18 April, 2009
Buses and claustrophobia
I had to go on a service call Thursday evening to repair a bus that was leaning severely to the right. No mechanic in his right mind likes working on buses. Just look at them. They're so low to the ground that a person can barely fit under it, if at all. Throw in an air suspension problem and the bus is even closer to the ground. I was the only mechanic available in the shop at that time and that meant I got stuck with this service call. We spoke with the owner of the coach and he told us what needed to be done. Apparently they've had this same problem with most of their coaches, so this guy new the fix for it. There's a leveling valve for each side of the bus which, in a primitive sort of way, gives the bus something like independent suspension. Most trucks have only one leveling valve to control all air bags (or air springs if you prefer) and it's adequate for the job. A bus on the other hand, is all about ride quality.
A leveling valve, properly called a "ride height control valve" or RHCV, is a very simple thing. The valve has (mounted to the vehicle's chassis), at the minimum, three ports. One port delivers an air supply to the valve, a second port will supply air to the bags, and the third port is an exhaust port. Sticking out of the valve will also be a shaft to which a lever is attached. Moving this lever moves the internal parts of the valve and either lets air flow to the bags, or exhausts air from the bags. There's also a neutral position that neither supplies air nor exhausts air. So, we have a lever with one end connected to the valve, but what about the other end? The other end is connected to a linkage rod. This linkage rod is connected to whatever axle the leveling valve controls ride height for. So how does it all come together in operation? It's very, very simple. Air bags are filled with, no surprise, air. Air can be compressed and when a load on a vehicle is increased, the air inside the bags gets compressed. As the air in the bags compresses, the chassis of the vehicle starts to lower towards the ground. Now since an axle (axles aren't considered a "sprung" component) will remain stationary and the linkage rod won't flex, as the chassis lowers the linkage rod causes the valve's lever to move, in turn the lever operates the valve. As the valve shaft turns it uncovers (in this case) the supply port allowing air to flow into the bags. Not surprising at all, as air flows to the bags, the air pressure inside the bags increases. As the pressure increases, the bags will raise the vehicle back up to a pre-determined chassis ride height (set with the linkage rod.) Are you confused a little bit? If you are, it's okay. To understand the principles of how a leveling valve works, just go to your bathroom. Seriously. Pull the lid off of the toilet's tank and look inside. Think of the valve inside the tank as a leveling valve and think of the float as the linkage rod. The water in the tank is like the air supply to the leveling valve and think of the toilet bowl as the air bag. Now, flush the toilet. Notice that the float drops as water in the tank drains away. When the float drops, water flows through the valve into the tank. When the tank reaches the full mark, the float shuts off the valve, stopping water flow into the tank. That's how a leveling valve works :) Got it? Good.
Anyway, this bus I was out fixing had a common problem. The owner told me to disconnect the air lines from the valve and look for debris blocking the ports. This would be a simple job if it were a truck, but it was a bus with all of the bags on the right side deflated. It looked like the leaning tower of Pisa, believe me. Guess where the leveling valves were? In between the drive axle and tag axle where there was a lot of room to work? Hell no! They were in front of the drive axle, above some really big air tanks. I thought I'd crawl in between the drive axle and tag axle (the tag is to the rear of the drive BTW) wiggle through some torque rods and be able to get at the offending valve. Nope, didn't happen. I could see the valve just fine, but I couldn't reach it. Hmmm, now what? I called the owner again and asked him how I could get at this valve. His suggestion was obvious, but I hadn't thought of it. Buses, usually, can be raised and lowered manually if the need arises. Sometimes a bus needs some extra ground clearance to clear, oh say, a steep driveway entrance. The driver can operate a valve which bypasses the leveling valve and inflate the bags to maximum capacity. It's not really safe to go down the road like that, but it's okay for low speed maneuvering. Back to the story. The driver raises the bus as high as it will go and as I looked in from the top of the right drive tires, I could see the leveling valve I needed to fix. There was one small problem though. With the bus raised I could fit between the top of the drive tires and the bottom of the bus's body, but disconnecting the leveling valve lines would cause the bags to deflate. If the bags deflated, the bus would lower. Crushing me like a rotten tomato. So, I had to find a way to do this job without ending up dead. I found that I could stack some blocks between the drive axle and the chassis. After doing so, I had to driver lower the bus. Of course with the blocks in place, the bus didn't go down, but the air bags deflated and I still had the clearance I needed to crawl in. Now I could get a wrench on the air line fittings. I couldn't, however, get my hand on the air line fittings. Not a big deal when removing the air lines, but I had to be able to get my hand on them to reconnect the lines. Twenty bolts later I had the fender flare removed and could put my hand on the leveling valve. After disconnecting the lines I used a blow gun to blast the leveling valve's ports and, sure enough, some big chunks of debris came shooting out. Not sure what it was, but it was white so I'm thinking it was pipe thread sealant. I reconnected the lines to the valve and had the driver start the engine to build air. After we had enough air pressure, the driver raised the bus and I removed the blocks I had put in. With the blocks removed, the driver lowered the bus and put it into "travel" mode. Woohoo!!! All of the bags inflated as they were supposed to. I bolted the fender flare back on and had the driver drive around the parking lot to make sure the leveling valve was still functioning. It did and that made me very happy. The service call ended on a high note because the driver paid in good 'ol cash! No comm check systems to deal with, no credit card hassles... cash is always nice. After a handshake and a "Have a safe trip" I was back on the road, heading for the shop.
I started to shake a little bit on the drive back because my fear started coming to the front of my mind. I have mild claustrophobia and being under a bus like I was scares me. Mix in the possibility of being crushed and things border on sheer terror for me. But the job isn't going to take care of itself. Trucks don't bother me because they're an "open air" environment you might say. Not buses. When I face situations like this I would rather be doing just about anything else, but somehow I manage to put my fear aside and scrape up enough courage to get the job done. I'll just have to deal with the fear later. Maybe one of these days I'll have a nice office job...