Major electrical problems

hiler62hiler62 Member Posts: 6
edited October 2016 in GMC
1st issue was the auxiliary air valve rusting shut, then the voltage drops, then the 4th speed hvac going out, now running at 19v not 14.4.
Replaced alternator, battery, cleaned all ground wires, replaced power train control module, no chance.
FIELD terminal is getting a 9 volt signal think that's wrong.
Going to replace the ignition switch next then the body control module.
Running at 19 volts burns out head lights and sure it's causing mire problems . 
Worst nightmare auto ever.
AMERICAN junk at its finest hour.
Gm unable to find a solution neither according to other customer posts wirh same oroblem.

Comments

  • thecardoc3thecardoc3 Member Posts: 5,533
    edited October 2016
    The field terminal signal is sent out of the alternator to the PCM that tells the PCM how hard the alternator is working to achieve system voltage. It is best viewed with an oscilloscope since what you want to see is the duty cycle, but at 9v it is spending most of the time high which would make sense if the alternator is overcharging.

    This isn't a difficult system to analyze, but you do need to have a full function scan tool that can provide codes, data and bi-directional controls.

    Why do you want to throw a BCM at this? If the vehicle uses RVC or SARVC (regulated voltage control, or stand alone RVC which according to the schematics I checked does not appear to be used in the Yukon line) the BCM is involved but it only tells the PCM what it wants, the PCM controls the alternator charge set voltage with the L terminal and as mentioned the alternator reports back to the PCM with the F terminal. Both the command and response as well as system voltage measurements can be seen in scan data, so you can see what the computer thinks it is seeing and compare that to direct measurements to get a diagnostic direction.

    Quick check, start the engine and after the alternator starts charging, disconnect the regulator connector. Did the output voltage change? If so what did it go to?
  • hiler62hiler62 Member Posts: 6
    It will drop to 12v.
    I had the old and new alternator tested off the vehicle.
    Both will deliver 14 vdc.
    It's the signal going into the alternator.
    Schematics show the signal comes from the pcm but the bcm also monitors the system voltage.
    I seen alot of complaints also about the ignition switch which could possibly give a poor feeding to the bcm and pcm.
  • hiler62hiler62 Member Posts: 6
    The pcm is getting a voltage reference somewhere my guess is from the bcm or ignition switch.
    Remember the earlier issues from the hvac blower pulling too many amps and the auxiliary air blower doing the same may of caused this permanent issue with over voltage . 
    I don't have the hvac blower on and the auxiliary air is just for 10 to 20 sec once you start.
    I belive thosee two sub standard unreliable high current motors may of caused burnt connections under the dash and through the wire run which it self will add resistance and more load.
    But won't know till I get under the dash for the ignition, under the Suv for the auxiliary air, & behind the glove box for the hvac blower checking resistance, voltage ,  & current draw.
    People that have just replaced the auxiliary air relay and hvac blower relay, controller resister still have the issue but some have replaced the motors em self cause they were drawing double there rated amps.
    Obviously defective parts sense it's so common but GMC won't admit to it.
    I'M worried my envoy will eventually have an electrical fire.
  • thecardoc3thecardoc3 Member Posts: 5,533
    hiler62 said:

    It will drop to 12v.

    It should drop to 13.68v which is the failsafe level set by the alternator's internal regulator.
    hiler62 said:


    It's the signal going into the alternator.

    The "F" terminal, grey wire, is a signal that comes from the alternator and is sent to the PCM that comes directly from the voltage regulator field current. The voltage regulator pulses the rotor current (field) and 400hz and varies the on time in order to change the alternator voltage output. The "L" or lamp terminal (red wire) is the one that the PCM sends to the alternator to control the charging command level. The duty cycle on the L terminal can command the alternator to out put anywhere from
    hiler62 said:


    Schematics show the signal comes from the pcm but the bcm also monitors the system voltage.

    But that doesn't equal a bad BCM. The BCM merely tells the PCM what it is seeing when it measures the ignition voltage as well as the battery supply voltage.
    Those values are available in scan data, what does it report?
    hiler62 said:


    I seen alot of complaints also about the ignition switch which could possibly give a poor feeding to the bcm and pcm.

    People often misuse information like that. Those reports reveal common issues but that's not enough to suggest replacing any parts. They are only enough to suggest that you need to measure those voltage levels while the problem is occurring, and then proceed from there based on what you find through testing.

  • hiler62hiler62 Member Posts: 6
    Thank you I will dig deeper.
    I'M also building a 5 volt references simulator to test all sensors there's a possibility the 5 volt supply could have stray voltage from a damaged wire bundle crimped or worn.
    Reverse of a grounded 
  • hiler62hiler62 Member Posts: 6
    Unfortunately can't afford a decent scan tool yet and dealership will charge at least 150.
    Saving up for a xtool 100 pad
  • thecardoc3thecardoc3 Member Posts: 5,533
    hiler62 said:

    Thank you I will dig deeper.
    I'M also building a 5 volt references simulator to test all sensors there's a possibility the 5 volt supply could have stray voltage from a damaged wire bundle crimped or worn.
    Reverse of a grounded 

    Building your own 5v reference makes for great study, but in practice today you have many of them available on any given car today that you can use to temporarily supply power when necessary. As far as some sneak circuit bleeding power into or pulling a reference voltage down, if you think that might be occurring then at some point you will probably need to measure that 5v reference. That is best done with a digital storage oscilloscope. With multiple channels you can take a suspect sensor and monitor the signal, ground, and reference for it all at the same time thereby taking all of the guess work out of the routine and greatly cutting down the diagnostic time.

    It takes a long time to learn to do diagnostics correctly but at it's most basic level it starts with a question. In the case of the 5v reference as you have mentioned lets say that you for what ever reason suspect that there is an issue with a reference voltage. One question you have to ask yourself is if that voltage goes high, (or low) what would I see the return signal go to and how would the PCM react to it? You see you not only would get a distorted signal but the computer is going to react to that signal. The easiest one is depending on how far that signal voltage changes and how long it is errant you could have the control module generate a code for that signal circuit. Another possibility is if it's the throttle position sensor circuit and the signal though moving stayed within a normal expected range the computer would react as if the throttle in fact opened (or closed) when it really did not. You should be able to feel that in how the car behaves as well as see it in the scan data. But here is the next question. Does the car in question have a 5v reference that only goes to the TPS or is that same reference voltage shared by other sensors? The answer to that is it's very rare for common sensors to have different reference voltage signals and armed with a schematic you should be able to identify those other sensor circuits. If they are not displaying the same type of a failure as the TPS in this example, what does that say about the 5v reference signal? Did you actually have to measure it to prove what it is doing?

    You say that you don't have the money to have a dealer's technician test and prove what is wrong, if you blindly tossed a BCM at this (which is very unlikely to fix it) how would that impact your ability to be able to afford to have a professional technician test and prove what is wrong? There are problems out there today that are so complex that a lot of professional techs aren't prepared to deal with them, so even they rely on specialists. It's actually less expensive for them and the consumer to do that then it is to fight a tough problem and especially if either of them have resort to tossing parts.

    Your trying to go about this the right way, but at this point in time you don't have all of the tools, training, and experience to work through it. Those are hard earned but if you really work it at are achievable in time. Maybe if the trade can get to the point that it offers the kind of wages and benefits that would be attractive to you, you could be a great technician. But it's hard work and you will deal with thousands of challenges that will have to be dealt with just like this charging system issue. You'll get beat up by them, but they will teach lessons that no discussion or classroom could ever match.
  • hiler62hiler62 Member Posts: 6
    Thank you for all your insight.
    I will defiantly shop around for a digital occiliscope, been 36 years sense I've used one in electronics class building a AM FM stereo on a foam bread board.
    I fully understand what your saying.
    I've just been so upset with all the electrical problems this Suv is having and I'm too stubborn to give in and go back to a Mitsubishi.
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