Low Air tell tale, pressure gauge seems fine...

[mqbus767]

My 2001 MCI 102 DL3 developed a new issue recently. The Low Air tell tale is illuminated and the parking brake refuses to release. Here are the relevant clues:

* Pressure guage on the dash shows 120psi (both needles)
* Low air tell tale/alarm on/sound constantly
* Bus suspension no longer seems to air up (bus stays on bump stops)
* Transmission will shift, but PB won't release
* Rear, engine compartment air tank is not airing up (see picture)
* Front air tank (spare tire compartment) is aired up; verified functionality of both pressure switches (see picture)

I suspect the valve on the top of the tank in the picture below, but I'd appreciate any other advice on troubleshooting/fixing the issue. Thanks!

[mqbus767]

Just closing this one out. It wound up being the pressure regulator in the HVAC compartment that I had absent mindedly closed when working on the equipment removal. Apparently it also shuts off the suspension and other non-critical air devices.

[Tedsoldbus]

Leave it out there! I just learned something. Again. That is why us new to having a bus guys stare at this thing. Thank you.
Ted

[mqbus767]

By close out, I just mean answer my own question and mark this one fixed, not remove it from the board. 

[Glennman]

Mq, I know you already figured out the trouble, but I believe there is an air solenoid valve on my 2002 D4500 directly above the spare tire compartment toward the passenger side of the bus that apparently keeps the parking brake from releasing if there is a door open (I believe the wheelchair door), or whatever. From what I understand, it is something that is easily dismantled. That is one of the things I'll be looking into during my ongoing electrical discovery process.

[mqbus767]

Good to know. But, if I disable the WCL door lock-out, I'll probably do it at the multiplexer input side of things.

[Glennman]

Good to know. But, if I disable the WCL door lock-out, I'll probably do it at the multiplexer input side of things.

When you do, please send a picture of what you did. That might be valuable information for me too!!!