Mitsubishi Mr. Slim not connecting with outside unit.

JackW

We have a Mitsubishi Mr. Slim Mini Splits system. There is one ODU and four IDU's. The ODU model number is MXZ 4B36NA, the IDU's are Mr. Slim Inverters. One of the IDU's is not working. When you first turn it on it run anywhere from 1 to 3 minutes, then shuts off, leaving the top light blinking. After troubleshooting via the remote, the top light on the IDU will blink three times stop, then repeat. Apparently, that means the IDU unit is not connecting or being recognized by the ODU. Any thoughts on what could be causing that this will be greatly appreciated.

Thanks,
Jack

ratio

Mitsubishi service manuals are available online at mylinkdrive.com, they have detailed troubleshooting procedures in them.

That said, did it ever work? The first thing to do is check the three conductors between the indoor unit & outdoor unit. I generally pull 'em off at both ends, ring 'em clear, & verify that they're not crisscrossed. That's by far the most common problem I run in to. But if it ran before, that's not the problem.

JackW

Yes, it ran fine until this year. Now I did have the people that installed the unit clean them in May, but not sure how soon after that this unit stopped working, so I can't go back on them. Thanks for the link, I'll check it out. What do you mean by the "three conductors" between the IDU and ODU?
Thanks,
Jack

PC7060

It’s a 3 wire interface so 3 conductor.  Use a multimeter to test the wire to verify continuity. For long run I disconnect both ends and tie two of the wire together on one end then check continuity for both lines and verify open on 3rd wire. Repeat using the 3rd wire and one of the verified lines and you should be good. 

As @ratio said, start by making sure the wires are properly connected at both ends.  One of the lines may be loose at the connection pint to the unit. 

JUGHNE

It is typically a 4 conductor cable, one (green) is a ground connection between units.
When checking the continuity as PC said you can check that all conductors have an infinity reading to ground.

Did they remove heads to clean them?

JackW

@PC7060, thanks for the info, I'll do that and see what I get.

@JUGHNE, I'll check the grounds and no, they used the water bag system.

GW

Ringing the wires——why not just check voltage and be done with it? 230v or so on 1 and 2, and a DC v swinging up and down (17 to 29 or so) on terminal 3. If the voltage is correct then you know the wiring is doing its job. If not, I would check the same process at the outdoor unit, with the wires disconnected. This tells you of the indoor unit, the wire, or the outdoor unit is or isn’t sending the correct power and signals. If all checks out you may need a new board on the indoor unit. You’re “supposed to” let the system chill for 5 minutes before sticking your hands in there. Careful, those wires are hot. 

remove the heads? No one removes heads, unless required 

JackW

@PC7060
@JUGHNE
@ratio
@GW

Okay, here is what I found. With all the wires hooked up at the ODU, the three units that operate the AC voltage between S1 & S2, and S1 & S3 is 247 VAC. Between S2 & S3 voltages jump from 3 to 30 VDC (+/-). So apparently that's the way it's supposed to operate.

On the IDU that's not working the S1& S2 voltage is 247 VAC. With S1 disconnected and S2 & S3 connected at the ODU, and S1, S2 & S3 disconnected at the IDU the voltage across S2 & S3 is 121 VAC.

With all wires disconnected at the bad unit block on ODU the voltage across is S2 & S3 is 27 VDC, across S1 & S2 it's 247 volts and across S1 & S3 is 247 volts.
With all the wires connected back up at the ODU and the IDU, the voltage across S1 & S2 is 247 VAC, across S1 & S3 it's 175 VAC and across S2 & S3 it's 38 VDC. This is the voltage at the block of the unit that's not working.

I disconnected all the wires at the ODU and the IDU, tied them together, and have continuity at both ends, so it's not the wiring.

So after hooking everything back up the other units work fine, so it appears to be the Electronic Control PC board in the IDU that's not working. I was shocked to see they are $495.00.

GW

It should be fluctuating at the outdoor unit, you're getting a steady dc v with wiring disconnected (on 2 and 3)?

Normally these have 10 or 12 year warranties on parts (depending on if the hvac co is Diamond dealer or not, and if it's been registered), who installed it? If it's under the table, you're on your own.

JackW

@GW. It fluctuates at 2 & 3 on units that are working, not at the one that isn't. Yes, I get a steady 27 DCV at 2&3 with the wires removed on the block that controls the unit that's not working.

I had it installed by a local plumbing contractor so I will have to call them. I replaced the main control board in the ODU about five or six years ago, that was covered, but I sure don't remember it costing as much as this board is going to cost.

GW

This may help:

https://www.registermehvac.com/WarrantyLookup_88973.aspx

Not sure if there's a log in, my pc goes right in.

JackW

@GW Put in the numbers but nothing showed up, I know it was under warranty, I replaced the main control board in the ODU in 2017, it didn't cost me anything. I'll have to call the installer tomorrow. Appreciate all your help.