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Thread: Glow plug relay question

  1. #21
    Join Date
    Mar 2020
    Location
    Long Beach, CA
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    Great, that’s just what I needed to know. If you could point me to where the temp sensor is, that would be extra helpful! Otherwise I’ll start reading up in the books and online. Thanks!
    1995 GMC Suburban C2500

  2. #22
    Join Date
    Apr 2001
    Location
    CA
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    13,370

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    The PCM temp sensor is in the thermostat housing, and is the only ECT (Engine Coolant Temperature) value the PCM "sees". If this sensor fails, it will mess with the GP cycles and cold start behavior (like timing advance and high idle staying on, or not activating at all). Disconnecting it will "show" the PCM a value of -40°, and a short between the polls will indicate +340°F. A short should trigger the SES and set a code for the sensor. The resistance between the sensor polls can be measured for a linear value of the coolant temp. Very simple test with a VOM (Volt/ohm Meter). I don't recall the range of values, but it should be easy enough to find.

    The temp sensor in the head is for instrumentation only, so disconnecting this will only change what you see on the gauge.
    1985 Blazer 6.2
    2001 GMC 2500HD D/A
    dmaxmaverick@thedieselpage.com

  3. #23
    Join Date
    Mar 2020
    Location
    Long Beach, CA
    Posts
    15

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    Great, I’ll check that out. Thanks very much.
    1995 GMC Suburban C2500

  4. #24
    Join Date
    Mar 2020
    Location
    Long Beach, CA
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    15

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    Thanks DmaxMaverick, you saved me. It wasn’t the PCM, as I had a spare that I tried, and it wasn’t the temp sensor, unplugging it didn’t help, I sure hoped it wasn’t the harness, it turned out to be the glow plug relay. I replaced it very recently when I was troubleshooting the initial starting issue, but things went back to normal when I put the old one in just now, which was still working fine. The new one was from Auto Zone, amazing it failed so quickly, what a p.o.s.! Anyway, burb is starting fine and running great again. Thanks!
    1995 GMC Suburban C2500

  5. #25
    Join Date
    Apr 2001
    Location
    CA
    Posts
    13,370

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    That's great! It's always nice when we get stuff figured out.

    I went through something similar on my 2001 Duramax a few months back. There's no difference between the old and new function, but it's more complicated and the parts are a lot more costly. I bought a high-dollar NAPA GP controller, only to find I had a ground issue. I like spare parts, but I'd rather not have $250 parts in the glove box if I can help it. I'd have just bypassed it, but I have to pass a smog inspection every 2 years, and a bypass won't turn off the SES lamp. The Duramax will start w/o plugs down to -8F (tested, albeit rough), but the EPA and CARB isn't happy with it, and they don't trust me with a button.
    1985 Blazer 6.2
    2001 GMC 2500HD D/A
    dmaxmaverick@thedieselpage.com

  6. #26
    Join Date
    Dec 2003
    Location
    Yukon Canada
    Posts
    1,446

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    The EPA might start to have a better disposition...
    Since the their hands got whacked by the supreme court.
    90 Chev 3500 c/c 4x4,6.2na,400 auto,4:10 gears.DSG Timing gears,main girdle, isspro tach, pyro,boost,oil and trany temp.Dual Tstats, High volume peninsular pump,on shelf, Custom turbo and intercooler 85%complete. Change of plans for the dually, it's going to get a Cummins. Both trucks are Blue 90 4x4 crews

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