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t-fran21
t-fran21, ASE Certified Technician
Category: GMC
Satisfied Customers: 1612
Experience:  GM Master Tech
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I have a 2004 GMC Sierra 3500 4x4 with the 6.6L Allison

Customer Question

Hello I have a 2004 GMC Sierra 3500 4x4 with the 6.6L Allison trans. I was getting code P0700 with my scanner which is a common code of no communication from the TCM to the ECM of the truck. So I took it to someone who could dig deeper into the trans and came up with U2104 & U2105 which is no communication on the cam bus side. Can I assume that the Range Mode Switch (Neutral Switch) is bad considering this is a high volume problem part with these trucks and the cam bus must run though it in order to talk to the ECM. If so how can I verify this before I go spend 200.00 for it or should I look somewhere else?
FYI the wiring behind the alt. is all good no cracks or signs of damage which this is also another known problem area for this.
Submitted: 1 year ago.
Category: GMC
Expert:  t-fran21 replied 1 year ago.

I can chat with you back and forth but please accept the live phone call it's will be faster and more convienent, I have all the exp. needed to figure this I specialize in the area but you will need a voltmeter and test light. If you like to chat that will stil be ok let me know. Thanks

Customer: replied 1 year ago.
I don't mind the phone call as long as it solves my problem but I can not tonight, it will have to be tomorrow after 3:30pm central time and hopefully before 5pm so I still have daylight to look for anything you may want to know. But first thing, do you have any thoughts to what I wrote out? And before we chat on the phone is there anything you want me to do, find out, or look for so I have anything on your mind answered already?
Thanks
Expert:  t-fran21 replied 1 year ago.

The problem is going to be a bad ECM, TCM or ficm not a neutral safety switch. But during testing you have to have a scanner than can read the 2104 and code? You need a voltmeter, small metal picks and small flat head and/or wire cutters and but connectors.

Customer: replied 1 year ago.
Well that wont work all I have is a the Matco MD1052 scanner and that will only do so much. I took it to someone to get the U codes out of it. Unless you have another way Ill have to take it in.
Customer: replied 1 year ago.
Well that wont work all I have is a the Matco MD1052 scanner and that will only do so much. I took it to someone to get the U codes out of it. Unless you have another way Ill have to take it in.
Expert:  t-fran21 replied 1 year ago.

Sorry man but that Marco scanner will only pull ECM codes and we will need them from the TCM ECM and FICM to be accurate

Customer: replied 1 year ago.
Hey there I did not forget about you :) had to go to work :( lol. So anyway I'm trying to get ahold of a scanner so we can do this phone call but right now my friends scanner is out for upgrades and he's hopping to get it back tonight. So please be patient and let me see if I can set this up and if I can't I'll pay the fee and I will just have to take it in.
Thanks Dave
Expert:  t-fran21 replied 1 year ago.

No problem just let me know. Thanks

Customer: replied 1 year ago.
Okay well he wont be getting it back for a few they had to send it out for something and I don't want to keep you waiting. When he does get it back can you give me a description of what to look for so I can narrow it down?
Thanks
Customer: replied 1 year ago.
Are you there?
Customer: replied 1 year ago.
Are you going to let me know so i can close out this question?
Customer: replied 1 year ago.
Are you going to give me an answer?

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