# Headlights dim at low RPM's ?



## pmarcuccijr (Sep 30, 2010)

92 Jetta Gl 8v 

When I start my car, the battery light on the instrument panel stays on until I give the engine a little rev, then it goes off, and stays off. 

At idle, my lights are very dim, but as my RPM's increase, the brightness increases. Driving for 20 minutes causes the lights to remain a little brighter, but there is still a noticeable "dimming" when the RPM's drop. 

No problem whatsoever with weak starts, the car only needs half a crank and it starts right up. Idle is stable. All in all the car runs great. 



new headlight assembly 

new bulbs 

new battery 

new ignition coil 

new headlight switch 

all grounds are good 

battery connection is clean/solid 

new spark plugs/wires 

new distributor cap 

 

I'm thinking the alternator is a bit weak, it appears to be original. 

Any suggestions?


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## audipanzerwagen (Jul 11, 2003)

*You have answered your own question....*

But it may be an internal alternator problem. Either the Voltage regulator or one of the Phase Diodes is giving up the ghost. Whatever, you need to have the Alternator put through a "Full Load" test. Autozone or PepBoys can do it for you.


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## cuppie (May 4, 2005)

And, it's also quite possible that the alternator is fully functional - just that the power isn't making it to the battery posts. Connections do break down, especially after nearly two decades..... 

Got a voltmeter? With the engine idling, check the voltage at the battery posts; also check with the engine at ~2000 RPM. Turn loads (headlamps, etc) on, and repeat (idle and 2k checks.) 
Next, put the voltmeter directly to the alternator (+ to the output post, - to the case.) Repeat above measurements. 
Under all circumstances, the voltage at the battery posts should equal (or, at worst, be ~0.2v less) than the voltage at the alternator. Any higher, and you have a voltage drop to fix. 
(note: pen & paper are helpful here.  ) 

Could you do the above tests, and post results?


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## pmarcuccijr (Sep 30, 2010)

Thanks cuppie, 

I will definitely give what you said a try, and post the results. I have a clutch to put in this weekend, so if I have extra time I will definitely do that.


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## gti dreamn (May 18, 2002)

*don't worry, new ones do it too...*

funny, my 2004 GTI does this same thing. As my goes to idle at night, the lights dim when sitting at lights and when the brakes are pressed. 

I guess VW hasn't quite figured this out in 12+ years. 

I think my culprit is the 760rpm idle speed. If I could raise it to 800-850, I think my problem would go away. However the dealer won't touch it cuz their VAG-COM says it is within parameters and won't let them raise. Time for me to get Lemmiwinks. 

When the A/C is on, the (factory) idle is 740rpm. Again, a bit low in my book, but I'm not a VW engineer. 

Every other manual car I've owned has had ~900rpm idle speeds, so this sub 800rpm is still tough to get used to.


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