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Super Mechanic
Super Mechanic, Master technician
Category: Chevy
Satisfied Customers: 16816
Experience:  29 yrs. exp., ASE master, cert. in G.M. electrical, a/c, susspension, brakes, electronic steering
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1993 GMC C1500, the gh beams are always on, even with the

Customer Question

1993 GMC C1500, the high beams are always on, even with the switch off and the low beams do not work.
JA: Sometimes things that you think will be really complicated end up being easy to fix with Chevy. The Chevy Mechanic I'm going to connect you with knows all the tricks and shortcuts. Tell me a bit more about what's going on so he can help you best.
Customer: Bulbs, wiring and fuses all seem to be in good shape
JA: How many miles are on the car? Do you happen to have the wiring diagram?
Customer: 294,000 kms on the truck. I looked for a wiring diagram on Mitchell/Prodemand websites but cannot find anything.
JA: Are you hoping to fix this yourself?
Customer: yes
JA: Great! What have you tried so far?
Customer: checked the fuses, wiring harness and bulbs, everything looks good up to the dash. Wondering if this could be a problem in the switch
JA: Is there anything else the Chevy Mechanic should be aware of about your Chevy?
Customer: It was recently purchased, not sure if anything has been modified under the dash
JA: OK. Got it. I'm sending you to a secure page on JustAnswer so you can place the $5 fully-refundable deposit now. While you're filling out that form, I'll tell the Chevy Mechanic about your situation and then connect you two.
Submitted: 4 months ago.
Category: Chevy
Expert:  Super Mechanic replied 4 months ago.

Hello and welcome. My name is ***** ***** you for visiting the site.

Are the high beams on with the headlight switch off?

Does the dimmer switch seem to click when activated?

Customer: replied 4 months ago.
the high beams are on as soon as the ignition is on, and the park lamp light (green) is displayed on the dash whenever the ignition is on. The high beam indicator does turn on with the switch, and the dimmer switch is working normally. Low beams do not work at all.
Expert:  Super Mechanic replied 4 months ago.

I see, thanks. From what I remember they only had daytime running lights in the Canadian vehicle in 93. In which case could be a ground issue. Does it happen to have DLR's?

Customer: replied 4 months ago.
it does.
Expert:  Super Mechanic replied 4 months ago.

I see. It may just be that the low beams are inoperative. The high beams are the DRL's (at half voltage)

When the Ignition Switch is placed in RUN, the voltage from the relay is provided a ground path through the LT GRN (11) wire through the module. This enables the relay to provide voltage to the High Beam Lamps in series causing the lamps to share the voltage resulting in the lamps operating at half intensity.


That could be in the headlight switch, dimmer switch or fuse link. Let me pull up some diagrams on it.

Expert:  Super Mechanic replied 4 months ago.

Where did you go?

Expert:  Super Mechanic replied 4 months ago.

If you accept my additional service request for your email address, you can give me your email in a private info box and I'll send you the test instructions in a PDF file, the file is rather large.

You will just have to accept the additional service offer and ENTER your email address in the “ENTER CONTACT INFO” section so that I can see it.

I do not have access to your email address,