QuestionI have a 1997 GMC C/K 1500 pick up that i just installed a new head unit to replace the factory delco radio. I found all colored wire diagrams i could online and i know i have my new deck wired in correctly and i know my new head unit works because i have used it before. Everything thing on the head unit works, however i am getting no sound out of any of the speakers. they all worked fine with the stock radio. What could be wrong or what could i do the fix this? Also i have three wires left over from the truck a blue with white stripe, a blue, and a purple. What would these wires be for?
AnswerHi Brian,
Sorry about the delay in answering your question.
Generally, when you have an after-market head unit that powers up but doesn't produce sound, there are two likely causes: a speaker wiring problem, or a factory amplifier.
In the 1997 GMC pickup, I don't think a factory amplifier is very likely; as far as I know, these trucks weren't available with GM's Delco Bose premium sound system. If you see a Bose logo on the door speaker grills or factory radio, though, then you'd probably need an OEM interface adapter to properly integrate the Bose amplifier with the after-market head unit--or you'd need to re-wire the speakers to bypass the amp.
A speaker wiring problem might mean a shorted or pinched speaker wire, or speaker wires not connected to the head unit properly. In a GM radio harness, it's common for different wires with different functions to share the same color. For example, there may be two or three dark green wires, but only one is connected to a speaker. If you connect the wrong wire to a head unit's speaker output, it could cause the deck's internal amp to shut down, resulting in no sound to any speaker.
If I were troubleshooting the problem, I'd use a digital multimeter with a "continuity" feature to test the speaker wire pairs in the radio harness. If you set a meter for continuity testing and touch the probes to both speaker wires in a pair, you should see continuity with between 4-10 ohms of resistance. If the speaker wire pair doesn't show continuity, you may have the wrong wires, or there may be a problem with the speaker. Also, you should test each speaker wire for continuity with chassis ground; a grounded wire can also shut down the new head unit's internal amplifier.
My references don't show the wire colors you listed in the GM radio harness, except for the blue wire; this should be a right rear speaker wire. There should be a violet/white wire that's an illumination wire, and a dark green/white wire that carries vehicle speed information for the factory radio's speed controlled volume feature.
Hope this helps!
Brian
