If you have a mechanical linkage for the throttle.... than there is a mechanical switch in series with the dash switch... in that the throttle has to be closed, switch contacts "made" and the dash switch on, to send 12 v to the solenoid.... if the throttle is fly by wire.... then there is a electronic switch, in the engine ECM that is in series with the dash switch.... the ECM sees that the throttle is closed... once the throttle is closed and the dash switch is on.... power is applied to the solenoid...
In some applications there is an optional foot switch also in series... so that the operator can call on demand for the exhaust brake to work....
One last thought.... if its been ages since the exhaust brake has been working.... either the Jacobs or PacBrake.... the butterfly can seize in the housing and the air cylinder is not large enough to push closed the butterfly... if you pull the air line, you should be able to push closed against the spring the butterfly...
On the other end of the air cylinder is a vent that allows air from the other side of the cylinder to escape and prevent dirt from getting into the bore of the cylinder.. this vent is a filter... I've seen this filter so plugged that it won't let the cylinder work...
If you have to take the brake off the vehicle to free up the butterfly, don't disconnect the butterfly from the shaft.... this is a critical adjustment that is factory preset... also because of the extreme heat any grease or oil used to lubricate the butterfly shaft will burn up and cause more problems... there is a specific high temperature lube... designed to lubricate this shaft and prevent seize up's.... I've always used the PacBrake brand.... expensive but it sure works..
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Retired Business Owner, Re-manufacturing HD Clutches, Brake Shoes, Air Compressors, Sales & Installation of PacBrake and other Industrial Friction
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