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Discussion Starter #1
We were pulling our Work and Play 34WRS this weekend and ran into the following... We were stopped at a traffic light, pulled through the light getting on the throttle to get up to speed. Truck seemed to downshift, and the old SES light comes on... Won't shift, but will rev, so stuck in Trans Limp mode. Pull over and pull the codes, P0700, P1093, and P1781. Reset, shut the truck off, start up and go on our way. No other issues on the trip.


Fuel filter was replaced about 100 miles ago with the last oil change, and trans filter change. Trans was recently rebuilt in the last 15,000 miles, added additional friction plates in packs was the only upgrade with HD converter. Truck is a 2002 2500HD with Allison, and about 290,000 miles. Basically stock, CAI, and added a Superchips DashPaq tuner in the last couple hundred miles. Only use Economy and Tow tunes. Had it in the Tow tune when this happened.


My real question is what came first (chicken, or the egg)? My guess is the trans codes hit first, since it was in trans limp mode. BUT... Could the P1093 caused it to think the trans was slipping? Just want to be sure the CP3 isn't dying... Appreciate any insight from the experts


Thanks,
 

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I would find an advanced code reader and pull the TCM codes to find out what the trans issue is and go from there
 

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Could the P1093 caused it to think the trans was slipping?
No expert, but I don’t think Any of your codes is a Trans slip.

Allison LCT1000 P1781 or P0802 Setting in the ECM - kw 6.6 8.1 code CEL DTC ECM GMT560 GMT800 intermittent L18 LB7 LLY light M74 MIL P1781 P0802 PCM repeat shift start TCM transmission #PIP3094 - (Aug 5, 2004)

Allison LCT1000 P1781 or P0802 Setting in the ECM

The following diagnosis might be helpful if the vehicle exhibits the symptom described in the PI.

Condition/Concern:

Some customers may comment that the Service Engine Soon indicator lamp is illuminated. Technicians may find DTC P1781 (model years 01-02) or DTC P0802 (model years 03-05) (Transmission Control Module - Incorrect Voltage Level on MIL Request Circuit) stored in the ECM. This condition will usually occur during engine start up. This code sets when the ECM and TCM do not communicate properly on the initial "hand shake" during power up. One module may be powering down during crank due to a low voltage situation.

Recommendation/Instructions:

Note: For 2001 vehicles refer to TSB 01-06-04-020 for the latest ECM calibration. Check the vehicle for a possible weak battery or a dead battery in dual battery applications. Also check for possible poor battery cable connections, poor grounds, UBEC connections, or possibly an ignition switch dropping out during crank.

Please follow this diagnosis process thoroughly and complete each step. If the condition exhibited is resolved without completing every step, the remaining steps do not need to be performed. If these steps do not resolve the condition, please contact GM TAC for further diagnostic assistance.

Models:

(01-05 Chevrolet Silverado) and (01-05 GMC Sierra) and ( 03-05 Chevrolet Kodiak) and (03-05 GMC Top Kick)


#01-06-04-020 Service Engine Soon Indicator Lamp On, DTC P1781 Set (Reprogram Engine Control Module)

This condition will usually occur during engine start up. This code sets when the ECM and TCM do not communicate properly on the initial "hand shake" during power up. One module may be powering down during crank due to a low voltage situation.

Check the vehicle for a possible weak battery or a dead battery in dual battery applications. Also check for possible poor battery cable connections, poor grounds, UBEC connections, or possibly an ignition switch dropping out during crank.

I would Put your Superchips DashPaq tuner back to stock to eliminate this.
 

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Discussion Starter #4
Thanks Rvn4me,


Looks like you are more than an expert, it says you are a Pro!!! :D


Just a quick question on putting the DashPaq bask to stock. From a troubleshooting standpoint, totally agree to eliminate a variable. Just trying to understand what you would expect that would address from the list of possible causes.


Taking it to my trans guy tonight to take a look at it. Hopefully he can shed more light on the trans side (and fix the pinion seal leak that I found yesterday...) It never ends!



Appreciate the the help.
 

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Discussion Starter #6
Ok, after resetting trans to relearn, it seems to be shifting fine (no more trans issues last two pulls). The P1093 code is still showing up. It doesn't trip a SES light, I only see it if I pull the codes. Truck starts up fine, idles fine, and runs fine. No real symptoms, or loss of power... I'm going to look for collapsed lines. Any thoughts on what could be the culprit? I cleared the code on my way home last night, and I checked it when I got there. The code was back, and this was just from normal driving. Weak pressure relief valve, high pressure sensor, etc.? I don't get the sense it's the CP3, since it seems like that usually has rough / long starts (based on all I've read...).


I seen some issues with clogged FCM that was blown out with compressed air, but that had a loss of power at WOT. Not something I'm experiencing...



Appreciate the insight.
 

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Discussion Starter #7
More info...
I found Fuel Rail Pressure on my DashPaq tuner, so I added it to the gauges to watch. Here is what I saw on my drive to work this morning.
Idle: FRP was at 5kpsi

55 - 60 mph: Range was 10 - 14kpsi
65 - 70 mph: Range was 12 - 16kpsi
I saw as high as 17kpsi. It would start to drop as I went up a grade, or added load on the truck. Say, it would start at 14kpsi, but would drop down to 11kpsi as I went up a hill. If load was added slowly (ease into throttle), it would rise, say 12 up to 16kpsi. Getting on the throttle pretty hard (not completely WOT), it would drop from say 14kpsi, down to about 10kpsi.

Hopefully that added info can help explain what the culprit may be. I don't have enough knowledge of this system to know what the response should be... Does the fuel pressure drop because the demand is more than the system can supply to keep pressure above X level?
Appreciate any insight.
 

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Discussion Starter #8
Checked fuel pressure at key on / engine off and it was 200 psi, so I believe that's where it should be and eliminates the high pressure sensor.

Anyone have any experience with the fuel pressure relief valve, or leaky fuel filter housing?
When you do a suction test for restriction with a J-44638 fuel pressure gauge, where do you connect it in the system?
Thanks,
 

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Discussion Starter #9 (Edited)
More info...

Here is what I noticed this morning...

Before the truck was up to operating temp it would build rail pressure up to 19-20 kpsi under hard acceleration, no problem. After it warmed up is when it would drop to 9-10 kpsi under hard acceleration. Not sure what that means just yet, but just something to note in the diagnosis. To me, that doesn't make sense for the CP3 to be going out. I would think it wouldn't build pressure under any situation. Maybe a rubber line gets warm enough to start collapsing, or a leak in the filter head starts to open up when it gets warm?

Balance rates also look good (all are less than +/- 2 in park with a fueling rate of 7.

I also checked MAF readings and it covered the range of 45 to 450 g/s. I know there were some other discussions on another thread about something causing de-fueling, and there were some thoughts it could be the MAF. That doesn't seem to be the case for me.

The biggest items I need to check off the list are:

  • Find a Tech II to see if the pump can make 21 kpsi at idle
  • Check the fuel vacuum level feeding the pump (and the potential of air in the system)
  • Depending on the output of the above, any kinking fuel lines, or other potential restrictions
  • Maybe a return test to see how much fuel is being sent back to the tank (crank for 15 seconds, and return should be less than 100ml)
If someone can help me understand the Fuel Pressure Relief Valve and how it works, I would appreciate it. Does it work like a "Hammer Arrester" in a home water line? Where the spring loaded plunger just depresses to adsorb the "shock" from a pressure spike (no flow back to return line), or, does the relief valve open to allow flow bypass back the the return line? If it's the former, I would expect the fuel pressure to recover and build, once the spring is compressed.
Any insight on where to take fuel pressure readings would be appreciated too (pressurize the fuel filter housing for leak down, and vacuum pressure on suction side.

Thanks,
 

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Discussion Starter #11
Thanks Rvn4me,


Just about everything you find is for LLY or newer trucks. I did find searching this time that the LB7 FPRV is in the center valley of the motor (great...), instead of at the back of the driver side fuel rail. Looks like it does dump to the return.
 
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