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Old 11-14-2022, 10:01 AM   #1
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Coleman Mach 15 Issue - Need Expert Advice

We have 3 Mach 15 A/C units on our motorhome, and the one over our bedroom exhibits some odd behavior (and is a heat pump).

Note that I am an engineer (electrical), but know only at a high level how the heat pump aspect works, and I suspect this is where the issue may lay.

At random times the compressor, when is should be running, will kick out (turn off) for a minute or two, and then come back on. I removed the cover over the weekend and watched this for about an hour. The first few times (when the issue occurs, which seems to be random) during on the initial "on" cycle two smaller lines to/from the evaporator will almost instantly develop frost, and soon thereafter (maybe 5-10 seconds) the compressor will turn off. After a few short cycles of this the compressor eventually comes one, and stays on, with no icing, and operation proceeds normally

I believe what is happening is the freeze sensor on the condenser is detecting a freeze, and subsequently de-powering the compressor. I *think* the reversing valve (for the heat pump) is perhaps getting stuck in some kind of "in between" position. There is a solenoid on the reversing valve...could that be going bad?

To be clear, this is 100% isolated to the Mach 15. The thermostat (a Firefly system in this case) never removes the control signal to the compressor (I've checked/monitored while on the roof with multiple multimeters hooked up), and the issue only occurs when the two smaller lines freeze up. Therefore, I am of the believe it's something to do with the heat pump reversing valve.

Image attached...red arrows point to the two lines that *sometimes* will freeze up just prior to the compressor kicking out. Condenser on the right, evaporator (not shown) to the left.

Interested in any thoughts...hoping it's a simple thing to check or replace (but hope is not a plan...).
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Old 11-14-2022, 11:06 AM   #2
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Reversing valves are not as prone to mechanically lock up as many seem to believe.

Do you have any way to measure the compressor current and voltage (between the neutral and the Run) wires? That would be a clue as to if it’s cutting out on current or loss of power.

If it’s kicking out on high current, you need to check/possibly replace, the run capacitor and possibly the PTC start device (if that’s what they used. Some use a potential relay across the start winding and an additional capacitor across between the Start and Run winding of the compressor).

If it’s the reversing solenoid - you’ll hear the piston slide and a swoosh of refrigerant through the refrigerant piping when the solenoid is energized/deenergized (assuming the compressor has run long enough to develop head and suction pressures).
Again - you need to measure the current and voltage of the solenoid to see what it’s doing.

Initial frosting of capillary or distributor tubes is not uncommon on compressor start.

Typically on a heat pump, only an outside coil frost sensor is needed. When it sees frost, it energizes/deenergized the reversing valve solenoid in order to defrost the coil. Depending on if the condenser and evaporator blower motors are separate or combined, the indoor coil will be turned off.
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Old 11-14-2022, 02:12 PM   #3
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MrMark52 , thanks for the response. Three questions:
  • Is the behavior I describe consistent with a relay issue (either a faulty relay, and/or wiring to/from relay)? The issue occurs randomly, and seems to "stick around" for a few hours, then always seems to fix itself.
  • Is the reversing valve relay mechanism spring-loaded in the cooling position, and the relay energizes only to pull it into the heating position? I believe the valve internals do not have a spring...but wondering if the solenoid does.
  • Anything I should (or can) do now, or should I just live with it?
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Old 11-14-2022, 10:09 PM   #4
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Quote:
Originally Posted by hilgert View Post
MrMark52 , thanks for the response. Three questions:
  • Is the behavior I describe consistent with a relay issue (either a faulty relay, and/or wiring to/from relay)? The issue occurs randomly, and seems to "stick around" for a few hours, then always seems to fix itself.
  • Is the reversing valve relay mechanism spring-loaded in the cooling position, and the relay energizes only to pull it into the heating position? I believe the valve internals do not have a spring...but wondering if the solenoid does.
  • Anything I should (or can) do now, or should I just live with it?
You need to find the owner, and possibly shop/service manual for the unit so you know where to measure voltages.

The issue could be a relay but could also be poorly crimped QD lugs on the ends of wires which causes wires and connections to burn. These units are notorious for poorly made crimp connections combined with road vibrations.

Reversing valves work off of pressure. If you look at it, there will be 2 small capillaries running from where the solenoid is positioned. The solenoid valve combined with the capillaries act as a pilot valve for pushing a piston back and forth in the body of the valve. High pressure discharge gas from the compressor (the single port on one side of the reversing valve) is directed to one end or the other of the piston). The piston is ported to direct the discharge gas to either of the two outer most tubes on the other side of the reversing valve, while the other port directs the suction port of the compressor (the center tube of the three) to the remaining outside tube.

When the compressor powers off - the solenoid (depending on how the manufacturer designed the system) is usually de-energized and that usually sends the unit into heat pump mode.

You don’t have to “live with it” without figuring out why it does it,

Get a wiring diagram, get your volt and current meter, and make some measurements. Let us know what you find and we’ll try to tell you what to look at next.
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Old 11-17-2022, 07:59 AM   #5
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MrMark52, again thanks for the information...I now better understand what to look for.

I watched a few YouTube videos on how reversing valves work...amazingly simple operation (not how I imagined they would work), and I can see your point that the valves are less prone to lock up. Not impossible, just less probable than a wiring/solenoid issue.

I have yet to hop back on the roof to do measurements...busy with conference calls and other work this week...will probably do that this weekend.
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Old 11-22-2022, 10:14 AM   #6
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MrMark52, I have not yet made it up to the roof (rained a bit this weekend...and power+rain=unsafe), and it's raining again today.

However, I did a few experiments, and I believe it must be the wiring (as you suggest) or the solenoid. I ran the heat pump several times with zero issues...pumped out nice hot air. So, if the valve is normally in the heat pump position, when in cooling mode the valve is either stuck, or the solenoid is not moving the valve to the cooling position (so a solenoid or wiring issue).

When I make it up to the roof I am hoping to either find a lose/poor connection, or a faulty solenoid (obviously I am hoping more for a wiring issue). If the solenoid is bad I'll need to find a replacement somewhere...any suggestions on where to look? Is the solenoid a 12V part? I can only find 24V solenoids online.

I do have a spare run capacitor in case it's that, although I would assume that to be unlikely given the above (but I now have it on-hand when/if needed).
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Old 11-22-2022, 11:18 AM   #7
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Assuming your heat pump doesn’t also have electric heat in it and you could actually hear the compressor running, I would say the run cap won’t do you any good for the moment.

The only Coleman Heat Pump wiring diagram I could find shows the coil to be 115 volts. Considering there is no purpose to drive the reversing valve when only battery power is available then I would say it unlikely to 12VDC.
And since there generally isn’t any real need to create 24VAC since 12VDC is available to drive the Class II circuits as needed (and when 120VAC is available to drive high current items such as the compressor and fan motor) it’s unlikely the coil is 24VAC.

But in the end - just look on the coil - they are usually marked.

You also need to monitor the power going to the coil to confirm it being powered with the thermostat in Cooling or Heating. There may also be a defrost board/control that isn’t playing well as well.

In any case, you need to systematically go through the steps it takes to determine if things are operating correctly. Keep in mind, there may be time delays involved when manual switching between heat and cool cycles - and those time delays can fool you into thinking something is bad when it’s not.

If you find the coil is energized in heat mode then just take the fastener off the coil while the unit is running in heat mode and slide the coil back on the armature (be prepared to hear a loud “swoosh” when you do that! �� And be quick about it - fast off, and fast on after a delay or shutting the unit off).
Alternatively, after determining its a 120VAC coil and the coil is not energized in the heat cycle - find the switched lead and disconnect off-road splice into it and feed 120VAC directly to it (you can jump from the hot lead going to the compressor Run terminal at the run cap).
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Old 11-22-2022, 12:03 PM   #8
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Thanks...

When (if?) it stops raining I'll hop on the roof. In addition to testing, I'll try to get a good picture of the wiring diagram.
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Old 11-25-2022, 09:27 AM   #9
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Wiring diagram and picture of solenoid (no other markings) attached from a visit up on the roof on Wednesday afternoon. From what I could find online this is a 120V solenoid.

I reseated all connections where possible, and pull-tested all other wires/connections. After that things seemed to work good for about 24 hours, and the the symptoms reappeared...cycling off after 30-45 seconds, then waiting the same, then restarting.

I think there is a pattern here. When the system has been off for some period of time (15 minutes or more), and then restarted things work good for several hours of continuous use (including several normal on/off cycles). However, when running for an extended period overnight (overnight the wife likes the fan running continuously when sleeping for noise and air movement) it starts misbehaving after 5-6 hours.

MrMark52, question: Would dirty condenser coils be something that might cause this behavior...the freeze protection tripping the system? I do keep them cleaned at least annually...last cleaning was very early this year...and they do not look dirty. However, easy to clean, so that will be my project for today/weekend. I don't believe the evaporator coils are dirty (cleaned earlier this year), and I clean the inside filters *very* regularly and inside airflow is very good.
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Old 11-25-2022, 10:35 AM   #10
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To get the terminology right - the coil accessed from the roof is called “outdoor coil” (aka as condenser on a regular A/C unit), and the coil accessed from the inside of the coach is called “indoor coil” (aka as evaporator on a A/C system).

Good that your trying to keep coils clean, especially the outdoor coil. Key thing about the outdoor coil if you don’t know it’s history before you - is that you have blown through the coil (reverse direction of air flow) with water to insure any embedded dirt has been flushed out. Once you know it’s clean in that way, doing an annual cleaning of the face is typically adequate.

Indoor coils generally only face load so depth cleaning isn’t generally needed.

It does sound like your are seeing a pattern - and further investigation is warranted to determine if the cause is controls related or some aspect of component failure is the cause.

While you pulled/tugged on wires to check that they were secure - it does happen that crimped connections on wires get crimped on insulation rather than the wire itself. This can make for an intermittent connection that could be the cause.
You might also keep an ear to the unit while operating - do you hear a click of any sort coming from the unit that causes a change in operation? If so, determine the source of the click can lead you to what is causing a change in unit performance.

Bryant rv.com is a good source of owner and service manuals for MoHo related stuff. Go there and see if you can find either or both manuals - they will go a long way in helping you to understand how the units are supposed to operate (especially the service manual) and what triggers what.

Let me know what you find!
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Old 11-30-2022, 11:36 AM   #11
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MrMark52, quick update.

First, thank you so much for your input...I learned a lot.

I have an extended warranty I negotiated into our coach purchase (at a 50% discount). Used it a few times, and am now using it again. $200 deductible, and the warranty will pay to replace the unit with labor included.

I did narrow it down to (I think) the freeze protection...and the technician I had out said for that there are no readily available parts, so unit replacement was the only option (yay!!!).

So, again...thanks for your help...and I will update thread when the new unit gets installed.

EDIT: I rarely buy extended warranties unless at a huge discount from list, but since I got this one (at a huge discount as a deal-closer) I've been happy. Before this event I was about even when what I paid versus what has been covered. After this event I'll be very much in the positive side of the equation (and still have a year left!!!).
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Old 11-30-2022, 01:50 PM   #12
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Good deal!

Thanks for the update!
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Old 12-07-2022, 03:15 PM   #13
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New unit installed today. The guy had a neat little lift thingy to raise the new unit all the way up to the top of the motorhome...he had no helper, and refused my help (other than supplying him with Dr. Pepper).

He simply tossed the old one off the roof (told me to stand clear during "express removal")...said there is no market for parts (I would have removed the fan motor if he had told me in advance).

Total time onsite was less than 90 minutes.
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Old 12-07-2022, 05:24 PM   #14
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He probably used what’s called a GenieLift. HVAC people use them all the time to get condensers up to the roof.
They’re expensive but very useful.

He’s right about much being salvageable out of the unit. Primarily the scrap value of the copper and aluminum is all that’s there.
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