Mountains and Tranny

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We’ve been traveling from NY to the Midwest, with Mesa Verde, CO, as our first stop. While driving west on Route 160, navigating through Colorado’s mountainous terrain, we started a 12-mile descent from 10,800 feet to 6,400 feet. The recommended speed for this descent, given our 33,000-pound vehicle, was 18 mph. I used second gear and the exhaust brake, but toward the end of the descent, I briefly lost the speedometer. About 5 miles later, both the tachometer and speedometer stopped working for a few minutes.

After another 50 miles of climbing a hill at around 45 mph, the speedometer, tachometer, and odometer all went out. The dashboard displayed a “no data available” message, followed by a check engine light when we stopped. I checked the oil, transmission fluid, and antifreeze, all of which were fine, but the speedometer still wasn’t working. We drove another 24 miles to our campsite and contacted Prevost. They instructed me to check the connections on the D Deck and the module on the Detroit engine and perform a reset on the dash computer. Everything seemed okay after that, and the speedometer registered 10 mph when we moved the coach forward.

Prevost suggested it might have been a glitch and may not recur, but this hasn’t completely eased my concerns. We’re planning to visit Zion, Salt Lake City, Yellowstone, and other destinations over the next month. Any advice or insights would be greatly appreciated.

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The DDEC system uses low voltage for communication with sensors and other components, so issues like intermittent problems can often be traced to something as simple as a bad connector, poor ground, or a faulty terminal. These kinds of problems can be particularly tricky because they may only occur sporadically, making them hard to diagnose until they become more serious.

In my previous bus, one weak spot was the 28-pin connector at the transmission. To manage this, I made it a habit to disconnect and reconnect it whenever I was under the bus, effectively “wiping” any corrosion off the pins and receptacles. After addressing this issue the first time, I didn’t experience further problems with that connector.

When cleaning around the engine or any area with plugs and receptacles, it’s crucial to avoid getting them wet and to steer clear of power washing these areas. Environments with salt—such as those near the coast or in rust belts—pose a significant challenge for our supposedly protected connections. In my case, corrosion from salty roads created a conductive path between two pins, leading to numerous erroneous error messages and eventually causing the transmission to lock in gear. Keeping the space between pins and receptacles clean and free of corrosion is essential. I empathize with your situation—it’s a real hassle.

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Jon,

Yesterday, the Prevost tech mentioned that the system operates at 60 ohms and suggested that excessive heat might be causing the issue. They noted that once everything cooled down, the system functioned normally. Does this make sense to you? I had a plastic storage bin in the storage area on the riverside that melted, indicating it was extremely hot.

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I’d defer to his expertise on this. I haven’t heard of heat affecting good metal-to-metal electrical connections unless the connection itself is generating the heat or if heat causes expansion that impacts the quality of the connection.

If he’s right, it could be a significant issue, especially as buses are required to meet stricter emissions standards, which likely increases heat generation.

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I had a similar issue recently. On the same day as your first post, I drove out to Rancho ORA and stayed for a week. When I started the coach to head to Pechanga for a wedding on the 7th, I noticed the speedometer wasn’t working. I figured it might be due to driving too slowly while leaving the ORA park. As I approached the street, the speedometer still wasn’t registering, and I saw a check transmission light. When I stopped, I found the transmission was stuck in first gear and wouldn’t respond to any button presses.

I checked the DDEC reader and found no codes. The transmission oil level looked fine, so I restarted the coach and managed to drive to Pechanga without further issues. Four days later, on the drive from Pechanga to storage (about 45 miles), the same problem occurred: the transmission was locked in first gear when I tried to put it in neutral upon arrival. Restarting the coach allowed me to park it.

This time, I ran a code-read on the transmission pad and found a 25-11 error, which indicates an output speed sensor error with 0 RPM in first gear. I’ve since purchased a new sensor and plan to check the coach again tomorrow. Other potential causes could be loose or shorted wiring to the sensor. I’m hoping it’s just a loose plug, but we’ll see. If this issue happens, it could definitely leave you stranded.

Ray

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Keep us posted Ray, I would like to know where the sensor is and what you found out.

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I found a couple of online manuals for the transmission, which might be helpful. There are actually three speed sensors in total. The one causing me trouble is the “Output Speed Sensor,” located near the shaft at around the 5 o’clock position if you’re looking from the transmission toward the engine. The other two sensors are on the right side of the transmission: the turbine speed sensor in the middle, and the “Input Speed Sensor” closest to the engine, which measures the input speed to the transmission.

I’m planning to get a closer look at everything tomorrow. It’ll be the first time I’m using the jack stands I bought from Jon two years ago!

Ray

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This weekend, using the jack stands I bought from Jon two years ago, I got under the bus and replaced the output speed sensor. The major challenge was that the sensor was located at the very front of the transmission, right at the drive shaft. Access was obstructed by two sheet metal panels, which nearly made me give up. I managed to remove one of the panels, allowing me to reach the sensor with one hand.

I successfully removed the old sensor and installed the new one ($80). I took the bus for a test drive today, and everything seemed to work fine. While I won’t be sure if the issue is fully resolved until it’s tested under the same conditions that previously caused problems, it worked flawlessly through several starts and stops. Fingers crossed that the problem is fixed!

Ray

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As a postscript to this thread, I found it useful to learn how to read the codes from the transmission. Although the check-tranny light came on the dashboard, it didn’t trigger a code in the DDEC. I had to use the transmission keypad to read the codes.

Ray

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Well done Ray, it’s always satisfying to know you have fixed something yourself.

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I’m finally back in a place with Wi-Fi. Since leaving Cortez, CO, we’ve traveled to Zion, UT, then Salt Lake City, and now we’re in Arco, ID. The tachometer and speedometer have intermittently failed, though only briefly. The speedometer has pegged itself twice before returning to 65. There hasn’t been much mountain driving recently.

We’re heading to Yellowstone next, with plans to pass through Jackson Hole and up to Cotter Bay. Prevost has suggested that the issues are likely heat-related and connected to a problem in the engine-transmission area. I’ll keep you updated if we find anything. Once I’m back on the East Coast, I plan to check all the connections for corrosion and apply dielectric grease.

 

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The good news is that everything has been running smoothly. I’ve been cautious about generating excess heat, using the exhaust brake sparingly. I’ve made it to Jackson Hole, the Tetons, Yellowstone, Cody, WY, Sturgis, SD, and now Cedar Point, OH, without any major issues. However, the lack of a persistent problem is also a bit concerning because it means we don’t know if or when it might resurface.

Do you think it would make sense to have the codes read for the transmission and engine now, or should I just wait to see if another issue arises?

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For what it’s worth—While at the Allison shop the tech told me that on my model if you back up it wipes out all of the transmission codes.

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Was he serious?

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That’s an interesting point. How would a transmission shop ever diagnose a problem if the codes are cleared every time you shift into reverse?

John, I’ve heard there’s a fantastic burger place in Nevada. Does anything come to mind? We frequently travel on Hwy 71 since the bus is kept in Peculiar, MO. Yes, that’s really the town’s name. But the best town name has to be Tightwad, MO. There was once a bank branch there that did quite well opening checking accounts.

AL

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