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Allison Retarder

Allison Retarder

(OP)
I have a bus with an Allison 4060HD transmission equipped with an integral hydraulic retarder. I have a switch on the dash to enable the retarder. I also have a six position modulation control stalk to select the amount of deceleration. If the modulation lever is set to off but the dash switch is on, when I apply the brakes I still get retarder braking force. I get 33, 66, and 100% use of the retarder when pressing the brake pedal to 2psi, 4psi, and 6 psi.

After coming to a complete stop and then commencing a tak-eoff I get a slight shuddering of the drive train, ever so slight. I've been told it's related to the rapid change from actively diverting fluid through the retarder and downshift mode and then going back to normal take-off shifting and shudding off the hydraulic fluid flow through the retarder portion. I also notice my acceleration from a dead stop is more brisk if the retarder has been off. Assuming the retarder has been on and in use, is there a chance that on take-off from a dead stop the retarder still imposes a load on the drive train? Can someone explain what the transition works from full retarder stop to full power takeoff?

RE: Allison Retarder

generally speaking what you do when turning the retarder on and/or selecting a certain amount of modulation is that you regulate the amount of fluid in the retarder . basically a torque converter with a clocked turbine wheel.

the reason for the effects you mentioned can be several - either some of the hydraulic circuitry that modulates the retarder does not function properly or the retarder itself that should be drained immediately when switched off still contains some fluid, and hence still give a braking effect.

that the retarder acts depending on the pressure in the braking system is odd. normally the air brake system and the retarder should be completely independent from each other since both are designed for specific, quite differing, purposes. the braking system should be used only for deceleration (incidentally depending on traffic conditions), the retarder is designed to prevent acceleration under specific driving conditions (and act as a system that can be used continuously), especially when going downhill.

RE: Allison Retarder

(OP)
Thanks romke,

Page 18 of this link talks about the retarder integration with service brake application: Link. In short there are a series of air pressure switches that send voltages to the TCM for retarder activation.

It's interesting they also mention the use of vehicle air pressure used to force fluid out of an accumulator and into the retarder.

RE: Allison Retarder

Thanks for the link. Obviously Allison gives the vehicle builder quite a few possibilities to activate the retarder - some of them actually may be legally required whereas in other areas then the US some may not be allowed at all.

Still, given the problems you encountered, something is not working as it should in the control system - either in the electronic part, the pneumatics or the hydraulics. For some reason or another fluid is not (completely) drained and hence some braking power still is applied where it should not be available.

How long has the fluid been in use? If the interval has been quite long the fluid itself may have deteriorated due to overheating or oxidation and that may have caused varnish like deposits on various actuators, perhaps limiting their free movement that can lead to difficulties when the retarder needs to be emptied rapidly, for example after a dead stop.

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