EGT/Turbo Cool Down and high temp alarm.
(a description of how and why it works)
The reason I’m using this specific EGT gauge (Aside from being very inexpensive) is because it has 2 programmable alarms with normally open and normally closed contacts. This allows 2 completely separate temperature thresholds.
The truck can keep running until EGTs drop to a safe limit with the key out of the ignition, and alarm when temperatures rise to an unacceptable limit. This is the heart of the system.
To keep the truck running in spite of the key being removed from the ignition, the fuel shut off solenoid is bypassed. To do this, an isolated power source is applied to the “maintain” coil as the ignition is turned off. This is activated by Alarm 1 seeing “higher than set point” temperatures on the EGT gauge and through control relay 1. As the truck runs with the key on, Alarm 1’s N.O. contacts close due to EGTs therefore supplying power from the “maintain” coil circuit to control relay 1. Energizing control relay 1 closes its N.O. contacts which in turn supplies power to itself, the other controls and the fuel shut off solenoid. To prevent electrical back feed in a key off situation, a diode is placed between the solenoid and the main wiring harness. The bypass voltage is then applied between the diode and the solenoid to keep it isolated. To help preserve the life of the EGT gauge’s alarm contacts, the outputs are run through control relays capable of handling the higher amperage draws. Power to the fuel shut off solenoid is supplied by a constant power source and as mentioned, through one of the N.O. sets of contacts on control relay 1. In the event that EGTs hover just above the alarm set point, a timer is integrated on the same line as the relay to prevent excessive run time. The timer would be set to the maximum desired idle time. However, to ensure the timer only begins counting only when the ignition is turned off, a second control relay is necessary. Control relay 2 is energized by an ignition based switched circuit in the vehicle. Therefore timer and control relay 1 are fed through the N.O. contacts of control relay 1, Alarm 1’s N.O. contacts, control relay 2’s N.C. contacts and the timer’s own N.C. contacts. This allows the timer to shut itself and the rest of the circuit off after cycling. Again, one of the N.O. sets of contacts on relay 1 maintains auxiliary power to the EGT gauge once the ignition is shut off to keep it active. This way, the entire control system automatically powers down once Alarm 1 is satisfied or the system times out. At the same time, an “emergency stop” button with N.C. contacts is put in place ahead of control relay 1 to stop the sequence therefore killing the engine if required. An indicator light identifying the unit is in cool down sequence is tied into the same circuit as the timer.
The high EGT alarm circuit is very simple. Power is taken from a constant power source and directed to a set of N.O. contacts on control relay 2. It is then linked to Alarm 2’s N.O. contacts which are then fed to a simple strobe/buzzer indicator. The moment alarm 2 is triggered, the strobe flashes and the buzzer rings indicating high EGTs.
A high EGT mitigation system could then be incorporated into the system using a 3rd control relay wired parallel to the strobe/buzzer. This could then be linked to AFC controls, governors, ect.
A simple form of anti theft system could also be integrated in with the E-stop switch. Simply incorporate a N.O. set of contacts in the switch and make your starter solenoid run in series with it. -or your clutch pedal switch in series with it. The truck will only start with the E-stop and whatever else pushed. Most thieves won’t have the time to figure it out...
Attached I have the basic logic diagram for the system excluding any High EGT mitigation systems or anti theft. Sorry it’s so crude.
N.O.= Normally Open
N.C.= Normally Closed
CR1= Control relay 1
CR2= Control relay 2
EGT= EGT gauge
AL1= Alarm 1
AL2= Alarm 2
FSD= Fuel shut off solenoid
TR1= Timer relay
ESD= Emergency stop button
S114 Splice= Fuel shut off splice in wiring harness
B= Blue indicator lamp
>| = Diode