PCM replacement

Lincoln LS

  1. Black_Sunshine

    Black_Sunshine LVC Member

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    Guys,

    I am still battling the transmission problems. I have the code P0795, which my Lincoln shop manual says "voltage through PCC solenoid C failed - shorted circuit." Basically, the car will not transition from 3rd to 4th gear, and gives me an E code on the dash when I drive in D5.

    Back in December, I swapped the pressure control solenoid pack out with a new pack, and it didn't change the problem.

    Yesterday I took looked at the cables going into the PCM module, to see if one might be shorting to ground. I didn't find a problem on that end. Earlier, when I changed out the solenoid pack, I saw the cables at the transmission end, and they looked fine.

    So, the manual tells me this is an electrical problem. I can't find anything wrong with the cables. The terminals at the PCM looked clean.

    I am considering the idea that the PCM unit itself may not be delivering the proper voltage to that particular solenoid. Maybe that one output has gone bad. Does that sound like a common problem? Or am I really looking at a transmission change?

    Thanks,
    Jason
     
  2. joegr

    joegr Dedicated LVC Member

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    This should be really simple. Unplug the connector from the PCM and test the resistance between the two terminals on the car end connector for that solenoid. If the resistance is correct there, then test between each of those two terminals and the ground of the car. If there is no continuity to ground, then yes it should be the PCM.

    It is easiest to send your PCM in for repair. Otherwise, you have to configure the donor PCM for you car, and then marry it to your cluster (PATS) and keys before it will even attempt to start.
     
  3. Black_Sunshine

    Black_Sunshine LVC Member

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    Good points Joe. Thanks.

    Question: If the solenoid has 2 wires, but one of them is the negative for the solenoid, would it not be at the same point electrically as ground? (Negative wire showing a dead short to ground)

    Thanks,
    Jason
     
  4. joegr

    joegr Dedicated LVC Member

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    No, not at all. If you have found that, then there's the problem. It is saying there is a short, and you are reading a short.

    The positives of all eight solenoids are connected together in the solenoid assembly. This power wire comes out at pin 3 of the connector on the solenoid assembly, is Green with black stripe, and goes to fuse 6 under the hood. Power is supplied all the time that the key is in the run or start position.

    The negatives all go separately to the PCM. The PCM switches them to ground using FETs. It does so at high speed and varying duty cycle (PWM) so that they are not just on or off, but it controls how open they are.

    Pressure control solenoid C is pin 4 at the solenoid connector, is Brown with white stripe, and is pin 12 of PCM connector C1751
     
  5. FDR

    FDR Dedicated LVC Member

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    Most other cars run switched positive to the component with a constant negative that's often "grounded" to the chassis. The LS does the opposite by sending switched negative for most items. It has to run dedicated positive wires though, since the chassis is still negative

    This is my gripe with the naming convention of negative = "ground" by the manufacturers. The chassis isn't Ground, it's Common. It doesn't mean anything for negative/positive polarity, but the generalized usage implies chassis=ground=negative=all negative wires connected

    [steps off soap box]
     
  6. Black_Sunshine

    Black_Sunshine LVC Member

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    FDR - I hear ya. My first degree was an associates in electronics, so I know what you mean. The term "ground" means about 4 or 5 different things, depending on the application, and just adds to people's confusion.
     
  7. Black_Sunshine

    Black_Sunshine LVC Member

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    This conversation leads me to wonder about the car audio amplifier issue I have been having in the Lincoln.

    I have tried 3 different car audio amps, and 2 different head units. All good quality name brands. Every system I put together in the Lincoln had a crazy ground loop engine whine noise. I tried alot of things to fix it, from scraping paint off the amp's ground, to running my own ground cable straight to the battery, to running a ground cable from the head unit to the amp, etc. Nothing worked. The Lincoln has some really insane electrical noise running through the chassis. Perhaps it is because some items, like the transmission solenoids, have negatives that are not at chassis ground potential, so perhaps that creates conditions for excess electrical noise.

    My solution was to ditch the amps, and just run a head unit. (for now.)
     
  8. FDR

    FDR Dedicated LVC Member

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    I have just a head unit, no external amp yet. I don't get engine whine in the doors, but I do occasionally get it in the deck subs. No change with volume or source, just engine rpm. Factory amp, factory wiring, aftermarket 8" round subs with adapter extensions. Wiggling the amp>sub and chassis>amp wiring around fixes it temporarily. I don't know if it has something to do with the head unit change, sub change, or wire routing to tighten up the extra length of amp>sub harness (wrapped around the trunk springs a couple times - maybe I made an induction coil)

    As for the source of the noise with amp installs, I don't THINK neg-switching would cause it by itself. The noise comes from the change in flow. Everything is still a loop from negative to positive, just with a switch on the negative side of the component. I could see the increased number of wires making it harder to find a quiet place to route wires though.

    The rear placement of the battery does cause issues because it has to run the battery cable all the way to the engine bay. Could you have routed near that cable? I believe a popular option to avoid that is to route along the center console and along the rear floor hump under the carpet
     
  9. Black_Sunshine

    Black_Sunshine LVC Member

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    My Lincoln LS shop manual does a great job showing me pin layout of the Transmission Vehicle Harness Connector, and it does a great job showing me which pins are the solenoids. Unfortunately for me, the dipshits who wrote the manual don't say or show any images on where the connector can be found in the car. Can someone tell me where the holey hell this connector is? I assumed it would plug into the back of the PCM, but its not one of those.
    IMG_2884.jpg IMG_2885.jpg
     
  10. joegr

    joegr Dedicated LVC Member

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    All those signals do go to a connector to the PCM. The connector they are showing is the solenoid assembly connector. It is on top of the transmission. You can get to it from under the car without dropping the transmission. There is a 10mm bolt in the center of the connector that you have to remove. You may have to cut a wrench down to be able to do it.
     
  11. Black_Sunshine

    Black_Sunshine LVC Member

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    Joe - I am reading 5.2 ohms from the green and black positive wire to Pin 12 on the solenoid connector (brown and white wire) at the back of the PCM center connector (which appears to go on to the transmission case).

    I read 19.2k ohms from Pin 12 to chassis ground.

    Can you tell me what the other solenoid wire colors are? I don't seem to have a wire color diagram in the manuals I have. I was expecting that 19.2k ohm reading to read as an open. I am not sure if I should read an open circuit to chassis ground or not. If I could check what the other solenoids read, that might help.
     
  12. joegr

    joegr Dedicated LVC Member

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    Are you measuring from the solenoid connector (car side) with the PCM still plugged in? The tests that you showed are for the solenoid assembly itself.
    What year is your LS?
     
  13. Black_Sunshine

    Black_Sunshine LVC Member

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    I am trying to isolate the solenoids and cable by themselves and test them. If they test good, then I can suspect a bad PCM, and send it off for repair. I am expecting to read the solenoid coils from the + to the -pin at the plug going into the back of the PCM (plug C175b).

    I get around 5.2 ohms across the coils. From the - pins to chassis ground, I am expecting to read an open circuit, but sometimes I am not. Yesterday I read 19.5k ohms the first time I tried from the - pin to chassis. That was for pressure solenoid C at pin 12. Then later in the same day, I read opens across all 3 solenoid-'s to chassis. Then I did a drive test, and measured around 800 ohms from - to chassis. It was inconsistent.

    A friend of mine let me borrow his Matco Tools software to monitor the PCM. The PCM is doing 2 weird things: First, its not able to complete it's own diagnostics scan. Secondly, if I monitor any of the pressure solenoids in a drive test, the car runs great, and the transmission shifts as it normally would. The car really doesn't seem to have a mechanical problem in the transmission. I am chasing some weird electrical gremlin.

    I am trying to isolate the solenoids and cable set to make sure no current is bleeding off to ground and making the PCM board act weird. The solenoid pack is new, so it really should be ok. I am leaning toward sending the PCM off for repair and seeing what happens.
     

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