Headlight brightness fluctuates on 1987 Golf

[vavavroom]

I have noticed that on my son's 1987 Golf the headlights sometimes dim noticeably while driving. Then they appear to get slowly brighter, which I don't notice until they go abruptly more dim again. I would guess the decrease in brightness at 20%. At idle, I notice a minor increase in brightness if I rev the engine a bit. The first time I noticed the lights dimming, I thought the street lights had suddenly shut off, so it was quite noticeable. The Voltmeter in the car shows just under 14V with the engine running and the headlights on. I drove this car 20 years ago and can't remember if my observations are expected as normal or not.

 

[WyrTwister]

Bad / loose connection ( including grounds ) ?

Alternator doing odd things ?

 

[Linctex]

The easiest task would likely be to try swapping the alternator with another one, for troubleshooting purposes.

If the dimness issue is ENTIRELY RPM-Related, I would look at the alternator, for sure.

If it changes when going over bumps and things, then it's definitely a bad bonding strap or ground wire somewhere.

 

[eljefino]

Turn on the blower fan motor when you're driving this. Ears are really good at hearing changes in pitch that happen with voltage fluctuations.

Some Bosch alternators have a little hole you can stick a screwdriver in to ground the field to force full power. This would be good to see if it's weak and giving all it can. Some also have removable voltage regulators. But another whole one should be cheap enough.

 

[SubieRubyRoo]

+1 on checking the headlight harness grounds... remove, sandpaper the mounting points and the ring tabs, reassemble, and squirt with WD40.

 

[Trav]

Do a voltage drop test, these things are known for the alternator to battery cable going flaky as they age.

https://www.aa1car.com/library/voltage_drop_testing.htm

 

[RooflessVW]

There's no headlight relay - your headlights switch could well be at the end of its life.

First thing to check is the major grounds and battery cables - and when I say check, I mean replace.

 

[Audios]

I would say test output of battery and alternator, and then look into the cables. I swapped out the cables on my 02 golf and made a huge huge difference in starting and cranking. Way more peppy now. I used these guys, great quality.

http://innovativewiring.com/?page_id=161

 

[Trav]

When I do these I get all the cables and most of the terminal lugs from AC/DC supply, all USA made wire and terminals and use tinned marine battery cable and lugs.
In the salt belt terminal corrosion is a real issue,I had a Ford F150 starter cable rot right off.
Remy has the 45 and 90 degree for the starters and for the MKIV military terminals are the best bet as you need 2 positive cables, one for the starter and the other for the fuse box on top of the battery.

These are real easy to make, I did use solder slugs but now crimp and solder. I buy (or cut my own) lugs with a peep hole in the front of the lug, hex crimp it then put a couple of drops of kessler rosin flux down the hole, heat it with a propane torch and put some rosin solder in the peep hole. This give a nice soldered joint in the front of the lug without wicking all the way up into the cable making it brittle then good heat shrink tubing.

Alternator to starter or fuse box AWG 4, battery + to starter 1/0, fuse box to battery + AWG 2, battery neg to engine ground 1/0, engine to chassis 1/0 and then a few extras.
I add battery neg to body 1/0, front alternator bracket to front engine mount 1" tinned copper braided strap, upper plenum to firewall 1/2" tinned copper braided strap and finally battery fuse box to interior relay panel AWG 4.

This seems to cure most of not all electrical gremlins in these cars, it takes about 3 hrs to do right, the key to lifelong performance of the cable is to use tinned cable and lugs if you live in high salt areas.

https://www.acdcwireandsupply.com/

https://www.remybattery.com/cable-c...s-lugs-and-terminals/lug-connectors.html

 

[vavavroom]

Thanks for the suggestions. I have checked all the ground connections. They look good. How stable should Voltage output be, and how much of a difference in headlight brightness would, for example, an 0.25 Volt drop make?