5/20/1999 1000-207-00
FloScan Instrument Company, Inc.
Tel: (206) 524-6625 Fax: (206) 523-4961
3016 NE Blakeley Street, Seattle, WA 98105 email: service@floscan.com http://www.floscan.com
III. ALTERNATOR
The alternator input usually comes off one of the windings of the alternator before it is rectified. This tachometer signal is
inconsistently inaccurate because of a varying amount of belt slippage depending on the load on the alternator, the dimension of the
belt and the belt tension.
Calibration
You can calibrate the tachometer by two methods:
1. Calibrate to the existing tachometer. Simply rotate the RED and BLACK knobs on the back of the instrument until the two
tachometers agree with each other. The accuracy of this method depends on the accuracy of the existing tachometer at the
RPM to which you calibrate. It is best to compare to the existing tachometer at cruising RPM or higher, not at idle.
2. You need the number of poles the alternator has to determine the number of pulses per revolution. Multiply this times the
ratio of the diameter of the crankshaft pulley. Divide this number by the diameter of the alternator pulley.
3. Locate the number of pulses per crankshaft revolution in Table 1. Select the number closest to your actual pulses per
crankshaft revolution.
Example: Motorola alternator has 12 pole pairs (12 pulses per revolution)
Crank shaft pulley = 10” diameter Alternator pulley = 4” diameter
12 x (10 ÷ 4) = number of pulses per revolution 12 x 2.5 = 30 pulses per revolution.
From Table 1 - 50 RPM synchronized resolution - RED switch = 8 BLACK switch = 3
20 RPM synchronized resolution - RED switch = 8 BLACK switch = B
Adjust the RED and BLACK switches for the pulses per revolution according to Table 1:
Synchronized
Resolution
BLACK Switch Setting
20 RPM 9 A B C D E F
50 RPM 1 2 3 4 5 6 7
0
1.00 13.9 23.8 39.5 63.5 111 190
1
2.00 14.3 24.5 41.0 66.0 114 196
2
3.00 14.6 25.3 42.5 69.0 117 202
3
4.00 15.0 26.0 44.0 72.0 120 208
4
5.00 15.4 26.8 45.5 75.0 123 214
5
6.00 15.8 27.5 47.0 78.0 126 220
RED
6
7.00 16.3 28.3 48.5 81.0 130 226
Switch
7
8.00 17.0 29.0 50.0 84.0 136 232
Setting
8
9.00 17.8 29.8 51.5 87.0 142 238
9
10.0 18.5 30.5 53.0 90.0 148 244
A
11.0 19.3 31.3 54.5 93.0 154 250
B
12.0 20.0 32.0 56.0 96.0 160 256
C
12.4 20.8 33.5 57.5 99.0 166 268
D
12.8 21.5 35.0 59.0 102 172 280
E
13.1 22.3 36.5 60.5 105 178 292
F
13.5 23.0 38.0 62.0 108 184 304
Table 1 Tachometer Calibration
