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HeadLight Trouble Shooting

The standard sealed beam headlights that came from the factory will turn yellow
& get dim with time, the more you use them the quicker that time comes.

The halogen type will burn white & brighter out of the box then a new old style
sealed beam headlight & they stay that way right up to the time they blow out.

Of course keeping the voltage at 12-14 volts at the light helps. You can test your wiring
by checking the voltage at the headlight plug, if it's anything lowered than about 1-2
volts then the out put of the charging system, you will want to check for corrosion,
bad switch & connections. This is were the relay system helps, instead of losing this
voltage through the wiring (Voltage drop) you run power to the lights through a shorter
wiring system with a relay system, getting less voltage drop, & brighter headlights.

There are a few WebPages on the net now with Headlight Relay Systems on them,
my site even has one here: Headlight Relay System.

Headlight systems can be tricky to troubleshoot over the internet. You need to know what
is happening in the system first to understand why they are doing whatevery it is
they are or are not doing. Power for the marker/park lights comes from a fuse, while
power for the headlights if not fused, rather they have a circuit breaker built into
the headlight switch. Looking at the below diagrams you'll see there is two styles
of headlight switches used on the 6066 GMC Trucks.

[1960-63 Light Switch] [1964-66 Light Switch]

[1960-63 Light Diagram] [1964-66 Light Diagram]

Power for the headlights flows from the battery to the switch, (post #4 on 60-63 or post #1 on
64-66 trucks), then the power goes through the circuit breaker. Once the switch is pulled
all the way out, power flows from the headlight switch (post #3 on 60-63 or post #6 on 64-66
trucks), to the dimmer switch common post. Next power is directed to the low or high beam
depending on position of the dimmer switch. Power leaves the dimmer & goes through the
bulkhead connector to the engine bay harness on the left side of the truck, then to the grill
support connector. From the grill support connector the power runs to the headlight connectors.
If the headlights are good, power will flow through them to the ground wire in the headlight
connector, then to the ground wire which is grounded to the back side of the grill. The grill
in turn is grounded to the grill support which is grounded to a small wire from the negative
side of the battery. If any of this flow of electricity is broken up or slowed down in any
way, the headlights will not work or be dim. Bad Grounds are a common problem with headlights,
running the ground wire from the lights to a negative ground post will take care of grounding
problems. I like to have a common ground post wired to the battery negative post.

The dimmer switch lives on the floor of your truck under the floor mat, so it can get wet &
corrode. If your lights don't have power past the dimmer switch, check for "Ugly Green Stuff".
If you see any, replace the switch & clean the dimmer plug. If you only have low or high
beams, check your dimmer switch, (after you have checked to be sure the lights are OK. I have
seen times were both low beams are out, changing the dimming won't help that).

Once your headlights are working as they should, you mite consider doing the Improved
Headlight System
modification. This helps your newer style headlight shine brightly. If you
have other headlight troubles, e-mail me, so I can help you and add your problem to this page.

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Lighting & Electrical Projects
&/or Demons in Your GMC

Installing Fog Lights

Installing Cornering Lamps

Rear Fog Light Project

GMC Trucks Cargo Lights

HeadLight Trouble Shooting

Park/Tail/Stop Light Trouble Shooting

Custom Instrument Panel

Turn Signal Systems

Install the Delco-Remy 12SI Alternator

Trans Temp Gauge

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