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Old 10-30-2012, 11:13 AM   #1
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Strange vibration

While driving through the mountains I noticed a strange vibration when down shifting to climb the hills. The vibration seemed to show up at about 1600 RPM and got stronger as the engine gained RPM and seemed to peak at about 2100 RPM. It did not always occur when repeating these conditions. When it did occur, it didn't matter what gear it was in...and therefore speed was not a part of the equation. It happened under load and when going down hill. Therefore I eliminated the cause to be tires/wheels, driveshaft, transmission, differential, and throttle position. I also checked the engine belts to see if they were vibrating at the indicated RPMs...didn't appear to be. I couldn't find any loose motor mounts either. Since it only does it periodically I cannot determine what it might be. I am convinced that it is tied to engine RPMs though...was able to test it several times to confirm this. Any ideas? Thanks.
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Old 10-30-2012, 01:33 PM   #2
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Have the U-joints and drive shaft checked. Make certain they are well lubed and tight.
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Old 10-30-2012, 03:21 PM   #3
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Engine speed?

By your belt check description, it looks like you may have determined the problem to be at a certain engine RPM. Maybe a few more engine checks are in order.

Always troubleshoot the inexpensive replacement parts first.

Remove the key from the ignition and put it in your pocket also capture the second set of keys; also disconnect the chassis (motor) battery at the switch in the side compartment before poking around the engine compartment. Yes, I over do it when it comes to safety!

You looked at the belts while the engine was running. Have you checked them when the engine was stopped? It's a dirty job but take a look, feel the tension, try to slide the belts and bend the belts sideways and look for cracks on the inside of the belts. They belts shouldn't slide, if they do they aren't tight enough or are glazed and need replacement, if the belts are cracked on the inside they need replacement.

Next possibility is the radiator fan. Your Alpine is similar to mine in that the engine compartment is open to the road on the bottom. Some debris may have flicked up and stuck to one of the fan blades. Possibly a plastic bag? Less likely to happen but possible--have you been driving on wet and/or muddy roads where some road debris became airborne and sucked into the fan blades? Check the fan blades when looking at the belts, turn the fan by hand--even more dirty than checking the belts and harder too. Look for nicks in the blades in addition to debris stuck on them. Also check the fan motor bearings by trying to rock the blades towards and away from the radiator--any looseness here and the bearings are suspect.
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Old 10-30-2012, 06:51 PM   #4
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Quote:
Originally Posted by Snuggery View Post
Have the U-joints and drive shaft checked. Make certain they are well lubed and tight.
I had read where another was having a tough time of locating a vibration issue.Turned out to be his drive shaft was out of balance.
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Old 10-31-2012, 11:47 AM   #5
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Thanks for the comments. I did not check the driveshaft or u-joints since the intermittent vibration is absolutely a function of engine RPM rather than vehicle speed. It is for this reason also that the driveshaft balance seems (to my thinking) to be an unlikely culprit...the problem is intermittent and RPM related. However, before the trip I lubed the u-joints and did a physical check. Something I didn't inspect though was the fan. Is it tied to engine RPM? I am not familiar with how they work. I thought they basically operated at a high speed or low speed depending on engine temp. Wouldn't a nick in the fan cause the vibration all the time if it were spinning at the proper rate?

When accellerating through each gear, when RPM reached about 1600 I would notice the vibration...sometimes. Also when "coasting" in a gear that caused RPM to reach the same level, vibration would occur...sometimes. As I said, it was intermittent. It would happen in every gear that caused RPM to reach 1600+, load or no load.
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Old 10-31-2012, 07:52 PM   #6
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Thanks for the comments.
. .
Something I didn't inspect though was the fan. Is it tied to engine RPM? I am not familiar with how they work. I thought they basically operated at a high speed or low speed depending on engine temp. Wouldn't a nick in the fan cause the vibration all the time if it were spinning at the proper rate?
. . .
You are correct about the fan not being connected to the engine and having its own power system. Sorry, I was thinking about a standard car engine configuration as I was typing.

So that path of problem solving might be out. But, still check the looseness of the fan rotating assembly. Also check all the fasteners on the radiator and all on all the accessories in the engine compartment. The noise you are hearing could be a sympathetic vibration picked up by a loose accessory.

If you don't know if or when the belts have ever been replaced. . .
I know it's a shot in the dark but I wouldn't pass on the engine accessory belts just yet. I've had several problems in my old car hobby from good appearing fan belts. The most problem I had was a combination of a good appearing belt (but needed replacing because it sat on the shelf too long and had a kink that wouldn't roll out) and with a really new with improper tension on a Model A engine and similar tension problems on Flathead Ford V8s.

It's your time and your money but, if you are handy with tools yourself I'd just replace them. If you have to have a shop do it, it'll be several hundred bucks. Then you might just wait until you have the Coach serviced and add it to the list.
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Old 11-01-2012, 05:38 PM   #7
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I would look to see if the engine,trans or the exhaust is rubbing up against the frame or body someplace under a load.
If it doesn't do it in neutral and its RPM related.
That's my best guess.
Good luck.
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Old 11-01-2012, 07:22 PM   #8
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Steve, take a look at this Cummins ISC belt thread over in the Monaco forum. With our Alpines we have access to the belts through the rear swing up cover.

You say sometimes the vibration is there sometimes and sometimes it's not. The air conditioning cycles on and off--the 'V' belt on the Air Conditioning just became the prime suspect.

The Air Conditioning belt on mine is pretty tight. I couldn't push it in more than a half inch. Loosen the two bolts on the right side of the compressor to set the tension. Loosen them up and pry up on the compressor with a wooden pry bar AKA short broom handle. While forcing the compressor up with a moderate pressure, tighten the bolts. Don't try to intentionally allow slack in the belt, after tightening it you will find you can push the belt in about a half inch. If you can push it in more than a half inch, try it again with more than "moderate" pressure.
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Old 11-02-2012, 01:37 PM   #9
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Originally Posted by SteveS View Post
While driving through the mountains I noticed a strange vibration when down shifting to climb the hills. The vibration seemed to show up at about 1600 RPM and got stronger as the engine gained RPM and seemed to peak at about 2100 RPM. It did not always occur when repeating these conditions. When it did occur, it didn't matter what gear it was in...and therefore speed was not a part of the equation. It happened under load and when going down hill. Therefore I eliminated the cause to be tires/wheels, driveshaft, transmission, differential, and throttle position. I also checked the engine belts to see if they were vibrating at the indicated RPMs...didn't appear to be. I couldn't find any loose motor mounts either. Since it only does it periodically I cannot determine what it might be. I am convinced that it is tied to engine RPMs though...was able to test it several times to confirm this. Any ideas? Thanks.
Hi Steve, Vibrations can drive anyone wild. You seem convinced that it cannot be in the drive line. You may very well be right. I am a retired cross country trucker and have had trucks give me several similar problems. Sounds like you have already got a lot of advise. I had never heard of belts causing this problem, but it does make sense. I would be checking the harmonic balancer, it is also called a vibration damper. It is located on the front of the crankshaft, behind the belts. I must admit that I do not know everything there is to know in reference to proper diagnosis. I do know that if you can wiggle it and feel any movement, there is a problem. Beyond that I would be talking to Cummins. I have never seen one of these give trouble under a million miles, the only one I ever replaced had 3 million miles on it. But all things are possible on these extreme high mintance motor homes. I have also had universal joints act like this. I have also found universals that you could grab ahold of and even use a small pry bar and not feel any slack, but would still vibrate. After disassembly I found it had ate up a couple of the needle bearings in the end caps. A word of caution; if you do remove the U joints, mark the joint with a marker tape or somehing. It is important to put it back in the right position, or it becomes out of time and will vibrate even with a new joint. Also check the slip yoke on the driveshaft, the drive shaft must be able to expand and contract, occasionaly it will be worn and loose. This will cause vibration. Be sure the balance weight (welded on the driveshaft) has not come off. Good wishes, hope you find your problem. old trucker.
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Old 11-02-2012, 04:31 PM   #10
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v8Dave - My first thoughts were motor mounts or belts. However, I replaced the belts last year so don't believe it is associated with old cracked belts. The air conditioning belt was somethng I also wondered about since the vibration was so intermittent. The tension seemed pretty tight but I will look at it again...I believe I get about half inch deflection when pushing on it with about 15 pounds force. That would be about 1 inch when pushing and pulling on it...is that too much deflection? I will also look at the flat belt tread to see if there seems to be an unusual wear pattern.

gatorv - I was able to get it to vibrate once while in neutral. Unfortunately, when getting SWMBO to help by accellerating to 1800 RPM while I inspected the engine, it wouldn't vibrate. Perhaps the solution is to always let her drive.

Oldtrucker - Good thought about the vibration damper. Not sure why it would be so intermittent but will check it. I still don't understand how u-joints or drive shaft issues would cause the problem since it is strickly linked to RPM. The drive shaft and shaft components are a connected to vehicle speed, right. In other words, say I'm in second gear with engine turning 1800. The drive shaft is turning at a much slower rate than it would be when in 6th gear and engine RPM at 1800. The vibration seems that it can occur at any speed if engine RPM is greater than 1600.

Thanks again. I have a couple more things to check now.
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Old 11-02-2012, 04:35 PM   #11
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If you have air bags check the ride height and for air leaks.
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Old 11-02-2012, 04:51 PM   #12
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Check your torque converter/flex plate bolts.
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Old 11-02-2012, 05:41 PM   #13
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Regarding your harmonic balancer.
I would draw a straight line with whiteout acrose the front of it.
If the rubber slips then you will be able to see it.
The engine could have an intermittent miss causing it to vibrate.
Possibly a dirty injector or fuel filter.
Just a thought.
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Old 11-02-2012, 06:44 PM   #14
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Quote:
Originally Posted by SteveS View Post
...
The air conditioning belt was something I also wondered about since the vibration was so intermittent. The tension seemed pretty tight but I will look at it again...I believe I get about half inch deflection when pushing on it with about 15 pounds force. That would be about 1 inch when pushing and pulling on it...is that too much deflection?
...
The AC belt deflection you describe should be good, it's hard to get much less than that.

Next time you get the vibration reach over and turn off the AC just to be sure.

I am not sure what the result of any of the answers to the next series of questions may point to, but it may help others here on the site. It will also give you something comprehensive to hand over to your mechanic should it come to that.

-Any thing else you have changed just prior to this starting?
-Have you checked the air pressure in your tires?
-Do you feel anything in the the steering wheel when this happens?
-Do you just hear this or can you also feel it, if so do you feel it in your seat and/or on the floor?
-Does it occur when in Cruise control and/or when not using cruise control?
-Are you using the "economy" mode of the Allison transmission?
-What is the engine temperature doing? I ask this because we have the "wax" thermostat on our older Cummins and the down shift bumps up the speed of the hydraulic pump and maybe the speed of the radiator fan. If the temperature is staying set then maybe this isn't the problem.

I am new to my Alpine so I am not sure about this. A previous poster asked about Air Suspension. Is the vibration possibly related to the air compressor cycling on and off more frequently? Which could be the case if you have the original Air Suspension Bags. According to EngineerMike we should be replacing these because they are getting old, cracked, and leaky (which by-the-way describes me too).
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