3406B Fuel Shutoff Solenoid

I have a customer that has a switch in his truck that he was holding up to start his truck. key on and push button to start with switch up. As soon as the truck started he could release the switch and the truck would run. Then turn key off to shut off. Now this switch has to be held up all the time or the truck will shutoff. The only thing I notice is when he holds the switch up the solenoid on the fuel pump clicks. I don't know why before once the truck started he no longer had to hold this switch up. I told him with out knowing what it was supposed to do the only thing I know to do is put a switch in that you have to flip on and off. Any thoughts?
91 Peterbilt 379

 

OK Thanks. When I pulled up the truck on Paccar it shows it came with a Kysor System on it. I will read up on it tonight now that I know what is installed.

 

I just wired in a new switch that he flips on when he starts the truck. I explained to him that IF the oil pressure and coolant shutdowns were working that they would no longer work. I don't think he even knew that was part of the system.

 

I figured the switch would give him security if someone tried to steal his truck. I know that if they want it they will get it but for some whack a doodle jumping in there they would not be able to start it with just a key.