wiring issue, 1991 4x4 suburban

Discussion in 'Chevy Suburban Forum (GMC Yukon XL)' started by G.A. Heath, Apr 26, 2009.

  1. G.A. Heath

    G.A. Heath New Member

    Electrical issue, 1991 4x4 suburban

    Earlier today my 1991 Chevy Suburban (4x4, auto, mostly stock electrical and drive train) blew the PCMB fuse. After replacing the fuse I tried to start it resulting in another blown PCMB fuse. After checking different things (and blowing a fuse with each try) I was able to get it started not knowing what caused the issue. I just now got it home, turned the switch to kill it only to discover it kept running. I grabbed a pair and insulated channel locks and pulled the coil wire from the distributor, and disconnected the ground on the battery.

    Now I am wondering what is between the PCMB fuse and the ECU. Is it a relay (makes sense and if its sticking would cause the issue I am having) or is it just the ignition switch (makes less sense, but if defective could cause the issue also). In the AM I plan on finding my Chilton and looking at the wiring diagram.
    Last edited: Apr 26, 2009
  2. unplugged

    unplugged Epic Member 5+ Years 1000 Posts

    The ignition switch is a good bet. They aren't expensive and they are easy to replace. For wiring diagrams you can check out autozone.com or dwill3015's post for online manuals here: GMTC
  3. G.A. Heath

    G.A. Heath New Member

    First chance I got I went and pulled the ignition switch. it checked out ok, so I reconnected it, connected the battery, started the suburban, disconnected tbe switch, and it kept running. so now I know I am either looking for a relay that I can not find listed anywhere or an ECU. I believe my mystery relay (if it exists) should be located near the ECU. My chilton's wiring diagram leaves much to be desired.
  4. G.A. Heath

    G.A. Heath New Member

    Today I picked up a new fuel pump relay, just to have a known good unit installed. Same symptoms were shown. I then decided to kill it by pulling the relay and got success. Just out of curiosity I decided to try turning it over with out the relay engaged, and it started. Next I turned the key off and it shut off like it is supposed to. It works just fine without the fuel pump relay installed, but this is not and can not be right. I also put my test light on the plug connectors and got the grey, orange, and the green/white were all hot w/o the relay installed and the engine running.
  5. GaryL

    GaryL Epic Member 5+ Years ROTM Winner 1000 Posts Platinum Contributor

    The fuel pump relay is only there to "charge" the fuel lines and pressurize the fuel injection system. Once it does this, it shuts off. The reason it restarted with out the relay was because you just had it running and it was still pressurized. The fuel pump will not run without oil pressure, so the relay makes it run for a few seconds so you can get it started and get the oil pressure up, then the fuel pump takes over.
  6. G.A. Heath

    G.A. Heath New Member

    I understand this, and had determined that this is probably the case from the wiring diagrams. But what confuses me is the fact that the ignition switch will kill the engine with the relay disconnected but will not when the relay is connected. Additionally disconnecting the relay with the engine running will kill the engine. These symptoms do not make complete sense to me. What I am really looking for is someone who has idea what can cause these symptoms. Suburban Parts

    Symptoms are (currently):
    Was blowing PCMB fuse.
    Quit blowing PCMB fuse and continues running after ignition switch was cut off.
    Engine dies when PCMB fuse is pulled (as it should).
    Ignition switch tested good.
    Fuel pump relay (FPR) appears to be ok (swapped brand new unit in its place).
    Engine dies when FPR is disconnected (will restart immediately, or hours later).
    With relay disconnected and engine running three leads are hot on FPR relay plug (including the green/white from the ECU that is supposed to engage the relay).
    Engine can be turned off with ignition switch if relay is disconnected prior to starting.

    Possible issues:
    Wiring fault (currently looking for this issue).
    ECU issue?
    Last edited by a moderator: Nov 15, 2009

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