2004 Chevrolet Trailblazer, 4.2 LT headlight issues.

norman52

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#1
Make
Chevrolet
Model
Trailblazer
Year
2004
Miles
101,000
Engine
4.2
High beams work, low beams work.
Problem is when high beams are on the low beams stay off. All 4 bulbs should be on when using high beams.
Any ideas? Relay or module issue?
 

grcauto

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May 29, 2014
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#2
Are you sure they all came on before?
 
Last edited:

big al

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#5
Im questioning if all 4 should be or not. I know alot of these gm vehicals turned off low beam side when the high beam switch is activated. I know alot of the full size gm trucks are this way
 

nickb2

Wrench. Diagnostic Tech.
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#7
My loliness is killingme, nope, odonèt need a sign right now, just hyjacked a thread just to say, I like being alone while my gf makes edouble time. Gives me time to check her shyter out.

Yup, this keyboard is due for a six pack,

 
Joined
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#8
Vehicles use to be very particular on headlamp circuitry, a dash switch with self cleaning wiping contacts and a super fast automatic resetting circuit breaker.

This all changed with daytime running lamps, a complex relay system is used to put either the dims or the brights left and right side lamps in series. These are point contact relays, the worse kind in the world, one bit of arc carbon, no contact, really done by bunch of idiots. DTR is not required by law in the US.

To save on relay logic as its called, later vehicles are using the BCM, code almost stored in flashram do the determination.

To really know what you have, need an alldata.com subscription. Actually prefer alldata, OEM shop manuals so much constantly safety information, can't find what you are looking for. Practically all of it is common sense.

Information you are looking for should be here.

https://my.chevrolet.com/content/da...rolet/blazer/2004_chevrolet_blazer_owners.pdf