Coleman Mach 10 Compressor Vibration and In-coach Low Frequency Drone - How We Fixed It

When we first got our View back in ’20, we were pretty disappointed in how loud the air conditioning was. It was loud enough that we actually would have a conversation about whether we really needed to turn it on. I did the well known DIY “block the flood dump to force the flow out of the overhead ducts” trick, and cleaned up the installation wiring cutting up the air intake flow – both to great success. After those mods, we actually could sit at the table and have a pretty close to normal chat. To see the details of those mods affecting the heat pump’s airflow, have a look here:

Quieter Heating and Cooling due to cleaned up Airflow

However, what we were left with after those successful mods was a very annoying low frequency vibration originating from the compressor on the roof, resulting in an unacceptable low frequency hum. The vibration would come through roof structure when the compressor was operating.

This photo shows what the compressor in our Coleman Mach 10 heat pump looks like, when the rooftop cover is removed - notice the damping pads that the compressor cradle is attached to:

So, first some history. At one point, quite by accident, while having a refrigerant port added to the system so we could troubleshoot a leak and add refrigerant, I noticed that one of the compressor’s feet was not bolted to its damping pad. It had come out of the factory missing the bolt. I got a bit excited, thinking that getting the foot attached to its pad might be enough to help us with our in-coach noise. I found a ¼” course thread bolt and washer and proceeded to tighten down the foot, and then hooked up shore power and turned on the AC…

No change.

Kinda disappointing – but not the end of the world. After all, this is an RV. But, I did recall that while cleaning out our household residential air conditioner’s condenser coils earlier in the summer, I noticed that the compressor’s feet were a bit loose on their damping pads. I tightened those up a touch and was pleasantly surprised to find that it quietened down the unit significantly.

That gave me the incentive to get back up on the roof of the View to see just how tight all of the compressor feet were in the Coleman Mach 10. All of the bolts used to tighten the compressor’s cradle to the damping pads were indeed, pretty tight – in fact they could not be tightened any further. BUT, the damping pads themselves were not compressed! For example, with the floating foot that is easily reachable, I could put light upward pressure on the bottom of the damping foot, and actually move the bolt and grommet up off the compressor’s cradle. This was true for all of the bolts. Have a look here to see the bolt being lifted off of the cradle, with very little force:

The damping pads designed to absorb system vibration were not being compressed, at all. That explained why compressor vibration was being transmitted into the roof structure and then into the coach as an annoying low frequency drone.

When I removed a cap bolt from one of the feet, and had a good look this time, all became clear. The barrel nut (binding barrel?) that comes up through the centre of the damping pad was extending almost to the surface of the rubber grommet on top of the cradle structure. When the cap bolt is screwed into the barrel nut, it impacts the nut shaft and cannot be torqued any further. If the barrel nut cannot be drawn up the bolt, it is impossible to have any compression on the pad. In our case, this explains why all of the cap nuts were tight, but the damping pads had no compression.

The rubber grommet underneath the bolt cap might get compressed a bit, but not nearly enough to place any force on the cradle to compress the damping pad. A compressor left to vibrate without any damping? I know, sort of hard to believe…

The solution to my perceived problem was pretty straightforward. I needed a bushing on top of the rubber grommet, with an inside diameter large enough for the barrel nut to be able to travel up as the damping pad was compressed. The bushing needed to be tall enough to keep the bolt cap far enough away from the top of the barrel nut, and also allow for the rubber grommet to compress enough to start applying enough force, in turn, to compress the damping pad. Originally I just thought I’d use an appropriately sized stainless steel nut as a bushing, but didn’t have enough of those on hand. I found some nylon bushings tall enough to work, and used some stainless steel washers as a base for the bushing.

There are six feet on the compressor cradle, but I could only get at five of them. (The hard-to-get-at sixth foot is down on the floor, forward, right behind the evaporator coil’s box.)

For each of the five feet I could get at, I removed the bolt cap, dropped a stainless steel washer - with a large enough inside diameter for the barrel nut shaft to slip through – on top of the rubber grommet, dropped on the nylon bushing - with a large enough inside diameter for the barrel nut shaft to slip through, and then screwed in the cap bolt. I torqued the bolt just enough so I could see the damping pad compressing a little bit. Here is a shot of two of the five modified damping pads; these two being located down at the base of the compressor cradle:

“What were the chances that all of this was going to do anything?” I could hear myself ask as I climbed down from the roof.

Well, you are probably saying to yourself that this guy would not be telling any of this to us – if it had not worked. You’re right. I was absolutely gobsmacked when I cycled on the AC, and could only faintly hear the far distant hum of a compressor running up on the roof. No roof structure vibration. Always the skeptic when it comes to troubleshooting a problem, I had a look at the ammeter on our EMS monitor to make sure the compressor was pulling a normal load. It was sitting at its normal 11 – 12 amps. Ok, it worked.

Well, I’ll be … four years later, and four mods later, maybe we’ve got what some other RV folks take for granted: a quiet air conditioner?

I’m hoping that we are the only ones that had this in-coach compressor vibration and noise annoyance – maybe our unit was assembled with the wrong barrel nuts, on the afternoon before a long weekend. But, if our problem rings a bell with you, a quick check on the roof to see if any of what I have described looks familiar might be worth the trip up the ladder. (If you have not been under the cover of your rooftop unit before - don’t forget to remove shore power, make sure the Onan is not running, or at the very least, make sure someone is not going to turn the heat pump on while you are up there. There are two Phillips head bolts securing the cover on top, and two Phillips head screws on the back end, down at the base. The cover lifts off easily.)

There are, no doubt, other solutions to this problem, but this was what worked for us, on our particular unit: stainless steel washers on the rubber grommets, nylon bushings sitting on the washers, and the factory bolt caps screwed back into their barrel nuts. I don’t have a torque wrench small enough for this type of job, so all I can tell you about the torque I applied was that it was enough to be able to see a bit of compression occurring on the damping pad, and not alot more. Note that you can see the rubber grommets compressed as well. All five of the accessible feet were modded in this fashion. If you have the bushings and washers on hand, it probably is a 30 minute job and easy to do. (The most difficult part relates to the lack of space near some of the feet to use a wrench, or driver, efficiently.)

A quiet air conditioner! How nice is that?


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