GMC 1976 ZEO 6083 Maintenance Manual page 750

Figure 13-Capillary Tube Obstruction Check
Figure 1 4-Superheat Charge Test Thermometer
ROOF MOUNTED AIR CONDITIONER 24E- 11
Problem Remedy
6. Superheat charge test 6. Install manifold gauge set. Tape an accurate
(figure 14).
thermometer to the suction line about six inches
from the compressor . Insulate the thermometer tube
and tip so that it will measure the line temper-
ature. Let the system run 10 to 15 minutes with
the shroud in place, then read the temperature and
pressure . Compare with the temperature pressure
chart for R-22 found earlier in this section.
Superheat is the difference between actual and
chart temperature. It should be from 15 to 25°F .
(8 to 14°C .) . High superheat indicates an under-
charge-charge the system, or it may indicate a
restriction . See checks 4 and 5 . Low superheat in-
dicates an overcharge-bleed the system .