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Replacement parts for KENMORE 10658134700 | AppliancePartsPros.com

See the attachment for the tech sheet.

You do not have a defrost timer.
Defrost is controlled by an Adaptive Defrost Control (board) it is in the control console in the fresh food section.

First remove the evaporator cover in the freezer so you can see the coils.
Do not let them de-ice.
If they are heavily iced/frosted over you have a defrost problem.

If yes.
Manually force a defrost cycle, see the tech sheet. When the fans and compressor turn off, you are in defrost.
Now check the defrost heater to see if it is on.
Be careful you do not want to burn your fingers.
If the heater is on then the board needs replacing, re:it is never getting into a defrost cycle.

If not on.
Unplug the unit.
Remove the wire for one side or both of the heating element and measure it for continuity, usually around 20 ohms or so.

If the heater is OK
Remove one wire to the defrost thermostat and measure it, should be 0 ohms when frozen. Note that it opens just above freezing so must be frozen to check it. Also inspect it, if it is bulged at all replace it even if it measures OK.

If both the above are OK then odds are the defrost timer contacts are toast.
Best way to test this is a live test to see if you have 120 volts across the heater/defrost thermostat combo.

[COLOR="DarkRed"]Do you have a diagram on how to disassemble the heater?[/COLOR]
No, but the following may help. Sometimes the screws that hold the cover on re under the gasket.
Whirlpool R-84 26 inch Side x Side Refrigerators and Freezers Service Manual - ApplianceDigest.com
[COLOR="DarkRed"]
Should I reach the heater from back side of the refrigerator?[/COLOR]
No, you get to it through the freezer compartment.

I am not sure.
I tried to find a procedure but could not.

You should not have to take the evaporator out.
You may have to undo it to get the heating element off but be very careful that you support it and do not force it.
If you stress the tubing going to the evaporator you will cause a freon leak and the fridge will be toast.

Perhaps someone else will jump in with the correct procedure or you could start a new post asking how to get it out.