Replaced All 4 Tranny/motor Mounts; Runs much better!

[Jazzy]

Hi Guys,

Yesterday I had a shop replace the three transmission and the one motor mount on my 2000 SLE with new DEA/Marmon ones. And that seems so far to have instantly fixed the ongoing problem that I've had with harsh shifting and a lot of vibration around 45-65 mph (which would go away if I got off the gas).

I had previously replaced the front tranny mount by the radiator a number of times, but that didn't really fix the problem for long. So I thought that replacing the other three, 21-year old, 202,000 mileage mounts was worth trying. I know from this forum that the other three don't go bad very often, but I began to think that even if they were not broken, they must have had some degree of wear and looseness after 21 years, and that the combination of the wear on those old mounts let the motor and transmission wiggle around some under power and caused the problems I was having.

The four mounts were $80 from RockAuto and the shop charged $150 to install all four. The Head Mechanic said the two side and rear mounts were not broken and did not look bad, and gave them back to me. I could not see any obvious signs of damage on them holding them in my hand, but then again, off the car they didn't have 800 pounds or so of motor and transmission weighing and torquing on them while going over rough roads.

But there is no doubt at all that my Bonneville is running much better now. That vibration at speed is gone and so far the transmission shifts feel right (I had it rebuilt 4 years ago).

[Jazzy]

After a week of driving around a lot on the three new transmission mounts and the one motor mount I had put in on my 2000 SLE, all the front end vibration of the last two years is completely gone at all speeds.

I had the shop give me the old mounts back, and the head mechanic said they didn't look bad. But yesterday, I put the two side mounts and the rear mount, all the original GM ones, on a vice and wiggled around the primary mounting bolts. It wiggled and bent like a cheap joy stick, and I could clearly see that the rubber in the two side mounts were split 240 degrees around the mounting bolt. That's something you could never see when the mounts are on car. The rear mount jiggled around like Jello.

My advice to you all having vibration problems is to just replace all four mounts instead of just replacing the front tranny mount by the radiator every six months. While that one may be the first to go bad, if your other three mounts are split and Jelloing, your motor and transmission is going to be jiggling around whenever you drive it, throwing off the CV axle alignment and suspension.

Smooth as silk now. Replace all four mounts and enjoy.

(Why am I sure it was the mounts? Because in the last month, I put on four new tires, two new front qwik-struts, new sway bar end links, new rear shocks, new rear trailing arms, several alignments -- and none of that made any improvement whatsoever to the front end's nasty vibration and shaking. Bu four new (DEA from RockAuto) mounts cured it instantly.)

[Erinyes]

Checking Rock Auto, I find myself getting confused by the various possibilities. One motor mount listed, front right. Transmission mounts, they have rear left, front left, left, center, rear, and various solid or hydraulic versions. I'm using a boat roller for the usual suspect in front, I assume that is the "center" mount.

Which specific mounts did you install? I tried checking through Napa as well, didn't help. Just more and more positions.

[Jazzy]

Here you go. Start with DEA's website, http://deaproducts.com/ , plug in your year Bonneville, mine was a 2000 SLE, and you will get pictures and part numbers of the 3 automatic transmission (AT) mounts and the one motor mount for the Bonneville. I used those DEA part numbers to search within Rockauto, by part number and not by vehicle. Here is what I got and used, but check the DEA website first to confirm.

AT center A2896HY (the one by the radiator in front)
AT Front left A2897HY (By driver's side tire)
AT Rear A2898 (rear by firewall)

Motor A2895HY (By passenger's side tire)

PS: I think these DEA Mounts are the best aftermarket ones out these today. Marmon bought DEA in 2016 and is a huge company: "Marmon Holdings, Inc., a Berkshire Hathaway company, comprises more than 100 autonomous businesses serving diverse industries and markets worldwide. Revenues exceeded $7.6 billion in 2020."

[Erinyes]

Thanks, very useful. I couldn't seem to get clear in my mind that there is one motor mount and three trans mounts...

[Jazzy]

I know what you mean. Most people just call them all motor mounts, and the rear, side, or front is confusing talking about a motor mounted sideways like in most front wheel drive cars like the Bonneville. But you need to know the correct name and part number when you order the parts. I know Amazon mislabeled at least one of the mounts and I had to return it. Go by the DEA/Marmon part number to be sure.

As to the labor in replacing all four mounts, I'm sure that you can do it yourself. Mostly all I've seen in forums like this is folks saying that the sides and rear are a major job. The tire shop that did mine said it took them almost 4 hours, but gave me a friend labor price of $150. I did not see how they went about it.

But I found a video last month of a young guy changing out all four mounts on his 2005 Bonneville in his driveway, using common Harbor Freight tools. He said it took him only six hours, and that was figuring it out as he went along. He filmed it on his cellphone but has great close-ups of what needs to be done.

He did them in order of front, passenger side, driver's side and rear. I think the trick/tip that he figured out and used was to not tighten-up any of the first three new mounts once they were in; he just got the nut started. That gave him the flexibility to move the motor/transmission around a bit when going on to the next mount. Pretty smart. Then on the rear mount, he unbolted/loosened the sway/stabilizer bar to get access to the mount bolts that needed to come off to get the rear mount off and a new one in.Then he went around and tightened up the other three mounts. Smart guy. Here's the link to his video:

https://www.youtube.com/watch?v=aONnwakKCGo&t=207s