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Thread: P1635 Code, 5V Reference, Not Sure What All Correlates

  1. #1
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    Default P1635 Code, 5V Reference, Not Sure What All Correlates

    Out of the blue, my 97 411 swapped 454 truck decided to roll coal in the dark when I couldn't see. The next day, the mess on the white truck and camper was apparent. The code is an enhanced DTC and claims to be "5 Volt Reference 1 Ckt". I checked my pinout sheet from Lextech, and what I found referenced in there were a MAP/EGR reference output and a TPS reference output. Obviously, more sensors than those utilize a 5V reference signal. I think a schematic for my truck would be the best bet, but I'm also hoping someone here knows a little more about this than I do. Unfortunately my laptop's HDD just took a dump, so I'm temporarily without datalogging ability, though I intend to have that fixed this week. In the meantime, I can use my neighbor's Snapon to monitor sensor voltages. I just need to know what sensors are tied to what 5V outputs from the PCM to narrow this down. I put eyes on the first foot or so of both outputs wiring and all was well and both pins were still fully seated. I also shot the ECT, MAP, MAF, and AC pressure sensors and all read 5.03V so either there's an intermittent wiring short or an intermittent sensor failure. I just need to capture that. Is there a way to monitor applicable sensor feedback voltages in EFILive once I get the laptop back online? Or can I just see sensor values?
    1998 GMC Sierra K1500 5.7/4L80E, longtubes, 411 w/COS 5, marine cam/intake, Whipple. 91 octane at 6000'.
    1997 GMC Sierra K3500 7.4/4L80E, 411 w/COS 3, Whipple, small cam.
    2004 Corvette Z06 with longtubes.

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    Joe (Moderator) joecar's Avatar
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    What is the year/model/vehicle/options...? Do you have a circuit schematic...?

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    Joe (Moderator) joecar's Avatar
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    I'm looking at a circuit schematic (probably for the wrong vehicle) and I see 5V reference A and B...


    Click image for larger version. 

Name:	1997 Chevrolet Series C 5.0L 5.7L 7.4L MAP, IAT, ECT, TP.gif 
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    I found a schematic in my Haynes that I forgot I had and found the same thing, a TPS pin and a MAP/EGR pin. Someone else mentioned the splices in the harness maybe being suspect. I may dig into that first while the HDD is coming. Since it's the MAP circuit that I suspect, it makes sense because it shares with the EGR that's no longer used. Although an open in the ECT circuit would cause a rich condition as well. I thought maybe the EGR plug made intermittent contact with the EGR valve to cause it so I taped it off and reset the code but it came right back. Not having the laptop is a real asspain.

    Just checked the schematic again, seems the rest of the sensors get their supplies onesie twosie from the PCM so it has to be one of these 2 circuits.
    Last edited by Supercharged111; May 21st, 2019 at 10:11 AM.
    1998 GMC Sierra K1500 5.7/4L80E, longtubes, 411 w/COS 5, marine cam/intake, Whipple. 91 octane at 6000'.
    1997 GMC Sierra K3500 7.4/4L80E, 411 w/COS 3, Whipple, small cam.
    2004 Corvette Z06 with longtubes.

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    Well I removed the convoluted tubing from the harness today, the wire looks good from the PCM to the factory splice to the sensors as far as I can see. It all ohmed good too. I also removed the plug from the PCM and the blue cover to verify the pin was fully seated and it all looked good in there. HDD should be here tomorrow, so once I get Windows on there with EFILive I can go get some data and see what's really happening here.
    Last edited by Supercharged111; May 24th, 2019 at 02:37 PM.
    1998 GMC Sierra K1500 5.7/4L80E, longtubes, 411 w/COS 5, marine cam/intake, Whipple. 91 octane at 6000'.
    1997 GMC Sierra K3500 7.4/4L80E, 411 w/COS 3, Whipple, small cam.
    2004 Corvette Z06 with longtubes.

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    I got my HDD yesterday and got Vista loaded back up, but it looks like I have some searching to do as the drivers aren't playing ball for some reason now. Windows claims they're not digitally signed and won't accept them even though they were just fine before.

    Click image for larger version. 

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    I've created a separate thread here with much greater fidelity.

    https://forum.efilive.com/showthread...017#post249017
    Last edited by Supercharged111; May 26th, 2019 at 05:13 AM.
    1998 GMC Sierra K1500 5.7/4L80E, longtubes, 411 w/COS 5, marine cam/intake, Whipple. 91 octane at 6000'.
    1997 GMC Sierra K3500 7.4/4L80E, 411 w/COS 3, Whipple, small cam.
    2004 Corvette Z06 with longtubes.

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    Decided to take a mechanic's approach rather than a tuner, borrowed a Snap On Solus and brought up my P1635 on Mitchell All Data. Turns out this code is set for either of the 2 5V reference circuits, so for the TPS or the MAP/EGR and you have to narrow it down between those 3. I looked at the freeze frame data and saw that the P1635 and the P1665 (evap) set at exactly 4 seconds run time. Now I was thinking perhaps they were somehow related, so off came C2 and the little red cover to isolate and ohm out that wiring. The pin right next door to the evap pin was pin 33, AC recirc/aux fan and he was looking pretty smooshed.

    Click image for larger version. 

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    I then turned to the PCM to see if it recessed a pin.

    Click image for larger version. 

Name:	20190530_185632jpg.jpg 
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    At this point, I wasn't just going to pull the pin back out. I wanted to really know if this was affecting anything, so I opened up the PCM and sure enough:

    Click image for larger version. 

Name:	20190529_165144.jpg 
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    The AC pin was backed into the TPS 5V reference signal, the smoking gun! Now why the hell I installed that smooshed pin into the 411 I'll never know. For the short time I had it running, long enough to get to operating temp, the idle wasn't being all wonky. The idle behaved over the winter, but last summer and as soon as it got warm this year it would soar to 1200 in P/N. I figured it was the piecemeal idle data I'd dumped in there, now starting to wonder if the TPS signal wasn't being boosted so the PCM wouldn't go into a proper idle? When I run the heat, the dial is over in the defrost/floor mode and the recirc light automatically goes out over there. Oh yeah, and the evap code didn't come back on that drive either. It would pop immediately before and was a constant code, not an intermittent one. Now I can finish figuring out my USB drivers on EFILive and start logging and making things better again.
    1998 GMC Sierra K1500 5.7/4L80E, longtubes, 411 w/COS 5, marine cam/intake, Whipple. 91 octane at 6000'.
    1997 GMC Sierra K3500 7.4/4L80E, 411 w/COS 3, Whipple, small cam.
    2004 Corvette Z06 with longtubes.

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    Glad you found the problem and thanks for sharing! Good advice to make sure the pins in your harness arent messed up as it can have other side effects.
    ~Erik~
    2013 Sonic RS Manual - 1.4L I4T E78, starting the mods
    2008 TrailBlazer SS 3SS AWD Summit White - LS2 E67/T42, bolt ons, suspension, etc.
    2002 Chevy TrailBlazer LT 4X4 Summit White - 4.2L I6 P10, lifted, wheels, etc.

  9. #9
    Joe (Moderator) joecar's Avatar
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    Good job, good detective work

    Makes a good case study for not dismissing any pin/terminal that doesn't look right.

    And +1 what Erik said.

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    Forum tyhee Site Admin GMPX's Avatar
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    Now that you've posted that middle photo it reminded me back when we used to unlock LS1 PCM's for people (haven't done that for many years now), at least once every couple of months someone would call up saying there was an issue with the PCM once they plugged it back in. I used to say look down the row of pins and check if one is pushed back in to the PCM, sure enough that was always the problem.
    It seems like a pretty good system for making sure the pins are aligned before tightening the bolt but I guess every now and then one catches the edge and gets pushed back. At least they are easy to pull back out and don't snap off.
    Need official EFILive help, please go here.
    For tuning support please post your questions on this forum (or other auto forums).
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