1995 GMC Sierra, Codes 35 and 36

Discussion in 'GM 6.5 Diesel Engines' started by konstan, Sep 9, 2011.

  1. konstan

    konstan Diesel junkie

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    All of a sudden the SES (check engine) light started coming on; it comes on under acceleration; after reaching
    cruising speed of about 50mph it would stay on, but if I let off the accelerator, the light goes away.

    When I got home, I scanned the codes. The codes are 35 and 36.

    The PMD is two weeks old (flight systems, pensacola), mounted inside the fender, on a giant heatsink with a silicon heat transfer pad.

    What can cause it to throw both those codes?
     
  2. NVW

    NVW Active Member

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    PMD, grounds, IP or lack of lubricity in the fuel can all cause those codes.

    Add some 2 stroke oil to the fuel at a rate of about 1/100.
     
  3. konstan

    konstan Diesel junkie

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    It did start happening right after a fill up. Previous tank was a 2% bio-d, this one i think is just regular ulsd. Gonna go to the coop tomorrow and top off with a few gallons of bio-d. I poured some 2 stroke oil in but I don't have enough.

    Keeping fingers crossed, I hope its bad fuel. Not bad IP.

    No hesitation, stalling or any other running problems.

    If this is indeed due to fuel lubricity - is there going to be long term damage?
     
  4. NVW

    NVW Active Member

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    Bad fuel could do it too.
     
  5. konstan

    konstan Diesel junkie

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    if its bad fuel (hopefully we'll see tomorrow after I top off with 2% bio-d) i am never filling up at that station again.
    Btw, I cleared off the codes, drove it around the field, code 35 came back, code 36 didn't
     
  6. Crankme69

    Crankme69 New Member

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    Check the connections on the PMD extension cable also. I was getting them codes when I had a new PMD failure on my Tahoe. Do you have a spare to try?
     
  7. konstan

    konstan Diesel junkie

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    Well, the spare is the old one, and it was on its way out - no hot start. I can try plugging it back in the morning.
    I've been meaning to coat the connectors of the extension harness with anti-ox; haven't done that yet. Adding to my to-do list for tomorrow
     
  8. Crankme69

    Crankme69 New Member

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    Google search stanadyne dealers in yr local area. I purchase used PMD's from the one here by me for a backup PMD. The dealer here sells them for 50 bucs with a no questions asked 2 week replacement. I buy em run for a couple weeks & then place that one in my glove box on a small computer like heat sink with some zip ties for a spare. No 6.5 should be without a spare PMD IMHO.
     
  9. WarWagon

    WarWagon Well it hits on 7 of 8...

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    Open the water drain and check to see what comes out in a glass jar. Settle it and make sure you don't have a slug of water. Make sure you don't have bad fuel before pushing it further.
     
  10. gnel

    gnel Member

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    My Sub was throwing those codes before the IP died. Have a lot of miles on your IP ?
     
  11. JMJNet

    JMJNet Recruit

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    If you suspect of getting bad fuel, change the fuel filter and have a couple more spare.
     
  12. konstan

    konstan Diesel junkie

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    Ok, so, I think its the PMD. Here is why:

    Hooked up the pump connector directly to my spare PMD, drove around. NO codes, NO SES light.
    Hooked up the pump connector directly to the suspect PMD. Got the code and the SES lights.
    Both of these times there was NO extension harness in the circuit; PMDs

    Gonna try to get Pensacola to send me a replacement...
     
  13. buddy

    buddy New Member

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    It would be very good to have GMTDScan or other scanner to read the injection pulse width. The code happens when it take more time or less time than it should for the fuel metering valve within the IP's fuel solenoid to seat once the PCM removes voltage. The PMD takes the pulse width modulated signal from the PCM and switched high power to the IP's fuel solenoid to close the metering valve and open it and close it and open it VERY quickly. A faulty PMD may cause the power to drop off too quick (DTC35) or too late (DTC36). Or the IP can be failing and lubrication can help. In that scenario the valve may not be fully closing so it seats too quick, or it may get stick or slow down and seat too late. There is a tolerance of like 1ms to 2.3ms. So its a very fast thing.

    The IP could be on the edge and a PMD that really is fine might set it off, or a faulty PMD combined with worn IP may actually allow it to work right. without the scanner info its hard to say, but its certainly a good chance that the PMD is causing it.

    Remember to always lubrciate the fuel to prevent the IP from becoming the issue.
     
  14. JMJNet

    JMJNet Recruit

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    He used 2% BioDiesel. I would think that is good lubrication. Just as long as the filter does not get clogged.
    Unless the BioDiesel is bad also and causes the IP to be on the edge.
     
  15. konstan

    konstan Diesel junkie

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    The fail code (SES) behavior is very consistent.
    With the suspect PMD plugged in:

    In park, idling, no code. Rev it up to about 2000 rpm, the code appears. Let it fall back to 700 rpm, code disappears.
    When driving, as soon as rpm reach about 1400, the code appears. Let it drop to below 1000 rpm, code disappears.
    Consistently, each and every time.

    With the spare PMD, no codes.
     
  16. buddy

    buddy New Member

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    And the PMD is probably bad, just would help with the other scanner data to see if any new PMD is likely to be an issue. Sometimes two wrongs can make a right is what I'm saying, and if either one were working properly they wouldn't work right together. Always hope thats not the case, but it can really confuse you when you don't have all the info available.

    The code happens at specific loads on purpose, the way its programmed.

    See if there is any difference in the way the PMDs are mounted.

    I think a lot of PMDs are junk, and the easiest thing to do is swap them, but I like to understand the failure modes.
     
  17. konstan

    konstan Diesel junkie

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    How tough are these PMDs physically? When we mount them to the inside of the fender, does that cause them to get beaten up too much?

    What is it that detects the pulse width? I mean, for the computer to compare it with a reference value, it needs to read that width from some kind of a sensor... what is that sensor?
     
  18. konstan

    konstan Diesel junkie

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    Well, the GMTDScan would sure be nice to have, but I don't think there's anyone in my area who's got one... So, for now, the suspect PMD is going to be exchanged.

    I have two used ones I picked up from a local Stanadyne dealer, will try them and see. Yeah Buddy I know what you're saying - I also hate the "throwing parts at it" fixing methodology, but with the PMDs, I have no other way without the scan tool...
     
  19. JMJNet

    JMJNet Recruit

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    Physically, PMD is tough like water probably will not affect it. Internally, it is questionable.

    Heat kills electronic, since PMD is electronic gadget, consequently, heat kills PMD.

    No matter where you put it, PMD will die eventually.
    When mounted outside the hood, it will hopefully stays alive longer.
    Since the temperature fluctuation outside the hood is not as big as inside the hood.
     
  20. konstan

    konstan Diesel junkie

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    Crankme yer a genius. So I find this stanadyne dealer here in Omaha, "Diesel Specialists", and I stop by around lunchtime. First he tells me that he does not have any used PMDs to sell, but then, when he realizes that I just want some to play around with, no warranty or strings attached, he brings two of them out of the desk drawer, and in exchange I offer him $20 for their soda pop fund. We say good-byes, both of us happy.

    Well I've attached the first one of the two to the heatsink and it's been running all week, with no problems. No resistor either, because I dont have a spare one (gotta order that on ebay). Still waiting for Pensacola, the replacement should be here some time next week.

    At the end of this week, the one I am running now is going to get labeled "good spare" and will take its rightful place in the Glove Compartment as my Official Spare Pmd.

    Good times.
     

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