With today’s newer buses being almost fully computer controlled in a multiplex setup, it’s hard to determine if a fault is severe or not while out on the road. Sensors monitor everything, and each subsystem (gearbox, body, compressor, brakes, electrical etc) has its own ECU all linked to the main engine ECU (Hence the Multiplex). But, even the sensors get it wrong, sending incorrect triggers to the ECU. Dirt, water, moisture, it all causes incorrect readings and triggers.
On the Volvo B7RLE and B9/B7TL it’s normally the brake ECU that never seems to stop notifying you on the in-cab diagnostic HUD to check its diagnostics at the next stop. But as a driver, you don’t have the training or equipment, and the message is generic, so you don’t know if it’s a minor thing or severe until the technician (that’s guys like me!) comes along and hooks his laptop up to the heart of the bus and coaxes ECU error codes out of it. To the driver it could be anything. The brake discs? Pads? Air system? Valves? The sensors even monitor brake pad and disc temperature! It might also just be a dirty sensor as in my case!
I devised a test while on the racetrack, out roadtesting a gearbox repair on Rikky’s ’08 B7RLE, I’d repaired the shifter rack, solenoids, and renewed the clutch because gearchanges had become erratic. The brake message started coming up every time I braked, and I’d changed the discs & pads not so long before, meaning it wasn’t the pad and disc wear error. So, I accelerated to the end of the green rev zone in 3rd gear, and just before the gearbox changed, pressed the brake slightly, and held it, still on the gas, hard. The gearbox ECU then holds off changing while the brakes are on, even slightly. I pressed the brakes harder, to heat the discs, pads and sensors up, while still at max revs for 15 seconds, and the warning came up every few seconds even with the brake released, because the system was nice and hot, the overheat code was running in the ECU log (I later checked with the laptop hooked to the ECU and it had logged the overheat code 23 times!). It cooled and stopped beeping constantly, and whaddyou know, the error went away in the HUD.
I’d forgot to clean the pad sensor, and it was dirty. This test also checks the high RPM gearchange, because if you’ve held the brakes at high revs with the gas to the floor and the bus still changes gear (upshift), the brake sensor has failed.
WARNING: This test MUST NOT be done by ANYONE other than an experienced bus engineer, and ONLY on a non-public road, such as a racetrack. The wear on the BRAKES and ENGINE during this test is IMMENSE, they will get hotter than normal use, and the brakes will need REPLACING following the test for safety, as imminent failure of the discs and pads may result otherwise, depending on their condition beforehand. I am an experienced and responsible mechanic, and CANNOT be held personally responsible for third parties trying out my tests.
YOU MUST check all brake components for damage after the test, especially the air supply system hoses for the above reason, BEFORE taking the bus back on public roads. Taking a PSV on a public road with possibly damaged brakes is careless, and puts you and other road users at serious risk, due to vehicle size and weight. Have the bus towed back to a garage with an inspection pit, as a bus has too low an overhang to do it anywhere else.