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P trouble codes

3746 Views 11 Replies 5 Participants Last post by  Mike L.
Hey guys,
New boot here. Just found out about your site and i like it. Everybody trys to help each other out. Now maybe someone can help me. The other day i was on my way home to south Louisiana from Texas and i went to pass 2 18 wheelers in a row. I got up to about 90MPH when all of a sudden I heard a loud noise and my truck stopped pullin. It coasted down to about 50MPH when it kicked back in to 4th gear and stayed there.:mad:Then the service engine soon light came on. So i pulled over and checked my fluid and it was fine,looked underneath and no fluid leaks. So i went to take back off and the truck was stuck in 4th gear. So I pulled back over killed the truck and did a good once over. I could not find anything wrong so i started the truck back up and this time it went through all the gears just fine. I went to Autozone and got them to read the codes for me. They r P0700(Trans control System) This code was in their twice!!! And P1720(Trans Control Speed Sensor).I took the truck in to my local gmc dealer and he reset the system and said i drove it to hard??????:confused: I think he is full of crap. Has anyone else ran into this problem? And if it helps i have 145,000 Miles on it and this is the 1st real problem ive had with it.If anyone could help a new boot out i would appreciate it.:help
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What power level were you running on, it sound to me like it was just slipping in 5th gear because of to much power. 5th gear dose not really like any more than 90 hp. the 50 mph limit it hit souns like limp mode due to either the trans slipping or even the engine starving for fuel but it would have thrown a p1093 code for this!

When was the last fuel filter change?
What power level? and were you under WOT when it shifted into 5th?


I was on power level 5 and im not sure about wot maybe but i dont think so. The fuel filter was replaced less than 2k ago. I am very anal retentive when it comes to maintaining the truck. So is it something i should be worried about or some upgrade to the tranny to keep it from happening again?:peelout
I'm gonna say that yu probally just limped the trans from the high power level in high gear, most people have problems running high power levels on a stock alison, very few dont have problems. the only fix for the problem is to either back the power down, go with an after market trans either suncoast, ats, or dtt?

Mike L might also chime in here he will no better what to tell ya as hes the man when it comes to the allison trans, you could send him a pm also here on the forum!

WOT=wide open throtle

P1720 is solenoid A ( trim solenoid ) control clutch stuck off. This means you might have a faulty output speed sensor ( rare ). More than likely you everpowered the C3 cutch in 5th and you limped the Ally. This code will cause the Ally to default to the next lower gear. Too much power.
Well thanks for all the input guys. I really appreciate the advise from you to.
This is a very common problem, and it is something you want to address sooner than later. The frictions that are in that clutch pack dont take to slipping very well. Once they slip they are a little easier to slip the next time and so on and so on. When the converter locks up in OD the tranny dumps line pressure. This is why when you get on the throttle fairly quickly in od the tranny cant boost the pressure quick enough to prevent the slipping. (This is a condesned version of what actually happens) The fix for that is to keep the apply pressure up, which we are able to do with our valve body.
Converter flow valve knocks down mainline pressure anytime converter clutch is applied. No matter whos valve body you use it won't help with a stock converter and high horsepower.
Mike L. said:
Converter flow valve knocks down mainline pressure anytime converter clutch is applied. No matter whos valve body you use it won't help with a stock converter and high horsepower.
That is right, but with one box or tuner on it say a 125hp increase the valve body is more than enough. We have quite a few customers in the 450hp range that are running our v/b with the stock converter, with no problems. The stock converter isnt that bad from the factory. Furnace braized fins fairly efficient stator. The big problem with them is the mounting feet for the bolts. On the stamped steel front cover the mounting feet are only welded onto the cover. They can cause flex in the cover and eventually a crack can form. This is quite commonly diagnosed as a front pump seal leak. A billet front cover is also the fix for that.

Mike please understand I am not trying to undermine you here, just add some more information to what you have already said.:beer
No problem.:D I think it all depends on how you use the power and not how much you have. As far as the Allison stock converter, I agree that the unit is solid with exception to the TCC piston design. There is nothing to stop the piston when the clutch applies and the center actually comes down a little too far giving you a ( stepping on a frisbee affect ) and causing the inside portion of the clutch to burn and the outside to hardly touch at all. The engineers at Allison admit to this. If you do not have a solid lock at the converter clutch your trans will not live.
so do you guys recommend an aftermarket converter. if so what kind would you go with and a ball park price. or just change the valve body. a ball park price for that would be nice to. thanks
I think you are beyond a valve body. '01s have other internal issues that need to be addressed along with high performance mods. I would go with a name brand triple disc converter to start with, and some pump, clutch and valve body mods. At this point I'll let the vendors jump in.
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