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Thread: The Definitive P0700 Thread

  1. #1
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    Question The Definitive P0700 Thread

    Ok guys... What are the common causes of a P0700. The Predator programmer I have won't display more detail on Allison codes. I had been driving normally today in a rural area when the service engine soon lamp popped on. Upon returning home I read the codes and saw the P0700.

    A couple of winters ago this same truck produced a P0700 after a cold start at near zero degrees F. I serviced the Allison with an internal and external spin filters plus fluid. That fixed the SES lamp, but now it's back, in warmer weather.

    Any ideas? Thx

  2. #2
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    P0700 = MIL commanded ON only, no specific fault defined (for those who don't know)

    Reasons I've seen it on my truck and others:

    Slippage in OD (also causes limp mode)
    TC drain-back (~2 weeks parked, may not engage into R or D until after restart, then normal, MIL stays on until a few restarts). There's a repair for this, but not worth the $$ or effort, IMO. Just idle for a few, then restart.
    Loose/dirty/failed NSBU or connector (may or may not be w/ flashing, wonky, or missing PRND). If the NSBU is Black, it's original. An updated tan model may be beneficial.
    Damaged shift cable or very sloppy lever joint(s)
    Low fluid level (low pressure)
    Parked/started on steep incline (overnight, may be TC drain-back)
    Overheat
    External filter never changed (350K miles, actually happened)

    Others may be actual faults, such as slow or failed solenoids, excessive seal leakage, or just due for overhaul.
    1985 Blazer 6.2
    2001 GMC 2500HD D/A
    dmaxmaverick@thedieselpage.com

  3. #3
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    Thanks Greg.

    This particular Allison in my 2001 GMC used to have a more noticeable drain-back (slow engagement after sitting for a time), but that has almost all gone away with time and miles. Now, engagement appears normal after sitting for up to ~2 weeks or so.

    Fluid level appears normal. Checked when hot yesterday. Was found to be at the marked hot level.

    This truck is still running the original black NSBU. I haven't seen anything wonky with the shift display.

    I'll be visiting a friend (transmission specialist) this morning. He has a scanner and wisdom. I'll report back as to what codes were present along with anything pertinent.

    Gotta say though that my heart sank when the SES lamp popped on, given the head gasket replacement project was completed just a week ago. I was a little relieved that it was a P0700...

  4. #4
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    Took the truck to my transmission specialist friend this morning. He could only see the P0700. After pointing out the TCM on the fan shroud, the code went away then and all during the ride home. We'll see if just touching the TCM was the fix...

  5. #5
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    Reconnecting power may have triggered the TCM, in some way. I've had some weird things happen after taking power away for a while. Usually, disconnecting again, then reconnecting, cures the weirdness, but if the P0700 MIL is triggered, it stays on for a few start cycles. When the drain-back issue happens, there will be no associated DTC, but the SES lamp stays on, with no code stored (I've never scanned before a restart when it happened). The TCM clears itself quickly when conditions return to normal, but the PCM insists keeping the MIL around for a while. The two computers don't handle faults and history the same. I suspect it's Allison's and GM/Duramax's engineers doing their own thing, only meeting when it's required.
    1985 Blazer 6.2
    2001 GMC 2500HD D/A
    dmaxmaverick@thedieselpage.com

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    As it stands right now, my firm guess is that the original 2001 black plastic NSBU was the cause of the P0700. Last week, the transmission went into limp mode, and was stuck in 3rd gear till I stopped and recycled the ignition. This is one of the key indicators for a failing NSBU (Neutral Start Back-up switch bolted to the side of the Allison). So, I installed a new one from Merchant-Automotive, and all is good once again! I'll report back if anything comes up that would indicate a yet ongoing problem. Jim

  7. #7
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    Well, turns out the NSBU wasn't the cause of the SES light. The SES comes and goes, along with a P0700. Right now, it's been off for nearly a week. My next likely suspect is pin continuity related to the main electrical connector on the Allison - the round one. I watched a video a few days ago where the mechanic worked on a 2005 3500 that was producing intermittent symptoms much like my own truck. In his case, the problem turned out being a pin continuity problem in that electrical connector (wiggling the wiring bundle just external of that connector would cause the light to go out). I'll see if that helps my situation when the SES comes back. Could this be a connector pin problem that is affected by time and vibration? We'll see.
    Last edited by More Power; 09-02-2021 at 11:50.

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    When I first got my 94 I read here about electrical problems. So one of my first mods was to disconnect all plugs and spray them with electrical contact cleaner with an oil base. Then re assemble them with dielectric grease. Never had any electrical problems with that truck.
    "The Constitution is not an instrument for the government to restrain the people, it is an instrument for the people to restrain the government."
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    94 6.5 4x4 5spd Sold

  9. #9
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    Quote Originally Posted by a5150nut View Post
    When I first got my 94 I read here about electrical problems. So one of my first mods was to disconnect all plugs and spray them with electrical contact cleaner with an oil base. Then re assemble them with dielectric grease. Never had any electrical problems with that truck.
    Good advice! I did that very thing with the two Allison TCM connectors under the hood. While in there I checked that all of the electrical pins hadn't pushed back into the connector shell, which can happen when unplugged/replugged. All of the pins were good. I'll do that to the Allison connectors at the transmission if/when the SES comes back. I suspect it will. Remember, this problem first appeared after a subzero cold start a couple of winters ago.

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