M1083A1 Air Gauge Troubleshooting

[Caelan]

I've been beating my head against the wall trying to figure out the gremlins in my A1's air gauges. Once I flip the master power switch, both front and rear air gauges will peg themselves as high as they'll go over 120psi and the low air buzzer starts screaming at me. If I start the engine, pressure will build in the tanks, the buzzer will turn itself off, and I can release the brakes. If I pump the brake pedal, the buzzer will activate like normal. Throughout all of this, the gauges are stuck at 120psi.

If I disconnect TL157 and TL156 from the pressure sensors below the dash and jump them to ground, the gauges work like normal up to 90psi then jump to >120psi again. The buzzer won't activate and the low pressure lights don't come on, but I'm guessing that's because I've disconnected the indicator panel (TL157 and TL156 are a straight shot into PX7 on the panel)

Do any of you have suggestions? I figured it can't possibly hurt to ask for some advice while I'm somewhat blindly jumping connections to find what I'm assuming is a broken wire or a bad switch somewhere.

 

[Suprman]

What year is your truck? If you have the 3126 engine you should be able to follow the first gen tm troubleshooting for that
issue.
Will

 

[Ronmar]

I think you have a bad ground.

The pressure sensor and pressure switch are combined into one unit. You can see this on Foldout 20 of 34 of the A0 schematics and on sheet 1 and 6 on the A1 schematic. Both seem to use the same TL numbers.

TL156 and TL157 are the lines to the pressure lights in the display panel and also activate the buzzer via the panel when the switch in the sensor closes and provides them a path to chassis ground thru the sensor bracket.

TL150 and TL151 are the lines that go to the pressure gauges. They go thru a variable resistor in the pressure sensor to ground thru the sensor case/bracket.

By your grounding of Terminal Lug 156 OR 157, you are providing a path from the gauges to TL150 or 151 thru the variable resistor and thru the pressure sw to 156/157 and thru your wire to ground. This of course only works up to about 90 PSI when the pressure switch opens and you once again loose your path to ground.

This indicates to me that your pressure sensors do not have a valid path to ground thru their cases/brackets. With 150-151 and 156-157 connected to their appropriate wires, try grounding the pressure sensor cases...

 

[Caelan]

Ronmar, if i ever see you face to face, I'm buying you a beer. It never occurred to me that the sensor and the pressure switch would be combined into one unit. That makes the schematic make a lot more sense.

As soon as I put everything back together and jumped the sensor case to ground, everything worked properly. I've covered a lot of the cab in spray on insulation and sound deadening that blocks an electrical connection from the sensor cases. A little bit of sanding, brushing, and chisel work should get me back up and running.

Thank you!