Written by the Motorplex Team
Serving Spokane, Washington with Emergency Mobile Fleet Repairs
The air pressure in a semi-truck is used for braking, supports transmission and clutch operation, and enables safety interlocks. Most Class 8 trucks, like Kenworths, Freightliners and Peterbilts, are designed to operate at an air pressure between 100 – 125 PSI. If that pressure drops, the truck’s safety systems kick in.
That’s exactly what happened at a recent emergency mobile repair call. A driver noticed that his air pressure dropped from 110 to just 70 PSI, and his transmission wouldn’t shift into gear. Unfortunately, this happened on a busy intersection off the I-90 in Spokane Valley, Washington.
Why the Truck Wouldn’t Go into Gear
When air pressure falls that low, several things can happen at once. For example, the air system can’t fully release the brakes. The clutch might not engage, and the transmission interlocks activate to prevent movement. You might also see dash warning lights and alarms. These are all safety precautions.
Modern transmissions are designed not to allow gear engagement when the air pressure is unsafe. It prevents the driver from moving a heavy vehicle, like a semi-truck and trailer, that might not be able to stop. If it didn’t do this, the driver could experience brake failure, loss of vehicle control and a bad accident. The truck is essentially immobile, so emergency mobile fleet repairs or towing are required.
Fast Emergency Mobile Fleet Repairs in Spokane
The driver called the FleetEasy dispatch asking for emergency mobile repair. He was stopped on the westbound exit of the I-90 onto Sullivan Road, blocking the right-hand turning lane. Our mobile technician arrived on the scene within 20 minutes and immediately checked the air compressor. It wasn’t building air.
At the same time, the Motorplex team contacted authorities to help with traffic control. The incident was blocking traffic, and if the truck needed towing, we’d need more room to do so.
Air Compressor Not the Culprit
There are a few issues that can cause air pressure to drop like that, with the most common being an issue with the air compressor. Other common causes include:
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- A major air leak in the system (lines, valves, fittings or discharge hose)
- Frozen moisture in cold conditions
- A valve stuck open or failing
- A failing air governor
To check whether the air compressor was failing, our tech removed the discharge line. After further investigation, we discovered that the purge valve had frozen over and was leaking air. Our tech was able to seal it enough to build air pressure and move the tractor off the road. They followed the driver to the unloading destination to do the repair.
Once we had the parts, we replaced the purge valve and air dryer cartridge. The AD cartridge is part of the air brake system that helps remove water vapor and contaminants from the compressed air before it reaches the tanks and valves. It helps prevent frozen air lines in the winter. Regular maintenance of these components can help you avoid needing emergency mobile repairs and keep your trucks moving.
Once all the new parts were in, everything worked perfectly after the repair, and we cleared the call.
Partner with Motorplex in Spokane
Motorplex by FleetEasy offers on-call fleet solutions in Spokane and the surrounding areas. We offer fleet repairs in our fleet services shop, on your site and wherever you might break down like we did in this incident. When you partner with us, you know you’ll have access to 24-hour emergency mobile repairs and towing, and a team of highly trained technicians who genuinely care.