applicable to readings taken in the engine compartment. Battery acts as a HUGE capacitor, filtering out most of the ripple content "up front", but cannot provide the same level of "filtering" all the way back in the engine compartment due to the milliohms of resistance inbetween.

While I agree with you in principle, the use of a meter helps in pinpointing an issue with eh charge circuit.

[dshepp806]

The voltmeter will certainly let you know what's up with charge level,..and a must-have..some way, some how

Never thought about mounting something to show me the waveform (kinda cool!)

BEST!

Doyle