Hi everyone. I recently purchased a 2001 Chevy 2500HD pick-up. On a recent trip I was towing my 23' travel trailer and on my trip was climbing some hills. Anyway, I had the setting on the tow/haul feature and had no problems maintaining the speed limit up the hills. Once I got to the camp ground I was staying at, I went to un-hook the trailer so I could level it. I happen to notice that there was a oily coating on my equilizer bars, which then lead me to notice splatter marks up the front of my trailer. At first didn't think too much of it since I was un-hooking the trailer. As I was leveling the trailer I happen to glance at the rear of my truck and noticed fluid dripping from the rear differential. Upon taking a closer look, I saw splatter marks all over the inside of the wheel well and on the axel. I inspected the brake lines for ruptures and didn't find anything. After further inspection, I found the transmission cross member support coated with a liquid and dripping. So, I ran my finger through the liquid and realized it was transmission fluid. I wiped everything down to try to determine where the leak was coming from to no avail. The next day, after checking the fluid level (showing mid-hot range on a cold tranny (I think the dealer over filled the fluid)), I drove the truck around the national park we were staying at all the while watching the transmission temp. Temp never went above 200-210. I checked the cross member through the day and when we got back to the campsite and there were no leaks. I then drove the truck back home (~380mi) with the trailer attached and saw no problems and once home checked for leaks and found none....

Now that I have the story outlined, here is my question... Do the Allison transmissions on the HD trucks have a pressure check valve to release fluid in heavy demand situations? I am not familiar with these HD trucks or the trannies on them so I thought I would ask the question here. This forum has been sooooo helpful to me in the past when I had my 1/2ton chevy, I am hoping to find an answer on this 3/4ton. Thank you so much for any feedback....

In the owners manual it states do not over fill transmission as the fluid will come out and there is a risk of fire...have transmission fliud replaced and always check all fluids for proper levels before towing....


air bubbles could have caused some damage to your transmission...

If it was overfilled, it may have vented the excess fluid and would guess that is has some type of mechanical pressure relief valve in it. On our 2003 Tahoe (4L60E...I know, not the same) there's a rubber vent tube near the top of the tranny where fluid will leak from. Ours needs a rebuild as the pump will froth up all the fluid out the vent on long trips and then stop leaking for days or weeks sometimes. In your case I'd just watch the fluid level and hope that it was due to the overfill done by the dealer.

If it was overfilled, it may have vented the excess fluid and would guess that is has some type of mechanical pressure relief valve in it. On our 2003 Tahoe (4L60E...I know, not the same) there's a rubber vent tube near the top of the tranny where fluid will leak from. Ours needs a rebuild as the pump will froth up all the fluid out the vent on long trips and then stop leaking for days or weeks sometimes. In your case I'd just watch the fluid level and hope that it was due to the overfill done by the dealer.


when you check the transfluid do so when hot on level ground....it is very important the the vehicle be level and hot ....the pump will create bubbles when it is over filled.....also if you overload your vehicle and the transmission overheats,,,,,,towing with the trans in over drive this also will occur so get a transmission cooler installed to reduce temps in transmission.