J&M 275ST - Manual (Page 25)

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2.0 Service 2.0 Service
Straight Edge
Normal Wear Abnormal Wear
(Replace)
Voltage in the system should begin at 0 volts. As the controller bar is slowly actuated, the voltage should gradu-
ally increases to about 12 volts. This is referred to as modulation. No modulation means that when the control-
ler begins to apply voltage to brakes it applies an immediate high voltage, which causes the brakes to apply
instantaneous maximum power.
The threshold voltage of a controller is the voltage applied to the brakes when the controller first turns on. The
lower the threshold voltage, the smoother the brakes will operate. Threshold voltage in excess of 2 volts (quite
often found in heavy duty controllers) can cause grabbing, resulting in harsh braking.
How to Measure Amperage:
System amperage is the amperage being drawn by all brakes on the trailer. The engine of the towing vehicle
should be running when checking amperage.
One place to measure system amperage is at the blue wire of the controller which is the output to the brakes.
The blue wire must be disconnected and the amp meter put in series into the line. System amperage draw should
be as noted in the table below. Make sure your ammeter has sufficient capacity and note polarity to prevent
damaging your amp meter.
Replacing brake magnet
1. Orient the magnet over the lever arm post such that the magnet leads are in the correct position for
routing
2. Push the magnet over the lever arm post by compressing the magnet spring between the magnet and the
lever arm.
3. Insert the magnet clip in the slot of the magnet. Be sure to orient the magnet clip so it will “snap” into
place.
4. Press down on the magnet and install the magnet clip.
5. Be sure that the magnet moves up and down freely on the lever arm post.
6. Route the wiring in the same manner noted on removal. Be sure that wires cannot bind, pinch, or rub.
Manually actuate lever arm to insure there is no interference.
7. Install strain relief bushing, allowing enough slack in the wiring to allow the lever arm to move without
straining the wires. Be sure the wire cannot come in contact with the armature.
8. Connect the magnet leads to the trailer wiring harness and then reinstall hub and drum.
Brake Size Amps/Magnet Two Brakes Four Brakes Six Brakes Magnet Ohms
12 X 2 3.0 6.0 12.0 18.0 3.2
Figure 2.7