Retarder 101

Topic: Retarder 101 (Read 3053 times) previous topic - next topic

Retarder 101

August 24, 2012, 04:14:59 pm

Hello!

I've driven 2 different Foretravels since 2005, in lots of mountains, and I've recently run into retarder issues I've not run into before.

For example, today I'm coasting down a 6% grade in 6th gear. As I reach 70mph I don't want to go any faster and I apply the retarder, and the transmission promptly drops to 5th gear. I'm thinking if I stay at 70mph in 5th that will heat up the transmission more than if I was able to stay in 6th. I'm also thinking if I can stay in 6th and retard to 70mph the retarder will heat up instead of the transmission.

Is the transmission working correctly? If so, what is its objective in dropping to 5th?

What am I missing here? There are other scenarios I'll go into later.

best, paul

Re: Retarder 101

Reply #1 – August 24, 2012, 04:18:07 pm

For a given amount of braking HP, it will heat the transmission fluid less to have it in a lower gear/less retarder than higher gear/more retarder.

The lower gear/less retarder both generates less heat AND because the engine water pump is turning faster, gets rid of the heat faster.

Brett

Re: Retarder 101

Reply #2 – August 24, 2012, 04:56:52 pm

Quote

For a given amount of braking HP, it will heat the fluid less to have it in a lower gear/less retarder than higher gear/more retarder.

The lower gear/less retarder both generates less heat AND because the engine water pump is turning faster, gets rid of the heat faster.

Thanks, Brett!

That's quite clear and concise.

I was not aware the engine was a major heat sink for the transmission/retarder. I thot they had their own heat exchangers.

It still bothers me that my retarder does not seem to be pulling its weight (because it rarely heats up much ;o)

best, paul

Re: Retarder 101

Reply #3 – August 24, 2012, 08:11:12 pm

Paul,

Remember that retarder effectiveness is proportional to engine rpm. If you stay in 6th and apply retarder you will get less braking. I would have manually downshifted first to get rpm up to 2000, then start applying the retarder.

Re: Retarder 101

Reply #4 – August 24, 2012, 08:41:04 pm

Brett isn't it a lower gear for engine breaking[back pressure on the exhaust becouse of the turbo ] and the retarder is a separate system in the transmission with it's own heat exchanger ? Gam

Re: Retarder 101

Reply #5 – August 24, 2012, 08:50:12 pm

Well, I am one of the "Bretts".

NO, a diesel has close to zero braking, as there is no throttle plate. The only exception is some of the smaller, newer diesels with VGT's. None were ever used in Foretravels.

Said another way, a non-VGT turbo will not generate any back pressure, so virtually no engine braking.

That is why the choices for engine braking on a diesel are:

Exhaust brake (basically a "potato stuck in the tail pipe").

Engine brake/compression brake (opens the exhaust valve at TDC to release the stored energy from compressing 17:1or so).

Transmission retarder.

Re: Retarder 101

Reply #6 – August 24, 2012, 09:43:07 pm

Quote from: amos.harrison – August 24, 2012, 08:11:12 pm

Paul,

Remember that retarder effectiveness is proportional to engine rpm. If you stay in 6th and apply retarder you will get less braking. I would have manually downshifted first to get rpm up to 2000, then start applying the retarder.

Brett H,

My recollection of the diagrams of the retarder show that it is on the output shaft of the transmission, i.e., on the drive shaft. It seems that the effectiveness of the retarder would not be dependent on engine rpm, except for the more effective cooling of the fluid at higher engine rpm. The retarder effectiveness would be proportional to drive shaft rpm. Do I have this wrong?

Quote from: wolfe10 – August 24, 2012, 08:50:12 pm

... NO, a diesel has close to zero braking, as there is no throttle plate. ...

Brett W,

My experience has been that a diesel engine provides significant braking even without a retarder. One can control speed on a downhill run by selecting an appropriate combination of gear and speed, even without an exhaust brake or retarder. I learned from my father, an OTR trucker, that you should descend a steep hill in the gear you would use to climb it. With the proper gear selection, you would probably not need to use brakes. To the best of my knowledge, his early trucks has no exhaust brakes or compression brakes. What am I missing?

Re: Retarder 101

Reply #7 – August 24, 2012, 09:58:46 pm

Keep in mind that the transmission temperature is not showing the temp of the fluid in the retarder, which is usually much higher when the retarder is engaged. I reported on this previously, but while coming down the long hill to the west of Durango, CO, at a speed of maybe 55-60 mph, my retarder hit 300 degrees. Don't recall the trans temp, but it was much lower. The second it hit 300 degrees, the trans automatically shifted down, I think from 6th to 4th. This caused an immediate decrease in the retarder temp due, I guess, to better cooling of the the retarder fluid. I now monitor the retarder temp on any long hill and keep it below about 280 by slowing down. Normally run about 40-45 mph on long steep hills. I really think going faster puts too much energy into the retarder and this causes it to to get excessively hot.

Re: Retarder 101

Reply #8 – August 24, 2012, 10:07:56 pm

Per Allison document about retarder operation:

"They generate the greatest braking at high drive shaft RPM and work independent of engine speed or gear ratio."

http://www.svfd.net/SVFD%20Files/Articles/Engineer/Retarder%20Operation%20How%20Does%20It%20Work.pdf

Re: Retarder 101

Reply #9 – August 24, 2012, 10:09:26 pm

Quote

Keep in mind that the transmission temperature is not showing the temp of the fluid in the retarder, which is usually much higher when the retarder is engaged. I reported on this previously, but while coming down the long hill to the west of Durango, CO, at a speed of maybe 55-60 mph, my retarder hit 300 degrees. Don't recall the trans temp, but it was much lower.

That's not been my experience. My VMS 240 CL shows me retarder and transmission temperatures. I've never seen a retarder temperature higher than my transmission temperature.

Normal base transmission temperature is 190F. I've never seen retarder temperature higher than even that.

best, paul

Re: Retarder 101

Reply #10 – August 24, 2012, 10:21:02 pm

Paul... I suspect your VMS 240 CL is not reading the PID for the retarder. Mine is always higher when the retarder is on... but is the same temp as the trans when the retarder is off. The second the retarder engages, the temp jumps. Is there any way to check the PID your VMS retarder temp is reading?

We drove today through Illinois and the trans and retarder temp were about 180 most of the time since I rarely had to use the retarder. Love this flat land driving. Got 9.4 mpg with the Mode switch on. Normal average is about 8.5 mpg.

Re: Retarder 101

Reply #11 – August 24, 2012, 10:21:18 pm

Quote

That's not been my experience. My VMS 240 CL shows me retarder and transmission temperatures. I've never seen a retarder temperature higher than my transmission temperature.

Even heading west out of Rocky Mountain National Park recently where my transmission temperature got very high.

best, paul

Re: Retarder 101

Reply #12 – August 24, 2012, 10:22:18 pm

Quote from: J. D. Stevens – August 24, 2012, 09:43:07 pm

My experience has been that a diesel engine provides significant braking even without a retarder. One can control speed on a downhill run by selecting an appropriate combination of gear and speed, even without an exhaust brake or retarder. I learned from my father, an OTR trucker, that you should descend a steep hill in the gear you would use to climb it. With the proper gear selection, you would probably not need to use brakes. To the best of my knowledge, his early trucks has no exhaust brakes or compression brakes. What am I missing?

Would the braking effect I have seen from diesel engines that are not using retarder, exhaust brake, or compression brake, be the drag of air compressor, water pump, hydraulic pumps, air drag, miscellaneous friction, etc.?

Brett W's excellent response sent me searching. There is a good description of engine braking at Engine braking - Wikipedia, the free encyclopedia.

Re: Retarder 101

Reply #13 – August 24, 2012, 10:27:17 pm

Quote

"They generate the greatest braking at high drive shaft RPM and work independent of engine speed or gear ratio."

So would this be basically "Greatest braking at high road speed?"

best, paul

Re: Retarder 101

Reply #14 – August 24, 2012, 10:41:36 pm

Quote from: Paul Smith – August 24, 2012, 10:27:17 pm

So would this be basically "Greatest braking at high road speed?"

best, paul

That is my understanding. That statement regards the effectiveness of the retarder only, not any other components of the coach's "braking systems."

Re: Retarder 101

Reply #15 – August 25, 2012, 01:00:44 am

RE " the retarder is a separate system in the transmission with it's own heat exchanger ?"

Retarder is not separate from Allison transmission. Retarder uses transmission fluid to 'Retard'. This transmission fluid is the same shared fluid source the transmission uses to function.

Heat is removed from transmission fluid with the same coolant that cools engine using a big round external heat exchanger.

Two large forward facing transmission fluid hoses connect to the heat exchanger.

Re: Retarder 101

Reply #16 – August 25, 2012, 01:34:18 am

Seems like more than Retarder 101 to me! To make simple, when probably I cannot not.....as I understand you Barry, if I monitor the transmission temp I am monitoring the retarder temp, right? I recall being told by the dealer to keep trans temp below about 240, maybe a bit less. And that if I do that, then the retarder is protected, right?

Further, excepting that it would be safe anyway so long as the temp stays in these ranges, it is nonetheless best to control my downhill speed by gear selection than use of the retarder.

Do I have this summarized correctly? Seems a valuable dialogue, I want to be sure I am getting this right so I can have a simple rule of operation in these mountains. Thanks, All......Mike

Re: Retarder 101

Reply #17 – August 25, 2012, 08:32:32 am

Quote

Keep in mind that the transmission temperature is not showing the temp of the fluid in the retarder, which is usually much higher when the retarder is engaged. I reported on this previously, but while coming down the long hill to the west of Durango, CO, at a speed of maybe 55-60 mph, my retarder hit 300 degrees. Don't recall the trans temp, but it was much lower. The second it hit 300 degrees, the trans automatically shifted down, I think from 6th to 4th. This caused an immediate decrease in the retarder temp due, I guess, to better cooling of the the retarder fluid. I now monitor the retarder temp on any long hill and keep it below about 280 by slowing down. Normally run about 40-45 mph on long steep hills. I really think going faster puts too much energy into the retarder and this causes it to to get excessively hot.

I'm assuming concern about fluid temp is related to insuring it has a longer useful life.

I also assume the retarder gets its fluid from its transmission.

Now I don't think folks would want transmission fluid to go to 300 or even 280 degrees. So does allowing retarder fluid go to 280 degrees effectively reduce the useful life of (at least some of the) transmission fluid?

In the above the trans shifted down from 6th to 4th and decreased the retarder temp. The downshift probably slowed the FT making the retarder less effective (according to the earlier Allison quote), in other words "worked the retarder fluid less" which, at least in part, helped cool fluid in the retarder..

I've emailed SilverLeaf asking where their VMS gets its retarder temp info.

best, paul

Re: Retarder 101

Reply #18 – August 25, 2012, 09:51:33 am

Quote

It still bothers me that my retarder does not seem to be pulling its weight (because it rarely heats up much ;o)

My trans temp according to the VMS is usually higher than the engine temp. On long 6 and 7 percent grades it will climb to 214 or so. According to Silverleaf, the VMS is reporting the trans sump temp just like the dash gauge. The Allison documentation states that the maximum safe temp for my trans (4060) is 230. I'm going a lot slower than Paul so maybe that's the difference.

I need to make the VMS change that shows retarder temp as soon as I figure out how!
jor

Re: Retarder 101

Reply #19 – August 25, 2012, 09:55:40 am

Quote from: Michael & Jackie – August 25, 2012, 01:34:18 am

Seems like more than Retarder 101 to me! To make simple, when probably I cannot not.....as I understand you Barry, if I monitor the transmission temp I am monitoring the retarder temp, right? I recall being told by the dealer to keep trans temp below about 240, maybe a bit less. And that if I do that, then the retarder is protected, right?

Further, excepting that it would be safe anyway so long as the temp stays in these ranges, it is nonetheless best to control my downhill speed by gear selection than use of the retarder.

Do I have this summarized correctly? Seems a valuable dialogue, I want to be sure I am getting this right so I can have a simple rule of operation in these mountains. Thanks, All......Mike

Mike,

Yes, indeed the fluid is shared between the retarder and transmission. My practice is to leave the retarder switch on all the time, except if the road might be slick. That couples retarder operation with operation of the service brake. I leave the joy stick in the "Off" position except to control speed on hills.

Like brakes, the retarder was designed to control the speed of the vehicle by converting kinetic energy (movement) to heat. Use the retarder to slow or control the speed of the vehicle. It is particularly useful on long, steep downhill grades. Operate at speeds that don't cause systems to overheat.

The engine is designed to control the speed of the vehicle by converting heat to kinetic energy. Operate it at speeds that don't cause systems to overheat.

Use and enjoy all the nifty systems those clever engineers have put together for us to use. They included a few temperature gauges to help us use them without abusing them by allowing them to get too hot. Don't worry! Be happy!

Re: Retarder 101

Reply #20 – August 25, 2012, 10:06:21 am

Quote

My trans temp according to the VMS is usually higher than the engine temp. On long 6 and 7 percent grades it will climb to 214 or so. According to Silverleaf, the VMS is reporting the trans sump temp just like the dash gauge. The Allison documentation states that the maximum safe temp for my trans (4060) is 230. I'm going a lot slower than Paul so maybe that's the difference.

My transmission temperature is also normally higher than the engine temperature on going downhill.

And my VMS also agrees with my dash transmission sump temperature.

best, paul

Re: Retarder 101

Reply #21 – August 25, 2012, 04:49:20 pm

1) Several years ago, Silverleaf told me they cannot display retarder temp because only Allison proprietary diagnostics have access to retarder temp sensor. Things may have changed.

2) For our transmission temp dash gauge, Foretravel installed a single-wire sensor located in the elbow of the hose coming from transmission cooler return. We later moved the sensor to the transmission hose going to the cooler, so we now display the actual fluid temp as it heads to the cooler, which increased the gauge temp reading.

3) Transmission fluid temps are often read from the sump where cooled fluid is mixed with a pool of fluid in the bottom of the transmission, which is where the internal transmission parts pump from.

4) Temperature readings from hot fluid going to cooler will read higher than sump. And probably fluid returning from cooler will read lower than sump.

5) Petroleum base transmission fluid probably starts to breakdown about 250 degrees, but synthetic fluid has a much higher temperature tolerance and is recommended for our retarder transmissions. Heat abuse is cumulative.

6) Allison transmission computer will automatically lower retard levels when it determines internal fluid temps are too high to keep transmission safe.

7) Transmission uses a modulated valve that allows more fluid into retard chamber at higher retard settings.

Initiating any retard setting, causes an external storage accumulator to quickly pump a quantity of fluid into retard chamber, ahead of the modulator doing its thing. A 12-volt valve opens to let motorhome air pressure force the fluid out of the accumulator.

9) High transmission temps will raise engine temp gauge as heat is absorbed by engine coolant.

Our transmission will usually down-shift when we activate retarder and we do not worry about relatively short burst of temps over 325. Anything to save our air disk brakes for emergency stops. We seldom touch service brake pedal going down mountains.

Since temp gauge readings may not reflect how hard the retarder is working, I would pay more attention to how well retarder is slowing down a motorhome to determine if retarder is working ok.

A few years ago, we found our retarder did not seem to slow us down, and after two days of diagnosing with a computer while driving, and replacing a few parts, a high-tech Allison repair facility could not find our problem. On day three, they found a small pin terminal connector at the computer was just touching, but not fully seated into the plug. This allowed diagnostics to think there was a good connection, but did not allow enough current to flow to fully open the modulating valve. We were happy to have our problem solved and Allison only charged us for 5 hours labor. All retarder electronic and mechanical control parts they examined were located on the outside and transmission pan did not have to be dropped.

Re: Retarder 101

Reply #22 – August 25, 2012, 05:15:11 pm

Has anyone installed a Jake Brake in conjunction with a retarder?... How did that work out?... just curious

Scott

Re: Retarder 101

Reply #23 – August 25, 2012, 05:49:16 pm

Quote from: Barry & Cindy – August 25, 2012, 04:49:20 pm

8- Initiating any retard setting, causes an external storage accumulator to
quickly pump a quantity of fluid into retard chamber, ahead of the modulator doing its thing. A 12-volt valve opens to let motorhome air pressure force the fluid out of the accumulator.

A picture of the hydraulic accumulator is here: Big Air Leak Hydraulic Accumulator
The picture was taken through the small access door on passenger side aft of the wheels. If you follow the big hose from the rear of the accumulator, it goes directly to the retarder assembly at the fore end of the transmission. The connection of the hose to the retarder is visible in the upper left corner of the picture.

Re: Retarder 101

Reply #24 – August 25, 2012, 06:08:39 pm

I descend mountains using the trans gauge. I select the initial trans gear based on my experience with the % grade placard. If my trans gauge moves above 200 degrees using the retarder, I go down a gear, applying service brakes firmly if necessary to keep engine rpm below redline. I continue this process to the bottom. No fuss, no fear! Never exceeding posted truck speed limits.

Affiliate Notice