2005 GMC 3500 UPDATE

You should also have someone with a good code reader look and see if there are any historical codes. An Autozone type (cheap) will not see the historical codes. You may end up taking it to a dealer for diagnosis and code reading. GM garages use what is called a Tech2 (or above) to work on these trucks. The Tech2 is a 2 way diagnostic tool and can program/set switches in the CPU. I have a good, but 1 way scanner that lets me see everything about my truck. Unfortunately a good code reader/diagnostic tool is a bit north of $500.00. We spent $1200 but I can see the injector balance rates, all the other PIDS, rail pressure, commanded vs actual etc etc etc. There is a lot to these trucks.

If you take it to a dealer to have them scan it with a Tech2 they will be able to tell if someone stuck a different program into the ECM. If they did, they will (for a charge of course) reprogram the truck back to stock.

As long as that truck has not been hurt, it will make you a good platform once you get it tweaked back into shape.

Here is a really good place to go and learn about that generation truck

http://www.dieselplace.com/forum/

If you have the transmission serviced, be aware that there is a serial nbr cutoff on the Allison transmission where you should not use Dexron VI in it, this is per Allison.

I run Transynd in mine. That is an Allison recommended synthetic fluid. It is a little more pricey, but once you get it all swapped over, you can run extended drain intervals. It takes 2 changes to get to the correct percentage, unless you do a complete drain, including the torque converter.

Allison no longer owns the part of the business that makes the pickup transmissions. GM sold off the remainder of Allison and kept that part. However, with a little digging, you can find the information on the Allison site.

There is a lot that can be right about that truck and a lot that can be wrong. Take a few nights and do some reading. It will educate you about Duramax/Allison combination. You will need to check the 8th digit of the VIN to determine which engine generation you have.

Here is a VIN code/Engine ID chart. At the tail end of 2005, there is some speculation that a few of the next gen engines slipped out. You will need to learn about the differences.

http://duramaxhub.com/duramax-vin.html

As an aside, have you cleaned the cooling stack ? Is the trans cooler dirty ? If it is an LLY engine and you are pulling heavy, the trans cooler that came on them is a bit undersized. If you look at the size of the 2006 ones, they are twice the size.

I put a new trans cooler in mine and I haven't seen over 180 on it pulling, even in 90+ degree, stop and go traffic due to the pig truck that took out the split to I-80 westbound this side of Chicago.

For a fuel filter, get the oem nbr and do a cross reference. They are made for GM (or were) by Racor. You can get Racor filters less expensive than what the dealer will charge you for the same thing, just with different numbers on them. The current filters are 4 micron water coalescer ones. Don't run anything micron wise above this. Back in the early Duramax days, they had 10 micron filters and a lot of grief was caused by them.

I have a diesel parts shop order the Racor filters for me. You will also need to get a WIF (water in fuel) sensor wrench. They are pretty cheap. You can change them from up on top. You probably will slop a little diesel fuel.

ceaser 22000 wrote:
Problem goes away when transmission reaches 200

Something is definitely wrong if the trans reaches 200 degrees while driving the truck around empty.

The only time mine gets to 180 is when towing, and a hot day.

The Allison is well known for running about 90 degrees above ambient temp when empty and about 100-110 above ambient temp when towing.

That's the temps I see on both of my Allisons.

Do you know the last time the trans. was serviced. I'd have everything serviced on a new vehicle that I didn't know the history of: Oil and filter, fuel filter, trans. fluid and filter, air filter, T-case fluid, diff. fluids, and coolant at a minimum.
Some people have no idea that vehicle require maintenance. Maybe the previous owner was one of them.

Though a Ford, my trans temp runs around 200.

Check the transfer case if 4x4. Pump rub through is a known issue causing the tcase to run dry. If it really goes away when trans gets 200f I'd look at getting it serviced. New spin on and dump then Fill with proper dex

Always good to do a new fuel filter and learn how to do the job yourself. 200 degrees for the trannie seems high. I don't remember 200 degrees towing our 35 ft alpenlite. If you don't know about the truck when towing use the tow mode everytime.

chevman