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I will find a pic tomorrow. It's too dark out now.

No I have never seen a manual that shows more than a block diagram of the system. Nothing that shows physical location. But it's down there by the other two. It has 2 wires coming out of it. There isn't much else down there like that.

The low air switches look like this except they are probably covered with green carc. I think the ctis one looks similar.

The part you circled looks like an air safety valve probably set to vent off excess air at around 150psi. Before the abs mods were installed I think more trucks had the safety valve there. The air dryer should have one also now.

There are 3 switches in the tank area by the air petcocks. One on each of the 2 air tanks they are for the low air warning and one switch for the ctis. If you look in the TMs at the low air warning system and identify which 2 are for that then the one left will be for the ctis. You can also let...

Did you find the correct switch and verify that it closes at around 120psi. The other 2 switches close around 60psi they don't affect the ctis. You can unscrew the ctis air feed filter and start the truck to see if you have a good air feed with no blockages. If the pressure switch is working...

Napa won't have one. Make sure both tanks go up to 120psi. You can jump the wires that go to the switch with a wire and see if the system does a pressure check. It could just be a bad contact. Some of the switches are actually rebuildable. I have seen controllers go bad also.

There are 3 switches on the tanks 2 are for the low air warning and one is ctis I dont remember which is which but you can see them when you look in towards the tanks from the passenger side by the air petcocks.

If the hwy light is flashing and the system is just idling it is probably waiting for the signal that the air tank has 120psi so it can do an initial pressure check. There is a switch on one of the tanks for this. There is also a small catch filter on the air feed to the valve body in the cab...

The governor is on the firewall the pressure protective valve is down between the air dryer and tank area. See if your wipers and horn work. If the ppv does not open then there is no air supply for the ctis. Replace the ppv.