Low Air Pressure Alarm Questions

[Barney-the-Duece]

I have an M925A2 and plan to use it for my CDL test. When building air pressure, the low air pressure warning light goes out at approximately 65 to 70 psi. When applying the brakes, to check the low air pressure warning light, the light turns on at about 53 to 55 psi on the primary pressure gauge (the secondary is showing approximately 65 psi). As the low air pressure warning switches are supposed to be set at 60 psi, I assume the light should be going on at 60 psi as well. Illinois also requires the low pressure warning to turn on at 60 psi (per CDL guides for the air test).

I first swapped the primary and secondary low air pressure switches to check them, and there was no change. I have replaced the primary low air pressure switch with a new one, and still no change. I have also replaced both air presure gauges (they were pretty scratched up anyway and hard to read) and still no change.

I have checked the TM's but they only refer to problems of the light/buzzer not turning off. Also searched the site for similar issues, but came up blank.

Is this actualy a problem? Are these pressures normal? Are there any adjustments or is there a way to increase the point where the low presure warning turns on? The various warning indicators seem to be working, other than when the low air pressure light turns on, however, could it be a problem with the failsafe alarm module?

 

[MtnSnow]

I would suggest you take a civi commercial truck to do your CDL test in as the examiners won't know how to deal with the military trucks 99% of the time. And the pressures you are seeing are quite normal as the gauges are not calibrated but your examiner will fail you for having a truck that isn't in "proper" working order.

 

[Barney-the-Duece]

I do not know anyone with a truck to use for the test and would prefer not to have to rent one for several hundred dollars for the test. I also assume I will still need to get this resolved to pass the semi-annual inspections.

 

[rosco]

In a DOT inspection, it is essential that the the warning buzzer & light (if it has a light), comes on at not less then 60 PSI, when there is a drop in pressure. MtnSnow has it exactly right. Taking a CDL test in your truck (in a truck that has to pass a test too), hopefully will be different then a semi-annual truck inspection. Usually the civi parts like the air pressure switches are right on the 60 psi mark. Hook the light & the buzzer to the same switch so they come on together. Most often MV's are "overloads" for DOT Inspectors, especially the older ones like the M35s with air over hydraulics.