Indicator Lamp/Initial Field Excitation Circuit Tests - Chevrolet 10 1971 Series Chassis Service Manual

3. Negative diodes (Test B) - connect ohmmeter oe-
tween " R " terminal and "GRD" and note reading,
then reverse the leads and note this reading. Meter
should read high in one direction and low in the other.
A high or low reading in both directions
NOTE:
indicates a defective diode.
4. Open Field Check:
a. Connect an ohmmeter from " F " terminal to
"GRD" terminal stud and note reading on the
lowest range scale. Meter should read 7 to 20
ohms.
b. If meter reads zero or excessively high resis­
tance the Delcotron is faulty.
5. If above tests indicate a defective Delcotron, remove
and completely check Delcotron as outlined in the
"Chassis Overhaul Manual" .

Indicator Lamp/Initial Field Excitation Circuit Tests

On standard models the indicator lamp circuit provides
initial field excitation (causing lamp to glow). The light
is cancelled by closing the field relay which applies
battery voltage to both sides of bulb (bulb goes out).
The indicator light should glow when ignition switch is
"ON" and go out almost immediately when engine starts.
Fig. 7 c-ln itial Field Excitation Circuit Tests
Ammeter equipped vehicles use the same initial field
excitation and control circuits as the indicator lamp ex­
cept the lamp is omitted. The continuity tests on both type
vehicles can be made as follows:
If Lamp Fails to Glow or Ammeter Fails to Function
the Possible Causes are:
1. Faulty bulb or bulb socket.
2. Faulty ammeter.
3. An open circuit in wiring, regulator, or field.
4. A shorted positive diode - (may also cause glow with
ignition switch "O FF").
Test as Follows:
1. Disconnect connector from regulator and turn ignition
switch to "ON" Connect a test lamp from connector
terminal "4" to ground (fig. 7c, Step 1) and note lamp.
a. Lamp fails to glow - check for faulty bulb, socket
b. Light goes on - failure is in regulator, Delcotron,
2. Disconnect lamp lead at ground end and connect be­
tween connector 'FF" and "4 " terminals (fig. 7c,
Step 2), and note lamp:
or open circuit between switch and regulator con­
nector. Repair as needed.
or wire between " F " terminals on regulator and
Delcotron. Go to Step 2.
10-30 CHEVROLET TRUCK SERVICE MANUAL
ENGINE-ELECTRICAL 6Y-11