Does anyone know if the brake pressure switches that control the retarder operation are shorted when pressure is applied? I think my coach has three switches.
Thanks in advance,Irwin
Irwin,
Are you referring to the brakes having a more intense feel while applying pressure on foot pedal when retarder is on?
David
Retarder has three air pressure switches that are normally open and close when air brake pressure is sensed. The retarder uses three pressure switches, each closing at different psi. The one that closes at the lowest psi turns on minimum retard, etc. all the switches close a quite low psi so maximum retard is probably on with most brake stops. Yes they short when pressure is applied.
There are two other air brake switches on front wall of street side forward compartment, one for rear and one for front brakes. These turn on brake lights. A relay near these brake light switches is closed when retarder is active, also turning on brake lights. Some coaches don't have the brake light retarder relay, as it was optional when joy stick was added.
Barry , is there any reason a person wouldn't be able to put a switch in parallel with the switches to activate the retarder without the brakes being applied? Like 3 switches ,one for each step of retarder operation, just wondering?
Thanks for the replies, Irwin
David , I was wondering if I can do the above, thanks for the reply, Irwin
Allison joy stick does that. It is a six position control for the retarder. It can be added if you have the retarder. It was the first "modification/enhancement" we did on our 1997 U295.
I have been told that when the brake pedal is lightly pressed (I have no joy stick), the retarder is applied without any coach braking. Only at stage 2 and 3 retarder are the coach brakes also applied. Does anyone know if this is correct?
Duane, That is how my 95 U280 operates. the more pressure on the treadle the more retard function.
Gary B
Yes, you are thinking correctly. Your manual switches would work, after all the retarder is activated by only the three switches. The joy stick pretty much operates the same, except with 6 steps.
Since air pressure switches would be progressively closed, you may need to close 2 psi switch contacts, then keeping 2 psi closed, close 4 psi, etc. You could find some rotary switch or make one yourself to experiment.
Keep in mind that the optional joy stick, is ergonomic and offer quick and safe slowing, without taking attention from driving. The cost of the joy stick will be paid back in saving brake pad / rotor wear. We have 150,000 miles and our original brake pads have very little wear. We go down steep mountains with retard only, never touching brake pedal. This not only saves brakes, but leaves the air brake system as a backup to retard braking, thus giving us two completely separate braking systems to prevent run away, in case of emergency failures, but engine must be running for both braking systems to operate normally.
Allison uses three non-adjustable 2, 4, 7 or 10 psi air pressure switches to activate the retarder, so all of them are quite low, showing partial & full retard come on with light brake pedal operation. Even at 2 psi, some air brake operation is taking place to slow down coach.
http://www.allisonelearn.com/LMS/scorm_host/scorm_modules/1.0/en/TT-34-RE01.0WBT/Documents/Principles%20of%20Operation.pdf (http://www.allisonelearn.com/LMS/scorm_host/scorm_modules/1.0/en/TT-34-RE01.0WBT/Documents/Principles%20of%20Operation.pdf)
Thanks , Barry I thought that was the way it works .