Persistent lights flicker

Durenol · Post by **Durenol** » 14 Sep 2018, 23:09

2007 XC70. For about a decade now all the lights (external and interior) in this car have a subtle flicker to them almost like a candle in a faint breeze. All the lights flicker in sync, and the flicker persists with a new battery. I've had it in the shop a few times but all they do is start the car and slap a volt meter on the alternator, which seems fine.

Questions:

1) This has been going on basically since the car was new (or at least since it was 1y old). Is this normal?

2) Is this something we have to worry about?

2b) if so, what are we supposed to do?

Post by **RickHaleParker** » 15 Sep 2018, 00:20

Does it not flicker when the engine is not running and flicker when the engine is running?

Sounds like you got a ripple in the alternator output that the battery cannot filter.
My initial suspicion is a voltage regulator that unstable. I could also be a loose connection that jitters with the vibrations of the engine.

A DC Voltmeter will not detect ripple because the ripple is too fast, it will display the average DC voltage.
An oscilloscope across the battery would be the best way to check for ripple or a unstable regulator.
You could put a voltmeter across the battery and switch the meter to AC voltage. It could read the RMS value of any ripple if the ripple is not too slow. However getting no AC voltage reading would be inconclusive.

You been running it like this for ten years, obviously it is an annoyance not a problem.

dj_v70 · Post by **dj_v70** » 15 Sep 2018, 06:34

About 20 years ago, I had a similar problem with a Volvo 240. It only happened with car running. It started after I was jumping someone’s car and I heard a popping noise. DC voltage checked out ok, so, I kept driving it like that. A couple of years later the alternator regulator brushes wore out. When I replaced them, I noticed that one of the two brushes wore a groove into the alternator commutator. Apparently, one of rectifier diodes in alternator had blown out. You can’t see it with dc voltmeter, but you would have with an oscilloscope and possibly even using ac voltmeter.

Georgeandkira · Post by **Georgeandkira** » 15 Sep 2018, 08:07

How many miles on your '07 XC-70?

I ask because my '02's lights began flickering at 185,000 miles.

Sure, I could've changed the regulator in situ for the price of the part but at that mileage I decided to carry the alternator to my local, friendly, 3rd generation rebuild shop. New bearings, freewheel pulley, accumulator ring (I think that's what it's called) and regulator done in one swell foop!

Durenol · Post by **Durenol** » 17 Sep 2018, 03:07

RickHaleParker wrote: 15 Sep 2018, 00:20 Does it not flicker when the engine is not running and flicker when the engine is running?

I don't *think* so..... if there's anything there when the engine is off it's so incredibly subtle it's certainly my imagination. It's definitely far more noticeable when the engine is running, although to be fair "noticeable" in this case was already low and no one else has said anything until I pointed it out to them.

RickHaleParker wrote: 15 Sep 2018, 00:20 You been running it like this for ten years, obviously it is an annoyance not a problem.

I'm more worried if this is an indicator that there's something wrong in the electrical system that might cause problems for us further down the line.

I'll have to figure out how to rent an oscilloscope for this or find a shop advanced enough to have one. Any pointers there?

Durenol · Post by **Durenol** » 17 Sep 2018, 03:12

Georgeandkira wrote: 15 Sep 2018, 08:07 How many miles on your '07 XC-70?

Practically nothing, we hardly drive it. But I noticed the problem around the 27-30K mark over a decade ago.

Post by **RickHaleParker** » 17 Sep 2018, 03:43

At a idle, any ripple would not be more then ~3000 cycles per minute. Even one of the cheap $25.00 DSO oscilloscopes on eBay can display 200,000 cycles per second, way more then fast enough.

Post by **jimmy57** » 17 Sep 2018, 06:46

The simple first check for diodes is to check to see what the AC voltage reads in system with engine running and some loads turned on (headlights on and blower on high). The diodes' job is to convert AC current to DC current. A failed rectifying diode lets AC voltage exceed 100 mV. But my suspicion would be the voltage regulator on alternator is faulty. On that car the CEM controls target of voltage regulator and there is a chance that CEM is signalling different voltage targets. The normal function on that year model is to use an algorithm and change voltage target for regulator for computed battery temperature. CEM can be the culprit for other reasons as it regulates external light bulb voltage by pulsing power supply to most of the bulbs (not fog lights, reverse lights, and bi-xenon) for longer life. If the dome lights or map lights inside are cycling then that would probably mean it's the voltage regulator.

Durenol · Post by **Durenol** » 17 Sep 2018, 18:48

RickHaleParker wrote: 17 Sep 2018, 03:43 At a idle, any ripple would not be more then ~3000 cycles per minute. Even one of the cheap $25.00 DSO oscilloscopes on eBay can display 200,000 cycles per second, way more then fast enough.

That may be true, but I'm not sure I'd trust a $25 device across the terminals of a battery that can deliver 500+ amps. My issue is doing whatever I'm supposed to be doing correctly and safely, not necessarily lowest price.

But either way, while I'm familiar with oscilloscopes in general I've never used one for this type of application before so I'm not really sure what I'm looking for or the exact steps I should be doing in what order.

Durenol · Post by **Durenol** » 17 Sep 2018, 18:58

jimmy57 wrote: 17 Sep 2018, 06:46 If the dome lights or map lights inside are cycling then that would probably mean it's the voltage regulator.

They definitely are, besides the headlights I notice the overhead interior lights doing it all the time.
Is the voltage regulator built into the alternator or something that attaches to it? How hard/expensive is it to replace one of these? Would it come with a new alternator or is it a separate part?