Coach moving at idle despite parking brake (split from Re: Handling Comparison:36' U270/42' U320 )

Topic: Coach moving at idle despite parking brake (split from Re: Handling Comparison:36' U270/42' U320 ) (Read 1280 times) previous topic - next topic

Coach moving at idle despite parking brake (split from Re: Handling Comparison:36' U270/42' U320 )

November 07, 2014, 12:29:15 am

Quote from: Gayle McNeece – November 06, 2014, 10:11:51 pm

I'm the one that usually starts the coach. And you're right. With the '96, you turned the key to the first click position, waited for the "Wait to Start" light to go out, turned the key the rest of the way, and the coach roared to life.

The '03, there is no "Wait to Start", you just start it but it takes a long time as it checks to make sure it thinks everything is ok and then the start seems a lot harder.

But, once the '03 is started, you better be ready because it wants to GO! I find that sort of disconcerting. The '96 seemed much more "patient".

Sounds like the idle speed is too fast.

Re: Coach moving at idle despite parking brake (split from Re: Handling Comparison:36' U270/42' U320 )

Reply #1 – November 07, 2014, 01:07:39 am

Tom,
The point of the observation is that the idle torque difference between the small block ISC or 8.3 and the big block ISM is something for the uninitiated to be aware of. If you are not alert, you will be creeping and doing modifications to your bodywork with the parking brake full on!
My idle is at 700 RPM (spec is 600 to 800) and it over rides the parking brake. When you slow for a stop light, the "push" is remarkably different also (stronger than that from a small block), and don't become lackadaisical about your service brake pressure while waiting for the light to change or the idle torque will collect you a Smartcar front cap ornament!

Re: Coach moving at idle despite parking brake (split from Re: Handling Comparison:36' U270/42' U320 )

Reply #2 – November 07, 2014, 02:42:30 am

Quote from: Neal Pillsbury – November 07, 2014, 01:07:39 am

Tom,
The point of the observation is that the idle torque difference between the small block ISC or 8.3 and the big block ISM is something for the uninitiated to be aware of. If you are not alert, you will be creeping and doing modifications to your bodywork with the parking brake full on!
My idle is at 700 RPM (spec is 600 to 800) and it over rides the parking brake. When you slow for a stop light, the "push" is remarkably different also (stronger than that from a small block), and don't become lackadaisical about your service brake pressure while waiting for the light to change or the idle torque will collect you a Smartcar front cap ornament!

Re: Coach moving at idle despite parking brake (split from Re: Handling Comparison:36' U270/42' U320 )

Reply #3 – November 07, 2014, 07:27:02 am

mine was at 600 and cummins said it should be 700 so that is where i set it. It does press a bit but the park brake will hold still. It is a torque machine, that is for sure.

Re: Coach moving at idle despite parking brake (split from Re: Handling Comparison:36' U270/42' U320 )

Reply #4 – November 07, 2014, 09:10:12 am

Quote from: Neal Pillsbury – November 07, 2014, 01:07:39 am

If you are not alert, you will be creeping and doing modifications to your bodywork with the parking brake full on!
My idle is at 700 RPM (spec is 600 to 800) and it over rides the parking brake.

OK Neal, I'm confused by this part of the discussion (happens a lot). It reads like you're saying you've got the coach in gear, not neutral, with the parking brake on. Am I misinterpreting what you're saying? Figured I'd ask because if I'm confused maybe someone else is.

Re: Coach moving at idle despite parking brake (split from Re: Handling Comparison:36' U270/42' U320 )

Reply #5 – November 07, 2014, 10:44:42 am

Quote from: John S – November 07, 2014, 07:27:02 am

mine was at 600 and cummins said it should be 700 so that is where i set it. It does press a bit but the park brake will hold still. It is a torque machine, that is for sure.

Both my tech gurus seperately turned my idle down to 550 versus overrun the brakes at idle. Makes something in the dash rattle once in a while although unless I add Lucas to the tank which smoothes the idle quite a bit

Re: Coach moving at idle despite parking brake (split from Re: Handling Comparison:36' U270/42' U320 )

Reply #6 – November 07, 2014, 11:41:02 am

Quote from: Michelle – November 07, 2014, 09:10:12 am

............It reads like you're saying you've got the coach in gear, not neutral, with the parking brake on.........

You are not confused, Michelle.
We are talking about differences between the two coaches. I think that in the spirit of making a comparison, the difference in idle torque between the small block and the large block Cummins requires a paradigm shift in thinking for most owners of 8.3 L Cummins:
Although poor SOP (Standard Operating Procedure), there is generally "No Harm, No Foul" if an 8.3L operator forgets to select neutral while momentarily engaged in some other activity, as long as you are keeping your maintenance up and the parking brake is set.
With the ISM-450, it is imperative that you not rely upon the parking brake to hold the coach in place while the coach is in gear.
Not saying this happened, but consider the scenario where on a test drive of an ISM-450, two experienced 8.3 owners pull over in a safe location to exchange drivers. Pull in, stop, shift to neutral, set the parking brake, unbelt and get up to change drivers.......What the? The coach is creeping ahead! This is the first time this has happened in over 150,000 miles of FT travel!
Focused better, it becomes apparent that the "drive to neutral" shift wasn't paid enough attention to and did not occur. The coach is still in drive!
PARADIGM SHIFT! It is not safe to rely upon the parking brake in an ISM-450, when the coach is running and in gear, at least until you do everything that Dave has suggested. If you do, there may be harm and there will be a foul!
I too thought that the idle was perhaps too high. FOT says the RPM is set properly and the brakes are adjusted properly.
Neal

Re: Coach moving at idle despite parking brake (split from Re: Handling Comparison:36' U270/42' U320 )

Reply #7 – November 07, 2014, 12:44:40 pm

Quote from: Neal Pillsbury – November 07, 2014, 11:41:02 am

You are not confused, Michelle.

Not saying this happened, but consider the scenario where on a test drive of an ISM-450, two experienced 8.3 owners pull over in a safe location to exchange drivers. Pull in, stop, shift to neutral, set the parking brake, unbelt and get up to change drivers.......What the? The coach is creeping ahead!

Ah - what you were commenting on makes sense now that I understand the hypothetical situation. At least it's not like someone driving away while still plugged in at a friends' house

Quote from: Dave M – November 07, 2014, 12:10:10 pm

maybe it would be a good time for a little experience, since my background is more with the MCI bus, can mention that with the S Cam brakes, when a driver starts engine, dials D and step on throttle without releasing the parking brake

That's apples and kumquats, Dave

. Different brakes, and we're not talking about actually stepping on the throttle. But good to know if one were to drive a machine with S Cam brakes (I'll have to go Google those now to see what they are).

Re: Coach moving at idle despite parking brake (split from Re: Handling Comparison:36' U270/42' U320 )

Reply #8 – November 07, 2014, 01:05:51 pm

https://www.youtube.com/watch?v=3mrUMTP4thI

Re: Coach moving at idle despite parking brake (split from Re: Handling Comparison:36' U270/42' U320 )

Reply #9 – November 07, 2014, 01:10:08 pm

In a nut shell Michelle (drumb Brakes)

Re: Coach moving at idle despite parking brake (split from Re: Handling Comparison:36' U270/42' U320 )

Reply #10 – November 07, 2014, 01:36:43 pm

I have a CDL in NJ with an air brake endorsement, and the among the many tests you are "supposed" to do on air brakes for you pre trip inspection is the parking brake test. This is where the parking brake is applied and the truck put into gear. and given some throttle. The truck is not supposed to creep, and creeping would indicate that the parking brake needs attention. If you cannot put your coach in drive with parking brake applied and have it stay still, I would think that is a problem, and although not earth shattering, is not the way your parking brake is supposed to work. I have more experience in trucks than with buses or motorhomes, but that is the way CDL drivers are trained, at least in New Jersey. BTW, since you are not driving commercial vehicles, you are not required to be trained on air brakes, but there are also other tests that you should know how to do.

https://www.youtube.com/watch?v=E-CFJDwwRyw

Re: Coach moving at idle despite parking brake (split from Re: Handling Comparison:36' U270/42' U320

Reply #11 – November 07, 2014, 04:08:23 pm

Mine does not move forward but it did and I had to back the idle off to 700 to prevent it. The park brake is adjusted properly too. It is just not as strong as it needs to be for the big coach. On the level it is fine but you really have to watch it if you are heading down hill and leave it in gear. I do not think it will move but a bit of throttle and it will.

Re: Coach moving at idle despite parking brake (split from Re: Handling Comparison:36' U270/42' U320

Reply #12 – November 07, 2014, 04:26:11 pm

Even with the parking brake adjusted, big torque at a slightly fast idle speed will move the coach if headed downhill. Normal idle will still move the coach if pointed down grade. Meritor disk brakes work much better if warm than dead cold. Use chocks like emergency services, public utilities.

Some coaches may have less vibration with a higher idle speed but will pay for it as the parking brake will be less effective.

Pierce

Re: Coach moving at idle despite parking brake (split from Re: Handling Comparison:36' U270/42' U320

Reply #13 – November 07, 2014, 08:29:15 pm

Turn down the idle speed. If parked more than a short stop in nuetral use the low rpm fast idle.

Cummins might be recommending the 700 rpm for oil pressure or smog issues or cooling fan speeds?

Re: Coach moving at idle despite parking brake (split from Re: Handling Comparison:36' U270/42' U320

Reply #14 – November 07, 2014, 09:34:30 pm

When we first got our coach it hadn't been used much in the previous couple of years, and I noticed that sometimes it didn't seem to want to stop completely while in gear. Shifting to neutral resulted in a pretty quick stop with the same brake pressure. Now that the coach has been driven more I don't notice that anymore. Maybe I'm just more used to how much pressure is needed, or maybe driving the coach has helped the brakes. We were told that the rear brakes were new when we got the coach, so maybe it is just that they are now broken in.

No matter what, I would be concerned about any coach that didn't want to stop properly.

Re: Coach moving at idle despite parking brake (split from Re: Handling Comparison:36' U270/42' U320

Reply #15 – November 07, 2014, 09:42:43 pm

Quote from: kb0zke – November 07, 2014, 09:34:30 pm

When we first got our coach it hadn't been used much in the previous couple of years, and I noticed that sometimes it didn't seem to want to stop completely while in gear. Shifting to neutral resulted in a pretty quick stop with the same brake pressure. Now that the coach has been driven more I don't notice that anymore. Maybe I'm just more used to how much pressure is needed, or maybe driving the coach has helped the brakes. We were told that the rear brakes were new when we got the coach, so maybe it is just that they are now broken in.

No matter what, I would be concerned about any coach that didn't want to stop properly.

Use of the retarder allows some glazing of the pads. But turning off the retarder requires a fair amount of concentration of whats going on around you. Regular heating of the brakes results in stronger stopping in my experience.

Glazed brakes symptom can be the parking brake issue mentioned.

Lower idle and deglazed brake seems to work well.

Re: Coach moving at idle despite parking brake (split from Re: Handling Comparison:36' U270/42' U320

Reply #16 – November 07, 2014, 09:45:26 pm

Torque has nothing to do with it at idle. A shaft spinning at 700rpm is a shaft spinning at 700rpm. It might be the torque converter on the HD4060R transmits more torque through at the same RPM. The 4060 has a slightly lower 1st gear ratio (according to specs), but the u360 has a higher ratio rear end--so this should be offset.

Re: Coach moving at idle despite parking brake (split from Re: Handling Comparison:36' U270/42' U320

Reply #17 – November 07, 2014, 10:56:02 pm

With a 2 stroke, we can idle as smooth as glass at 600 RPM but with cold brakes, I have to shift into neutral going down our driveway or I can't stop with the service brakes . When warmed up, I can lock up all six on a flat road.

Bigger cans on the rear might be OK at very low speeds as the engine is fighting the rear brakes but at any other time, the engine/transmission/retarder/Jake are helping the rear cans/brakes slow the coach. Bigger cans on the rear can mean rear lockup in ice or snow or even in a panic stop. If the rear comes loose going downhill in the snow, no one is going to catch it except another vehicle or guard rail.

Pierce

Re: Coach moving at idle despite parking brake (split from Re: Handling Comparison:36' U270/42' U320

Reply #18 – November 07, 2014, 10:57:09 pm

Here's a mostly off topic question for the FT brain trust.

Why is it, that my 1987 Mercedes 300D acts like this: No creep at idle when I take my foot off the brake, and no backwards slip when aimed uphill unless the hill is really steep. As long as it has traction, it will go up just about any hill with only modest application of throttle. The engine is diesel, so I know torque is higher than similar gas cars. None of my gas automatics have ever behaved like this, no matter how powerful the engine.

Re: Coach moving at idle despite parking brake (split from Re: Handling Comparison:36' U270/42' U320

Reply #19 – November 07, 2014, 11:07:52 pm

It's called "stall speed." Here is Bank's page that describes it better than I can. Banks Power | Understanding Stall Speed

Tom, you are lucky. Mercedes are about the only automatic transmissions that you can pull start or coast down a hill and do a 30 mph "bump start". All diesel and gasoline.

Pierce

Re: Coach moving at idle despite parking brake (split from Re: Handling Comparison:36' U270/42' U320

Reply #20 – November 07, 2014, 11:48:18 pm

Quote from: Pierce & Gaylie Stewart – November 07, 2014, 11:07:52 pm

It's called "stall speed." Here is Bank's page that describes it better than I can. Banks Power | Understanding Stall Speed

Cool. So, if I always parked the Benz on the top of a hill, it could be fully drivable with the battery and every electrical component and wire removed. As long as I can keep the tank filled.

Tom, you are lucky. Mercedes are about the only automatic transmissions that you can pull start or coast down a hill and do a 30 mph "bump start". All diesel and gasoline.

Pierce

Re: Coach moving at idle despite parking brake (split from Re: Handling Comparison:36' U270/42' U320

Reply #21 – November 08, 2014, 12:31:59 am

Yes. The warmer the transmission is, the faster you have to pull it or coast it. I have done this on all years and models except for the newer ECU diesels that need a little juice. One time with a gas V8, the battery 99% died miles from town up by some old gold diggings. The engine lights would barely glow. It was a three speed auto so I just coasted it and it started. The shorted battery gave the alternator bad advice and the engine barely ran once it started. I was worried I might not be able to get back to town but tried turning on the headlights and the engine ran great.

Some of the Mercedes diesels did not have a priming pump either on top of the secondary fuel filter or down on the side of the injection pump so when you changed filters, you either had to wear out the starter motor or pull start it. I always pull started.

So, you attach a tow strap, have someone pull you to to about 30 mph while in neutral with the key in the run position. At about that speed, you pull the lever back into low and in a few seconds, the engine spins and starts. The mechanical Mercedes diesels don't use a fuel solenoid like most other diesels do to turn on the fuel. They use a vacuum actuator to turn OFF the fuel so it will keep running until it runs out of fuel without any electrical system. That's why your door locks, trunk, gas filler all operate off of the engine mounted vacuum pump. You do have to have the transmission fluid between the high and low marks on the dip stick for towing to work.

Old Hydra-Matics or Power-Glides that GM put into their cars would do the same thing along with Dodge Fluid Drive automatics. Daimler Benz may put in an extra pump but you can't tow four down as the owner's manual gives the number of miles possible to tow.

Pierce

Re: Coach moving at idle despite parking brake (split from Re: Handling Comparison:36' U270/42' U320

Reply #22 – November 08, 2014, 12:43:27 am

Wow.

Thanks, Pierce. That was a lot more than I bargained for.

I've known all along that old Benz is a keeper.

Re: Coach moving at idle despite parking brake (split from Re: Handling Comparison:36' U270/42' U320

Reply #23 – November 08, 2014, 09:12:13 am

Regarding creep, I think what Pierce said is relevant and logical (imagine that!).

I have never driven a truck that has disc brakes. I think the drum brakes that you tend to find on trucks have a lot of disadvantages. Drum brakes are more likely to lock up than disc brakes. When you're idling they are probably more likely to stay locked and not allow creep than disc brakes, even more so when they haven't been used or are cold.

I would think that ALL truck drivers would have the same opinion on creep (i.e. there should be none). But these are not trucks, they don't have truck brakes. So the horror a truck driver would feel at the thought of creep is not relevant to a motor coach driver.

All that being said, putting your coach in to neutral is something simple that you should always do when you're not sitting in the drivers seat.

Re: Coach moving at idle despite parking brake (split from Re: Handling Comparison:36' U270/42' U320

Reply #24 – November 08, 2014, 09:57:07 am

From experience with trucks..... If your service OR Park brake will not hold vehicle in drive at idle you have a MAJOR problem. You should drive CAREFULLY to a shop that can inspect and adjust brakes , in the case of Disc's the calipers , and slack adjusters .
A DOT inspection that found that the vehicle would creep in drive would put you out of service and just common sense a MH driver should use the same air brake criteria as a trucker.
On second thought please DON'T drive it, tow it.

Gary B

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