Question about 1992 GMC Sierra

2 Answers

I am the original owner of a 1992 GMS Sierra Z71 pickup. It is just a full-size pickup, 5.7 liter motor with A/C and standard transmission. I currently have 2 problems. The 1st problem is the blower fan for the heat and a/c. It only has high speed now, no low or medium and was quite noisy. I replaced the blower fan motor with a new after markert blower fan thinking this would solve both problems. All it solved was the noise as I now have a quiet blower fan but still with only high speed. I would like to have medium and low fan speeds too. 2nd problem is with the radio unit. Several years ago it started to quit playing when turning on the power. Currently with the truck power on, and the radio powered off the clock shows 1:00 o'clock and the time is not adjustable. With truck power on and radio power on it displays a radio station and the volume is about 1/2 way up but there is no sound. Also the station cannot be changed either using another channel button or the seek features, nothing happens. The volume does not adjust either, up or down. I replaced it with a radio from a salvage yard that is guaranteed to work but the same problem still exists. Any ideas on these problems. I have 288k miles on my truck and other than these 2 problems there is nothing else wrong with the truck. Thanks, Ray

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  • Ray Howerton Apr 11, 2011

    Ok. I will check the resitor and see what I can determine. I guess this means I take apart the dash part way again, oh joy!! what fun!!

    As far as the radio problem, that is not my wife who posted the question on the link you provided, The pictures in the link show exactly what mine does. As far as the radio is concerned there is a seperate cassette player and graphic equalizer in the center of the dash located directly in front of the stick shift. Y found this answer earlier

    I have no idea where output amplifier chip is located unless its on the power amp that the radio and cassette are wired to that is located right above the accelerator pedal and slightly to the right.

  • Ray Howerton Apr 11, 2011

    When I replaced the radio I found 2 sets of wires coming from the back. One of the sets (just 2 wires) went to the cassette play/graphic equalizer located in the center of the dash. The other set of wires went to the what I am thinking is the power amp (CDM) (thats what it looks like to me) and ithis is located above and slightly to the right of the accelerator pedal.

    What does CDM stand for?

  • Ray Howerton Apr 11, 2011

    Thanks. I will look into the CDM. No mention of it in my Haynes manual and not much luck searching the internet for its location. I am a techie so I understand the principle and binary code used from what I read about this on the internet,

    I'll be back in touch once I can find the resitor and CDM and see what happens with these


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  • GMC Master
  • 75,822 Answers

Let us deal with the two very separate problems here.
The problem with the blower motor is that the blower motor speed control resistor is burned out, when this happens all you will have is high speed setting. The blower motor speed control resistor is located right next to the blower motor in the air ducting. It has an electrical connector going to it with 3 wires, the resistor is roughly oval shaped and would easily fit in the palm of your hand. It is held into the air duct work by two screws. See picture below.

I am the original owner - emissionwiz_150.jpg I will deal with the radio issue in a separate reply

Posted on Apr 11, 2011

  • yadayada
    yadayada Apr 11, 2011

    Next to the Radio problem.

    The radio most likley does not have any "KEEP ALIVE POWER" 12 volt power to it, the radio has has two battery (+) wires to it, one is ALWAYS hot and supplying 12 voltsto the radio connector, this is the keep alive power, the other is the main power that turns off and on with the ignition switch. Pull the the radio out and make sure that you have 12 volts at the radio with the ignition turned off and the key removed. If it turns out you do not have power to the keep alive circuit then you have likely found the problem, loss of power to the keep alive can be a fuse in the inside fuse box is blown or a loss of power to the fuse from its battery source. Below is the wire colors of the two power leads to the radio.

    Car Radio Battery Constant 12v+ Wire: Orange (this is the keep alive)
    Car Radio Accessory Switched 12v+ Wire: Yellow


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  • GMC Master
  • 8,206 Answers

Hi, the problem with the blower is probably the resistor. I think it's by the blower on your truck. You may have seen it there. The blower wires go to the resistor. There are additional wires in the resistor that go to the switch. The resistor is held into the heater box by a couple of screws. Take the connector off, remove the screws, and pull the resistor out. If it is bad, it will show heat damage. You can test it with an ohmeter for continuity. Sometimes the switch goes bad first and then causes the resistor to fail. After installing a new resistor, listen carefully as you change the blower speed to see if it has 2 distinct med and low speeds. If it doesn't hold a distinct speed when you giggle the switch, replace the switch or it will burn up your resistor again.

For the radio, I have never heard of that behavior. Are there separate controls for the radio anywhere else in the truck? Like on the steering wheel or in the back seat? That is the only way I can imagine 2 radios would present the same issues. I will see if I can find a wiring diagram on that and get back to you.

Posted on Apr 10, 2011

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  • Jeff Turcotte
    Jeff Turcotte Apr 11, 2011

    On the radio, I have found others with similar issues. There is a post that is exactly like your issue--maybe this post was put in by your wife? There are others as well, though--not exactly like yours but similar.

    Anyway, I have not found a solution. Apparently there are other components that connect to the head unit. It may be that these other parts are to blame, and that would explain why you can change out the radio and still have a problem. Perhaps the part you are swapping out does not have the parts that change the volume, clock, and stations, etc. I will look some more.

  • Jeff Turcotte
    Jeff Turcotte Apr 11, 2011

    Back to the radio, I'm reading there is something called CDM that's by the steering column. See if you can trace the radio wires there. If that unit is a multiplexer, it may be your problem. You may be able to get one of those at the salvage yard--maybe trade in that extra radio you have for it.

  • Jeff Turcotte
    Jeff Turcotte Apr 11, 2011

    code division multiplexing. I don't know much about it in this application, but multiplexing is a way to send multiple signals across one wire, like the internet works but different. I got the impression that CDM was printed on the module, but not really sure. If it's just an amplifier, it would not explain the behavior you are seeing. It has to be something that does the functions of tuning, etc. or at least have the ability to take control of the display and interface.

  • Jeff Turcotte
    Jeff Turcotte Apr 11, 2011

    By the way, I saw that answer on the wiki site too, but I didn't give it any salt, because he gave no explanation for why the amp would cause that problem.


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