Air Pressure Gauges/ Voltage/

[Lugnuts]

I am working on a Fire Department M1087. The reason I start with that statement is so you will know that I do not know what happens to these vehicles before I get there.
I have been correcting issues with the truck overall and I believe I am about to resolve that this unit took a shot of voltage that wasn't suppose to be there, like a polarity change or 24 instead of 12 volt.
Question: what is the correct voltage that the Air Gauges are supposed to work on? And does anyone have originals that read Front Brake Air and Rear Brake Air? I believe I am interested in buying them.
Thank you
Lugnuts

 

[Ronmar]

You didn’t say what year/model, A0 or A1?

On the A0 All of the gauges run on 24V provided by CB77...

The air gauges can get wonky if the senders are not grounded properly... the sensors are under the drivers dash and are screwed into T fittings and into a 2way check valve that provides park and trailer control air to the dash valves. That 2way is mounted to a bracket up to the bottom of the dash. That bracket is the ground path and if it gets dirty or loose the gauges won’t work right... quick test for this is take a piece of wire and ground the pressure sensor transducer case to the chassis. They also have 2 screw terminal lugs on top, things get weird if those come loose also. The senders are combined variable resistors to drive the gauges and a low pressure switch to drive the annunciator lights and air buzzer...

On the A1 all the gauges are data bussed to a control/sensor module under the drivers dash and sample pri-sec air there. Not sure what that controller is powered with, but it is probably the same 24V from CB77...

 

[Ronmar]

oops, double-tap...

 

[Lugnuts]

You didn’t say what year/model, A0 or A1?

On the A0 All of the gauges run on 24V provided by CB77...

The air gauges can get wonky if the senders are not grounded properly... the sensors are under the drivers dash and are screwed into T fittings and into a 2way check valve that provides park and trailer control air to the dash valves. That 2way is mounted to a bracket up to the bottom of the dash. That bracket is the ground path and if it gets dirty or loose the gauges won’t work right... quick test for this is take a piece of wire and ground the pressure sensor transducer case to the chassis. They also have 2 screw terminal lugs on top, things get weird if those come loose also. The senders are combined variable resistors to drive the gauges and a low pressure switch to drive the annunciator lights and air buzzer...

On the A1 all the gauges are data bussed to a control/sensor module under the drivers dash and sample pri-sec air there. Not sure what that controller is powered with, but it is probably the same 24V from CB77...

View attachment 834468

Thank you. To do a simple check on the gauge itself can I provide 24 volt to the ignition and ground and then ground the sender through a test light? My gauge is reading full all the time wether it has power or not. And truck is A0.

 

[Ronmar]

Well if it is already reading full, grounding the sensor lead won’t change anything, as low resistance to ground would indicate full needle deflection. If you disconnect the leads at the top of the sender then turn on the main power the gauge should fall to zero with no circuit thru the sender... meter not moving usually indicates no power reaching the gauge or bad gauge. Or in the case of needle always at full, bad sender or shorted circuit...

Now if someone wired the sender leads backwards, the gauge would read full until the tank pressure went over 85PSI and the low pressure switch opened, but then the low air buzzer and idiot lights would not work correctly...

 

[Lugnuts]

Well if it is already reading full, grounding the sensor lead won’t change anything, as low resistance to ground would indicate full needle deflection. If you disconnect the leads at the top of the sender then turn on the main power the gauge should fall to zero with no circuit thru the sender... meter not moving usually indicates no power reaching the gauge or bad gauge. Or in the case of needle always at full, bad sender or shorted circuit...

Now if someone wired the sender leads backwards, the gauge would read full until the tank pressure went over 85PSI and the low pressure switch opened, but then the low air buzzer and idiot lights would not work correctly...

Thank you. My question is answered except for where might be the best place or individual to buy 2?

 

[Lugnuts]

So I actually performed the test with gauge out and here is how it went.
Applied 24 volts in correct polarity on the gauge/ Red lead to ignition/ Black lead to ground/ Gauge read 120 psi/
Then applied green lead from sender to ground and gauge read 0 psi.
Gauge is not at fault/ wiring from gauge to ground through sender has the problem.
That was all just for clarification and to report my findings and yes, when you remove those slide on fittings on the back of the gauge it is amazing the corrosion that sits back there.
Thanks Ronmar

 

[Ronmar]

So I actually performed the test with gauge out and here is how it went.
Applied 24 volts in correct polarity on the gauge/ Red lead to ignition/ Black lead to ground/ Gauge read 120 psi/
Then applied green lead from sender to ground and gauge read 0 psi.
Gauge is not at fault/ wiring from gauge to ground through sender has the problem.
That was all just for clarification and to report my findings and yes, when you remove those slide on fittings on the back of the gauge it is amazing the corrosion that sits back there.
Thanks Ronmar

Interesting, that is backwards from how I am used to most electrics working... An open in the circuit or failed sender should give you a low reading so you know something is wrong, not the 120PSI reading you are liking and expecting to see on a properly operating air aystem

 

[Ronmar]

Interesting, that is backwards from how I am used to most electrics working... An open in the circuit or failed sender should give you a low reading so you know something is wrong, not the 120PSI reading you are liking and expecting to see on a properly operating air aystem

I am not a bg fan of these electrics, maybe it is the old school in me liking full sweep mechanical gauges. I would be very inclined to install mechanicals if a gauge breaks here and 85PSI pressure switches to drive the horn and idiot lights... The twitchy sticky nature of these gauges may drive me to do that anyway