Length of glowplug operation is governed by the ECT sensor (coolant) reading: 

Maximum pre-glow: 

-30c=25 seconds 
-20c=25 seconds 
-10c=22 seconds 
0c=19 seconds 
+10c=12 seconds 
+20c=8 seconds 
+40c=4 seconds 
+60c=2 seconds 

Maximum glowing time during start: 

-30c=60 seconds 
-20c=60 seconds 
-10c=40seconds 
0c=20 seconds 
+10c=10 seconds 
+20c=5 seconds 
+40c=2 seconds 
+60c=0 seconds 

Coolant Temperature Sensor Response
Temperature (Degrees Celsius) | Resistance (Kohms) | Voltage (Volts)
-40 | 925 | 4.54
-30 | 496 | 4.46
-20 | 277 | 4.34
-10 | 160 | 4.15
0 | 96 | 3.88
10 | 59 | 3.52
20 | 37 | 3.09
30 | 24 | 2.62
40 | 16 | 2.15
50 | 11 | 1.72
60 | 7.5 | 1.34
70 | 5.6 | 1.04
80 | 3.8 | 0.79
90 | 2.9 | 0.64
100 | 2.08 | 0.49
110 | 1.56 | 0.38
120 | 1.19 | 0.29
130 | 0.918 | 0.22
140 | 0.673 | 0.17
150 | 0.563 | 0.14