GMC Repair: Yukon headlights too dim, daytime running lights, gmc yukon

Question
QUESTION: I have a 1999 GMC Yukon (old body style 5.7L).  I bought it new. Lately I have noticed that the low beams are too dim to drive with at night.  I have to use the highbeams all the time to see.  There doesn't seem to be much change in intensity when going from daytime running lights to headlight/low beam.  I checked the voltage at the lowbeam headlamp.  When DRL is on, the voltage is 12v.  With lowbeam on the voltage is 12.5v.  How much voltage should I be getting?  Is my DRL module bad?  thanks!!!

ANSWER: Hi
I would check grounds and all related circuits and connectors for high resistance.
Any extra resistance will reduce flow and make loads have operational problems.
I hope this helps.
Good luck
Pawl

---------- FOLLOW-UP ----------

QUESTION: Is my voltage correct?  What voltage should be present at the headlamp connector for DRL and for low beam headlight.  Obviously, low beam headlight has to be a couple of volts higher than DRL, right?  Should DRL be 12v and low beam headlight 13 or 14v?  thanks!!!

Answer
Hi
The voltage is not changed.
Here is the EXACT GM service manual circuit description for the drl/headlamps


Headlights/Daytime Running Lights (DRL) Circuit Description
Voltage is available to the headlamp and panel dimmer switch at all times. The headlamp and panel dimmer switch includes a self-resetting circuit breaker. The circuit breaker opens when the headlamp and panel dimmer switch draws too much current. When the circuit breaker opens, the circuit breaker interrupts the current flow. Without the current flow, the circuit breaker cools and resets automatically.

When the headlamp and panel dimmer switch is in the HEAD position, the headlamp and panel dimmer switch directs voltage to either the low beam headlamps or the high beam headlamps. This depends on the headlamp and panel dimmer switch position. When the headlamp and panel dimmer switch is in the HEAD position, not only is voltage available for the headlamps, but the daytime running lamps (DRL) module receives a HEADLAMPS ON signal. When the DRL module receives this signal, the module disables the DRL operation, if the ignition switch is in the RUN position, the BULB TEST position, or the START position.

When the headlamp and panel dimmer switch is in the HEAD position and the headlamp and panel dimmer switch is in the LOW position, voltage is available through the closed contacts of the headlamp and panel dimmer switch to the left low beam headlamp and the right low beam headlamp. Ground G112 supplies ground to the right low beam headlamp Ground G113 supplies ground to the left low beam headlamp in vehicles with the base headlamps and in vehicles with the quad headlamps. With both power and ground applied, the low beam headlamps light.

When the headlamp dimmer switch is moved to the HIGH position and the headlamp and panel dimmer switch is in the HEAD position, voltage is available through the closed contacts of the headlamp and panel dimmer switch, and the headlamp and panel dimmer switch to the DRL module, the left high beam headlamp, and the right high beam headlamp. With the headlamp dimmer switch in the HIGH position, voltage is available to the high beam indicator. With ground G103 in vehicles with gasoline engines, or ground G104 in vehicles with diesel engines, supplying a ground to the high beam indicator, the indicator lights. Ground G112 supplies a ground for the right high beam headlamp. Ground G113 supplies a ground to the left high beam headlamp. With both power and ground applied, the high beam headlamps light at full intensity.


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  Document ID# 264991
1999 GMC Truck Yukon - 2WD     

So the voltage should be about the same at alll times, whether on DRL, low or high beam.
The intenisity is changd PROBABALLY through resistance given from the DRL module.
I Hope this helps.
Good Luck
Pawl