Chevrolet 10 Series 1968, Service Manual

Page: 438 / 713

ENGINE-ELECTRICAL 6Y-13
Indicator Lamp/Initial Field Excitation Circuit Tests
5 .5 " -6 .2 " Systems
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.
Ammeter equipped vehicles use the same initial field
excitation and control circuits as the indicator lamp
except 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 igni­
tion switch to “ ON” . Connect a test lamp from
connector terminal “ 4” to ground (fig. 9c, Step 1)
and note lamp.
a. Lamp fails to glow - check for faulty bulb, socket
or open circuit between switch and regulator
connector. Repair as needed.
b. Light goes on - failure is in regulator, Delcotron,
or wire between “ F ” terminals on regulator and
Delcotron. Go to Step 2.
2. Disconnect lamp lead at ground end and connect
between connector “ F ” and “ 4” terminals (fig. 9c,
Step 2), and note lamp:
a. Test lamp glows - problem is in regulator. An
open circuit in regulator or relay is stuck closed.
See “ Service Operations” for repair.
b. Fails to glow - problem is in wire between “ F ”
terminals on generator and regulator or in field
windings. Go to Step 3.
3. Disconnect test lamp at connector “ F ” terminal and
connect to “ F ” terminal on Delcotron (fig. 9c,
Step 3), and note lamp:
a. Lamp glows - an open circuit in wire between
“ F ” terminals - correct as needed.
b. Fails to glow - Delcotron field has open circuit,
see “ Service Operations” to repair.
NOTE: This test lamp procedure may be used
on lamp type circuits and on ammeter circuits.
If Lamp Fails to Go Out, or if Ammeter Shows Dis­
charge the Possible Causes are:
1. Loose drive belt - adjust as necessary.
2. Faulty field relay - (see relay test and adjustment).
3. Defective Delcotron - (see Delcotron output test).
4. At normal idle - parallel resistance wire open (See
Resistance Test). Ammeter models the initial field
excitation wire to “ ACC” terminal is open.
5. Switch off - positive diode shorted (See Diode Test).
Field Circuit Resistance Wire Checks
The resistance wire is an integral part of the ignition
harness. However, the resistance wire is not solderable;
it must be spliced with a crimp-type connector. It is
rated at 10 ohms, 6.25 watts minimum.
The check for an open resistor or field excitation wire
(connected to the ignition switch “ ACC” terminal) is as
follows:
1. Connect a test lamp from the wiring harness con­
nector terminal “ 4” to ground as shown in Figure
9c (Step 1).
2. Turn the ignition switch to the “ ON” position and
note test bulb.
a. Test lamp glows - resistance is O.K.
F ig . 9 c —In itia l Field Excitation C ir c u it Tests
CHEVROLET TRUCK SERVICE MANUAL