Evaporator Icing Condition

Contributed By: Costas Papayanopoulos

There is a service bulletin about an "Evaporator Icing Condition" for the 1986 GW, J/10 and J/20. Quoting from the bulletin:

"Evaporators on some early production 1986 GW's, J/10 and J/20 trucks may have a problem with the A/C unit putting out extremely cold air for some time, then, gradually warming up until only ambient air is emitted from the air ducts.

Possible Causes:

This problem may be the result of evaporator icing. The are two possible causes of this evaporator icing condition:

1. An incorrectly calibrated temperature control module (TCM), and/or,

2. A thermistor probe which has shorted-out.

NOTE: Depending on the heat and humidity, condensation can enter the thermistor probe (thermocouple), causing the thermistor to short out and cause the compressor..."

End of quote.

According the the bulletin, the TCM should be marked either "31(5)F" or "32(3)F". If not, should be replaced.

As for the thermistor probe, their recommendation is to re-install a new probe (P/N 8956 001 959) as far away as possible from the evaporator air inlet. To do this you have to drop the under-dash A/C unit to gain access to the evaporator core.

Costas Papayanopoulos   "If your oil pressure is low and your blood
'87 GW                   pressure is high you probably own a FSJ"
Long Island, New York