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Jeremy, Certified Truck/Bus Technician
Category: Medium and Heavy Trucks
Satisfied Customers: 3318
Experience:  ASE Med/HD Truck Technician, NATEF Diesel Engines and Electrical/Electronics Technician, EPA 608/609 HVAC Certified, Steering/Suspension and ABS/Brakes Technician
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Have a 2003 GMC C7500 with the 6hk1 LG4 with the 200 pre-

Customer Question

Have a 2003 GMC C7500 with the 6hk1 LG4 with the 200 pre- 4th generation allison automatic transmission. have no forward gears only reverse.
JA: How many miles are on the car? Is it the original transmission?
Customer: 242454 miles, and no has been overhauled about 6 months ago by another shop that has went out of business.
JA: What is the model of your '03 GMC?
Customer: c series c7500.
JA: Are you fixing your vehicle yourself? What have you tried so far?
Customer: Yes, came in for a rear output shaft bearing blew up and blew out seal. customer brought in due to leak. Had to remove rear cover to replace bearing. during disassembly found 1 thrust bearing cracked, dropped pan to see if there was any heavy metal pieces found part of the thrust bearing. Also, could not find any of the roller ball bearing for the output bearing, suspect that their on the road. found light metal on magnets for in pan and spin on filter and in fluid, also smelled a little burnt. Found that valve body harness was shorting out causing some solenoids the click as i performed a wiggle test. replace valve body side harness and also connector for main harness. Tested the park neutral switch found connector C1 pin C circuit 776 shorted right as i plug into TRS sensor. Checked main line pressures found N/P 225psi @ idle, 250 @ 2100 RPM, R 225 psi v@ idle, 250 @*****225 psi @ idle, 250 @ 2100 RPM. Acts like it is not going into drive, Scanner says when i shift to D that shift inhibited on the data stays in neutral have no codes except for the ones i had created during limp mode test. still would not drive.
JA: Anything else you want the mechanic to know before I connect you?
Customer: no, thank you.
Submitted: 8 months ago.
Category: Medium and Heavy Trucks
Customer: replied 8 months ago.
Anyone there?
Expert:  Jeremy replied 8 months ago.

My name is ***** ***** I will assist you with your question today.

I'm an independent contractor and I don't get paid until a rating has been made.

We can go back and forth as needed to help resolve the issue you are having. Follow ups are free after ratings, so rate in confidence!

I'm presuming there were no issues with shifting at all before the bearing blew out? Did the transmissioin vibrate like crazy when this happened? I ask because there could be some internal damage done not allowing the fluid to build behind the shift plates due to there being too much spacing now. I've seen that many times.

What throws me off though is you wrote taht you put it in D, but you are not showing the shift into D, but you are getting a shift inhibit with no codes? Are you using the Allison software to check for the codes?


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Customer: replied 8 months ago.
Yeah, I just figured it out. Found that C1 clutch burned up and just slipping. was able to get all electrical problems figured out then went for mechanical. Could see that it was trying to shift into D and no inhibit was happening, but still had no forward gears.
Expert:  Jeremy replied 8 months ago.

Usually C1 slipping will allow some sort of engagement. It must have been burned pretty bad to not have anything forward at all.

Glad it was resolved.

Let me know if you have any other issues to work out. I'm sure you have them squared away though since you found the clutch burned and ordered a friction kit.

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