First time poster and need help desparately.

Discussion in 'Performance & Fuel' started by Rascl93, Apr 19, 2012.

  1. Rascl93

    Rascl93 New Member

    I have a 98 Tahoe that had a lot of aftermarket stuff on it (alarm, sound system, etc) at one time, but it was all removed before we bought it. Of course it didn't run because of the passlock system when the previous owner removed all the bypass equipment. Anyway, we have solved the "tamper mode" issue and are down to a fuel/starting issue and I'm at a loss where to go from here. We have 12v coming in on 87 to the fuel pump relay and if we tap the relay it will trip and start the pump, but it will not shut off. If we crank it we loose all fuel pressure and the relay trips back out. On 85 we only have 6.9v. I believe this is the pin the heats the coil inside the relay to trip it. Not enough voltage to cause it to trip, but when we tap it, it trips. If we jumper 87 & 30 the fuel pump runs and we can start and run the vehicle. We can also jump orange to gray at the oil pressure switch and the pump runs. So I have come to the conclusion that I have an issue on the green/white wire that comes to pin 85 on the relay for one. Everything I have read says this wire comes from the VCM, but I can't find the VCM any where. Can any one give me some input or a wiring diagram or something?

  2. donyms

    donyms Epic Member 5+ Years ROTM Winner 1000 Posts

    Wow, you really have patients. I don't have an answer for you but I wanted to welcome you to the club. I know someone on here will see your thread and help. Also, try looking in the technical part of the forum, there may already be an answer there if not try posting your question there. Good luck and welcome to the GMTC :glasses:
    Last edited: Apr 19, 2012
  3. Rascl93

    Rascl93 New Member

    Not a man, but thanks for the welcome. :)
  4. mfleetwood

    mfleetwood Epic Member 5+ Years ROTM Winner 5000 Posts

    Which engine do you have? I'll check a few sites and what I can dig up for you.

    btw...welcome to the Club!!!

    Oh, and have you checked for codes? If so and a number was generated, please let us know.

    Last edited: Apr 19, 2012
  5. Rascl93

    Rascl93 New Member

    5.7 engine We have checked for codes. The only thing we had was the security issue because of the alarm system bypass that was removed - I'm sure for "fast money". We have taken care of that. From what I can gather, the fuel relay is not getting the signal it's supposed to. We have constant 12v on 87 via the ECM B fuze. I believe we are supposed to have ground on 85 coming from the PCM - which we do not - we have something like 6.9v. Then, if I understand the diagram correctly, the ignition is supposed to send 12v to 86 to energize the coil in the relay causing it to trip, which would close the circuit between 87 and 30 sending 12v to the pump. Anyone jump in and correct me if my brain is not looking at this right.
  6. mfleetwood

    mfleetwood Epic Member 5+ Years ROTM Winner 5000 Posts

    Here is a wiring diagram....Let me see if I can find you a pic of where your VCM is located. Sorry, I don't know enough to confirm how it all works.


    ---------- Post added at 10:56 AM ---------- Previous post was at 10:51 AM ----------

    I found this explanation regarding the green/white wire. Not sure if it is absolutely correct or not, but it may make sense to you.

    That green wire with the white stripe comes from the computer to activate the relay only. It will have power when the key is turned on for 2 seconds or until the computer gets a crank signal. That wire can not handle a load. The relay that wire energize, uses very little. The pump is powered by the ECM B fuse when the relay is energized.
    If the relay area is messed up in the [FONT=inherit !important][FONT=inherit !important]fuse [/FONT][FONT=inherit !important]box[/FONT][/FONT], You can put an external relay under the hood to work off the fuel pump.
    The fuel pump relay should have nothing to do with the fuel gage. That would be another issue.

    ---------- Post added at 11:01 AM ---------- Previous post was at 10:56 AM ----------

    Here you go...I hope this helps!


  7. Rascl93

    Rascl93 New Member

    thanks - knew that was the computer, but was looking for something else that triggered that relay. Some searches told me there was another module of some kind under the dash. Guess I shouldn't always rely on what I read on the internet. LOL So my next quest is to figure out why I'm getting voltage on 85 instead of ground. What pins in the VCM does the ignition come in to? Maybe it is an ignition problem. Somehow I doubt it. I have a feeling the VCM is bad and not sending the right signal to the relay. Any other suggestions?
  8. mfleetwood

    mfleetwood Epic Member 5+ Years ROTM Winner 5000 Posts

    From the research I've done, it's surprising that another code is not being thrown. I did read that when there have been problems stemming from the VCM that a reflash of the VCM was needed (there was actually a Service Bulletin put out on it).

    I'll check to see if I can find any diagrams specific to the VCM connections.....

    ---------- Post added at 11:38 AM ---------- Previous post was at 11:14 AM ----------

    I found a wiring diagram (wow, lots of wires)....try clicking the link below. If it does not work, let me know and I will provide instructions on how to find. I would copy and paste but for some reason, it won't let me do it from the Chilton's site.
  9. Rascl93

    Rascl93 New Member

    No problem opening it. You're the best!! Am I safe to assume that the blk/wht wire on the coil side of the fuel pump relay is a ground? If that is the case, I should get voltage on the dk. grn/wht coming from the ecm to the fuel pump relay for the 2 second prime when the key is in the "on" position but engine not running.
  10. mfleetwood

    mfleetwood Epic Member 5+ Years ROTM Winner 5000 Posts

    Hopefully someone (an expert) will chime is soon. I wish I knew :glasses:

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