Allison transmissions w/Hydraulic Retarder

[3408cat]

I've noticed that you can have a hydraulic retarder on the model 740. How do they compare to a Jake brake as far as hold back power. Is engine HP a factor? Is the extra heat that it adds to the oil a problem? Does any know if they are about the same as the old Caterpillar Brake Saver ?

[TomC]

Allison made two different models of hydraulic retarders-one on the input shaft between the torque converter and the transmission (similar to the Caterpillar brake saver) or the output shaft hydraulic retarder mounted at the end of the transmission.  The input shaft retarder was the first version to come out and works very well absorbing around 500hp.  The later output shaft retarder also absorbs around 500hp, but also added a multi wet disk brake to it where it could actually stop the vehicle (hydraulic retarders don't work well under 10mph).

The best usage for a hydraulic retarder is in stop and go traffic-like for transit buses and trash trucks.  For long down hills, you have to watch the temp gauge.  It takes only 5 or 10 seconds for the oil to overheat, then you have to "dump" the oil and recharge with new cool oil-or just release the retarder and reapply-like pumping the brakes.

For long down hills, nothing is better then the Jake type exhaust brake. It can be run continuously without harm, it creates heat in the engine (not enough to overheat it) to keep it warm so you don't have a cold engine at the bottom of the hill (compared to the hydraulic retarder where the engine does nothing).

There have been a few trucks made that had both the Cat brake saver and a Jake brake.  But-there was also a cut out to only allow operation of one or the other at a time.  One of the problems we first had at Freightliner when we introduced the Mercedes-Benz 4000 series (12.8 liter 350-450hp), was that the turbo and compression brake was too powerful (up to 600hp) for some of the drivelines, and were dropping the drive shaft.  That was easily cured with larger drive shaft.  The MBE4000 had an extra valve in the head hydraulically operated for the compression brake and the turbo shifted vanes to increase boost at the same time to get the 600hp of braking.  Most new engines now have 5-600hp of braking with dedicated camshaft lobes for the Jake Brake.  I highly recommend the Jake brake over the hydraulic retarder.  Good Luck, TomC

[opus]

Jake by far is the best.  I have the output retarder and it actually works very well.  I have never had the temp go up on me.  That being said, you do have to watch it.  I am amazing at how well it holds back.

[junkman42]

I have the Allison HT70 with the retarder and Jake's.  They will both activate at the same time if desired!  You can slide the rear wheels on a wet road as I found out on a long downhill.  Was told that the HT70 was a failure waiting to happen but so far it has not skipped a beat.  The transmission temp will rise on a long application of the retarder.  John L

[3408cat]

With an 8v-71, or any 2 cycle DD, I'm not impressed with the performance of a Jake brake - about the same as a Mack Dynatard. Would the hydraulic retarder work better with the 8-71 ?

[belfert]

Aren't transmission retarders a somewhat common cause of bus fires, or am I remembering the wrong thing?

[TomC]

The main reason the Jake Brake on the 8V-71 seems weak is that it is just plainly not adjusted correctly.  Don Fairchild in Bakersfield did a precision adjustment (a bit tighter then the book says-but lots of experience at that setting) on my bus.  Now with my car in tow going down the Grapevine (6% grade for 5 miles) and my bus and car weighing 34,750lbs, I have to switch between one head and both heads in order not to slow down to much.  The Jakes work great on a 8V-71 or 8V-92 when adjusted right.  Good Luck, TomC

[eagle19952]

Very interesting....
What then would be the thought of putting jakes on one head now and maybe (if needed) the other later.
Just thinkin.....

[TomC]

One head at a time is doable-as you can afford it.  Good Luck, TomC

[TomC]

As a matter of fact, the Series 60, when first put into a bus (I believe a Prevost) was so large that the bus manufacturer didn't have enough room for the entire raised valve cover.  So they compromised, the rear two cylinders had a flat top valve cover without Jake Brake and the front four cylinders (towards the rear of the bus) had the Jake brakes since they came in two cylinder castings.  Worked well-but of course, 6 cylinders is better.  Good Luck, TomC