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air conditionning? Answered

failed after lightning ,change fuse , start then after 3 min green led flash and unit stop
Friedrich MR12C1E ..cannot set timer OFF is it the mpu ?



6 years ago

I was ask to select the best answer !
cannot do this at this point in time as the problem is not resolved yet

I have ordered a piece of equipment and when installed I will know

some answers describing the circuitry guided me to narrow my search

to two possibility and I do not know yet wich one is the cause

when problem resolved I will let you know

in order to help others with a similar problem



6 years ago

Is it a house window or ground unit ??


it is a split system
the outdoor unit has a fan and a compressor
the in door unit has loovers fan and electronic boards
one supply board with fuses and one mpu board receiving instructions
from a remote control

this old model does'nt have electronic board on the outside unit
it is about 10 years old

the new units have also an electronic board on the outside unit

Good description. Try again to totally disconnect power to both parts
for two minutes by removing the fuses or tripping the circuit beakers.
Make sure the controls are set to off and re-apply the power to both
parts and try turning it on.

If it still fails you have to replace at least on if not both boards.

This was done
the unit start (red LED) and about 3 minutes later
green LED flashes and unit stop

Green LED reflect the timer mode wich seam to shut down the unit
based on a Timer error

I try to set Timer OFF but does'nt seam to accept this command

maybe it has to be set in the first 3 min before timer take action
will try next time I go there for tests ( this is my son unit

But it does puzzle me that the unit accept start command from the remote control but not the timer OFF command

in ,my opinion if mpu was realy bad it would not accept start command ?

Lightning does strange things to electronics. 
You would not be here if something did not change.
If its 10 yrs old it could have a separate IC on the PCB for timing. 
There may be a master reset feature.
As you say in the first minutes something may have to be set up.
Your instruction booklet if you still have it may be useful to review.
You could Google it for experienced solutions.

in the inside unit there is two boards
the supply board for 12 and 5 vdc
contain the solid state relays for compressor and fan

it is connected to the mpu board
has inside termistor , display ,stepper moter and fan motor

now i suspect the timer on this one

as you can see in the center of the board I have changed the blown fuse
and I will change the relay under white and brown wire
this is the compressor start relay


I wouldn't think a relay susceptible to lightning as much as the
integrated circuits which are the worst to diagnose and change.
I understand you have found the relay which should start the
compressor. Can't hurt to change a relay.

this is a triac type probably affected by high voltage ?
resistance of the coil is too low about 10 ohms instead of 300
and contacts in path of burned fuse ....it is only $9

if can't reset timer will order a new mpu board $ 125

similar brand new unit installed sell for about $3800

Yes, a triac can be harmed by High voltage with enough energy.
The concern is what can harm a triac will certainly do other damage.
Replacing the mpu is a good decision.

The 115ac fuse blew up but the low voltage fuse did'nt
this is why I was expecting the mpu to be still good ?

A circuit board fuse protects faults that cause prolonged
( millisecond ) excess current through the fuse path.

Lightning's damaging voltage effects on semiconductors
occur in microseconds, possibly followed by a secondary
failure like a stalled compressor rotor that can pop a line fuse
in a 1/4 second.
A stalled motor can occur when the starting rotation switched control
does not engage or fails to terminate in spin up time. Compressor motors
are usually thermally protected by mechanical means.

it is pointing now to the inside fan speed

when a speed is selected ( hi med low ) the mpu look for rpm feedback

in no match then it goes into lock mode and give me the 6 pulses error red LED

If one winding is bad and I do get medium only speed

then HI or LOW selection will give me the Error code

to be confirm by measuring the motor winding resistance and see if one is open

with jumpers the outside unit work fine compressor and outside fan

in testing my circuitry I am using a Mastercraft multimeter 52-0055-6

I am not sure the ohms reading to be correct

Notice 2 pot on the circuitry

How to recalibrate the Ohms range

any idea where to find service manual for this meter ?

I just hope that my reporting may hepl someone else with similar problem

compressor has an overload relay ,will check that too

but an overload is a pull by the compresor .the overvoltage was a push

one nanosecond is consider a spike

more than 3 nanosecond is consider a surge

possibly the surge passed before the fuse melt

how many Amps went thr ? fuse 3.15 Amps

after replacing the fuse and the Varistor I was surprise to see the unit accept

the start command from the remote control

may do some more tests next week

it is not a rush , here the season is over with 50 deg at night

working for next summer

next time will also check for error codes
this unit ,from litterature has only 2 errors codes
given by the red LED
5 pulses = problem with the interior fan
2 pulses =bad thermistors

but as the unit did start I suspect those two to be good ?


6 years ago

If it was surged by lightening then its probably got more burned out than the fuse. Lightening damage can be very strange. It will burn out some things and leave other components untouched. If you have house insurance it usually covers damage from lightening strikes. I would also check everything else very carefully including any phones. You could have hidden damage that will not show up for a while.

That does'nt help me diagnose this mpu unit
trying to see if it does accept some commands but not all of them ?