2000 GMC C2500 NO REVERSE

The problem is internal to the trans, and there are several possibilities. Does it have all forward gears? Here is the list of probables for No reverse, first chart is for 4L80E trans, and second for 4L60E. This is usually a broken internal part in the trans.. common to these Chevy's

https://www.2carpros.com/articles/automatic-transmission-problems

No Reverse
Checks
Cause

Case (7)
The rear band anchor pin is broken or the pin is not positioned.

Center Support (640)
Leaking at the case, or the support is broken.

Center Support Seal (639)
Leaking

Center Support Bolt (25)
"Â The bolt is loose or broken.

"Â The feed hole is blocked.

Rear Band (657)
Broken, worn, or not anchored

Rear Band Apply Pin (73)
The pin is too short or the pin is binding in the case.

Piston (65)
Binding in the case

Seal (66)
Leaking, damaged, or worn

Gasket (63)
Damaged or displaced

Cover (62)
Damaged

Bolts (61)
Broken, loose, or missing

Checkball
Missing

Fluid Pressure
Too low

Direct Clutch Components

Reaction Plates (618)
The splines are worn.

Friction Plates (611)
The splines or the friction are worn.

Spring Assembly (607)
Jammed

Housing (623)
Cracked

Piston (619)
Leaking

Seal (620, 621, 622)
Leaking

Ball Check
Leaking

No Reverse or Slips in Reverse
No Reverse or Slips in Reverse Checks
Causes

Input Housing Assembly
"Â 3-4 Apply ring stuck in applied position

"Â Forward clutch not releasing

"Â Turbine shaft seals missing, cut or damaged

Manual Valve Link
Disconnected

Valve Body Assembly
"Â 2-3 Shift valve stuck

"Â Manual linkage not adjusted

"Â Spacer plate and gaskets incorrect, mispositioned or damaged

"Â Lo overrun valve stuck

"Â Orificed cup plug restricted, missing or damaged

Reverse Input Clutch Assembly
"Â Clutch plate worn

"Â Reverse input housing and drum assembly cracked at weld

"Â Clutch plate retaining ring out of groove

"Â Return spring assembly retaining ring out of groove

"Â Seals cut or damaged

"Â Restricted apply passage

"Â Porosity in piston

"Â Belleville plate installed incorrectly

"Â Excessive clutch plate travel

"Â Oversized housing

Lo and Reverse Clutch
"Â Clutch plates worn

"Â Porosity in piston

"Â Seals damaged

"Â Return spring assembly retaining ring mispositioned

"Â Restricted apply passage

Unfortunatly it is a common story with transmission fluid changes. If a fluid looks brown it is not dirty but burned. I suspect the change and flush caused a transmission failure.

Your tranny needs to be changed every 30-45K miles.

You do no have to believe what iI say about transmissions. After 40+ years as a garage owner I can tell you some specifics about hundreds of failed transmissions.

Flushing a transmiision with discolored fluid is an "iffy "service at best. The flush chemical loosens the deposits that have been resident in all the corner, cracks and pockets of the unit. This creates a debris field that will loosen when new fluid is added and within 5K or so miles will start to create transmission woes.

The torque converter cannot be drained unless you drill a hole and put in a bolt to seal the unit. (Ford has one built in) This leaves several quarts of transmisson fluid unchanged. Flushing is often a "gimic" transmission shops use to sell an over haul. Read the fine print on the work order and it relieves them of all liability.

The act of flushing loosens deposits on all the internal working components especially the bands and valve body. These loosened deposits get in the valve body chambers and the flush fluid often distorts the small plastic balls that chamber fluid to the correct band assembly. I have seen bands come apart as they are held together in many cases by burned material.

Internally the transmission has a plethera of working parts including seals, O rings, gaskets and most important the bands. Every time a transmission shifts a minor amount of slippage occurs. This happens so the driver doesn't feel a jerk when the gears change.

Most all transmission experts will tell you that the dark deposits are from the internal parts of the transmission and torgue converter that have been loosened by an additive. Oil change and most surely from a flush.

Remember your torgue converter also has a clutch that locks up giving you a 200 to 300 RPM drop and acts as an over drive. It is a very dark color and a little bit of debris goes a long way to discolor fluid.

Sadly the only way your problem can be corrected is to rebuild your transmission. I tell every one with a 2500 or larger vehicle to insure they have an aux transmisison oil color because heat is the real killer of most of the transmision problems as well as failure to service the units.

Sir; you are not alone in you transmission problems with forgetting to service a unit or expecting a magic bullet to cure everything when a high mileage unit is serviced.

Believe or don't believe what I have written, as a I am passing along facts based on my experience and beliefs. You are the end decision maker and very good luck to you.

Losone

Scan it for both hard and soft codes. As for the temperature, do you mean the engine temperature? It wanting to die might be a vacuum leak.

It varies by scanner. Read the manual with yours. You want it to do a deep scan and find everything, not just emissions-related codes. You want it to scan for transmission codes. What you could be experiencing is excessive converter slip creating the excess heat. Also, did you blow out all the lines when you replaced the transmission?

Ok. Also check to see if your scanner can give you live data and if it can, use it to check for slippage in the trans.