View Full Version : Master Bilt MBGFP48 Freezer Defrost Problem

01-09-2013, 03:15 AM
Hey all,
This is my first post, so please let me know if i'm doing this in the correct area.

Looking at a Master Bilt MBGFP48 freezer that the evap was a solid block of ice. Not frost, BLOCK OF ICE!!! When thawed out it filled a 5 gallon bucket.

The bi-metal checked out ok, cut circuit at about 80 degrees F.
Heat element tested at 15 ohms.
Both evaporator fans are OK.
Drain is clear.
Door seals are perfect.
Defrost schedule is every 6 hours.

When I let unit come down to temp @ -5 F, I actvated manual defrost. The defrost mode came on for about 2 minutes then went into recovery mode. In the 2 minutes in the defrost mode the heater element had 7amps showing. I also put a temperature probe on the heat element, but it did not show any noticable rise in temp.
I'm thinking the controller might be defective since new. This unit was installed about 2 months ago, maybe it never correctly defrosted since then.
I ran the electrical for it, used 10AWG for 60' run and 20AMP 120V circuit. The delivery people turned unit on when it was delivered.

Any advise or help????? Thanks

install monkey
01-09-2013, 02:09 PM
got sensor values at bottom of the page
usually defrost on a freezer should be for 30-45mins

01-09-2013, 02:30 PM
Hey "install monkey", I have used some of your advice from other forums I have read.

I will check these 3 Temp Sensors per Mfr recommendations. Thanks

01-09-2013, 03:25 PM
In the 2 minutes in the defrost mode the heater element had 7 amps showing. I also put a temperature probe on the heat element, but it did not show any noticeable rise in temp.

I must be missing something! If you checked the defrost heaters and they ohm out, have proper voltage and show amperage there is no way that you will not feel or measure a rise in temperature on the elements. If the evaporator was a block of ice at the time and you only ran 2 mins perhaps the ice acted like a heat sink and that's why no measurement was available. I would say that you need to confirm the defrost settings. You will need to schedule multiple defrosts per day and like install monkey said, they need to be a minimum of 30 mins per cycle.

01-09-2013, 05:39 PM
Hey PassandScore,
I let the unit thaw over night so there was no ice at the time I took the readings.
This unit is scheduled to defrost every 6 hours. The defrost time is approx 30 min on the same unit next to it. They are both new.
The unit that froze will only go into DEFrost mode for 2 min then go into RECovery mode, then comes back on line. I can change and set the defrost cycles, but can the length of defrost time be changed??? Its not getting warm enough for the bi-metal switch to shut down defrost.

And YOU RIGHT, I do not understand why Voltage and Amperage that the Heat Element did not show rise in temp with a probe I attached to it. Is 2 min not enough time to show anything? Why is it kicking into recovery so fast?
There was a wire pulled out of a connector but I think a piece of ice or the ice itself pulled the wire loose, I connected back and triple checked all others.
I am going to pass by and check the temp sensors "installmonkey" showed me.
Unit is on line now and works fine besides defrosting.

When it goes into defost mode the evap fans shut down like they are supposed to and evap fans come back on when it goes into recovery then when recovery is done the compessor comes on and runs fine. Last I checked the unit was at -7F.
I will INPUT ALL advice to help me figure it out and get the problem fixed.


install monkey
01-09-2013, 05:51 PM
got a manual
check defrost termination setting on controller- default 55 deg
check coil probe not close to element

01-09-2013, 05:56 PM
I found a manual for this unit and the defrost duration does have a setting 1 to 120 min, factory setting 30 min. And defrost termination temp settings -50F to 120F, factory setting 55F.
I will check everything

Thanks again all!!!