New to me, used GMC C6500 wrecker CAT 6.6, Allison trans

RichieB9280 · January 16, 2019

Hello Everyone,

My name is Rich, I'm an ASE certified mechanic with over 21 years experience repairing passenger vehicles. I recently purchased a used GMC C6500 with a Vulcan wrecker body because I'm trying to start a towing business on my land. She's got a CAT 6.6 turbo diesel motor, and an Allision transmission from what I found searching the VIN and what I saw when I flipped the hood forward and saw a big yellow inline 6 cylinder diesel motor. This truck while much of it is similar to GM products I've encountered over the years, has a powertrain that I'm not familiar with. I've watched a few service videos on Youtube to learn general CAT service procedures, but that doesn't help me with the problem I face today. I bought her knowing the brake lines were in poor condition, and knowing that the motor needed a top end overhaul at the minimum because I got a decent deal on her. She has a good solid rust free frame, and I took her on an extended road test, 45 minutes all over town and the highway, no problems that day. I towed two cars then as happens with old equipment I had a failure. She started struggling to shift into higher gears when enroute to a tow destination, then the transmission began to slip. I checked her out as soon as I stopped. I discovered my transmission cooler had ruptured and began to leak, I managed to get her home and I think the trans is okay, probably got a little hot as it was slipping, but she still shifts.

I need two new lines and a new cooler... so I called GM. GM quoted me around $880 for two lines and a transmission oil cooler!! Considering I just fabricated all new brake lines under this thing over the course of three days in constant snow flurries, making a few more lines even if I have to have them fabricated should be cheaper than those factory prices.

I'm looking for help with aftermarket parts from people who have dealt with or worked with these trucks in the past. NAPA had no listing, so I'm not going to bother with Autozone and stores like it. I was thinking of calling 1-800-RADIATOR to see if I can get some sort of universal fit kit.

Has anyone encountered this problem before and repaired with more affordable aftermarket parts? If so please respond.

Thanks - Rich

Doug_Scott · January 16, 2019

Don't have any experience on the 6500, but, unless Allison is different from most everyone else in the transmission game, the pressure in the lines and cooler is not very high.

As for the slipping, depending how many miles you drove with it slipping you may have burnt the clutch discs enough to require a rebuild.

Check wreckers in your area for the lines and cooler.