From: william f cox (grntrks@juno.com)
Date: Sat Nov 23 2002 - 21:12:49 PST
Scott,
On the M35 series trucks there have been 2 types of brake light switches
used
The original style was hydraulic and mounted on the master cylinder.
These were replaced by an upgraded air operated switch on some trucks
during depot overhaul/upgrade. I believe you can find drawings of both
types in the -20P or -34P parts TM's. The illustration will show where
they are mounted and what they look like. Once you locate your switch
you will be able to check the wiring.
Most likely cause will be a faulty switch or disconnected/broken wire.
You can check the switch by disconnecting the wires and useing an ohm
meter across the terminals while an assistant applies brake pedal
pressure.
CAUTION!! You'll be under the truck. Chock the wheels, set the hand
brake, transfere case in neutral, transmission in neutral. Start the
engine to bring air pressure up, then stop the engine. Turn the master
switch on. Turn on the service light switch and check the brake light
switch across the terminals with the meter.
NO READING==FAULTY SWITCH==NO JOY==replace the switch.
Reading on meter==good switch==begin following wires to find faulty wire.
Be sure to turn every thing off when finished with the continuity check.
I sometimes have forgotten to do this and killed they batteries.
Happy hunting,
Frank Cox (grntrks@juno.com)
MVPA 14530
'45 GMC 353 DUKW
'66 Kaiser-Jeep M35A2 w/w
'68 M105A1 trailer
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