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Transmission Discussion of preventative/corrective maintenance and other technical issues regarding your coach's transmission.

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  #1  
Old 12-11-2013
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Default 5-4th Speed Shift - Air Throttle vs Cruise Control

My 84 PT36 shifts from 4th to 5th at about 54 MPH using the air throttle, but shifts from 4th to 5th around 45-48 MPH using the cruise control. I thought something in the air throttle is causing a difference in the shift point.

While using the air throttle to accelerate with the transmission shifted from 4-5 and then have to ease up on the throttle due to traffic, the transmission always downshifts to 4th at 53 or 54 MPH, even when the throttle is even gently pressed. If I re-engage the cruise control to speed up it does not downshift to 4th unless the speed has dropped below 45 MPH.

Does anyone have any idea what the cause could be. I thought maybe the air throttle has different pressures in the two lines from the air throttle and is causing the problem.
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  #2  
Old 12-11-2013
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Shane is our resident PT expert.
He works on the Bendix cruise control stuff,maybe he can help?
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  #3  
Old 12-12-2013
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Less throttle position angle on cruise control results in earlier upshift due to less transmission throttle valve (TV cable) motion. Best method is use cruise control at speeds over 55 mph. I've noticed that the MD 'puter trans in my FC and some other MD rigs I've driven will spin up a little on a 4/5 upshift on cruise, due to a more relaxed TV position.
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  #4  
Old 12-12-2013
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Default Throttle vs Cruise Control

Mike,
Thanks for the information. Is there an adjustment for the transmission throttle valve cable?

Is this a separate cable from the air throttle cable or is there just the one cable that comes out of the cruise control and goes over to the engine throttle? I have not seen the second cable.

I would like to eliminate the downshift when using the air throttle that pulls the cable connected to the engine throttle.

Thanks
Dave
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Old 12-12-2013
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Dave, Can you take a picture of the engine? Since you have an Air Throttle you may also have and air operated kick down. We would know if you can post that picture.
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Old 12-12-2013
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Rick, Yes I would be glad to. Where would the air operated kickdown be located? Is it on the transmission or around the cruise control?
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  #7  
Old 12-12-2013
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Dave, Open the engine compartment. Take a shot of the Johnson Control area. Let us see how the air throttle is hooked to the pump. We may be able to see if you have a cable there. If not you will need to take a shot of the transmission. The cable or air tube will be on the drivers side about 3/4 of the way back and right above the transmission oil pan.
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Old 12-12-2013
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Dave- There is a throttle position sensor which may need to be replaced on the side of your transmission. Our non-computerized MT654CR tranny has a "modulator" on the right side near where the shift cable is attached. It has a 1/4" air line going to it from above the trans and is used to "sense" the throttle air pressure- telling the trans to downshift or upshift. I have a spare one onboard the coach, but can't remember the part number. Maybe someone else has it and can chime in here....
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  #9  
Old 12-12-2013
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Shane is right. For some reason I thought we were dealing with an FC. Sorry if I caused any confusion.
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  #10  
Old 12-14-2013
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Default 5-4th Speed Shift - Air Throttle vs Cruise Control

Shane, Randy, Mike - I have attached pictures of the cruise control panel with the actuator cable and pictures of the driver's side transmission.

I only see one cable from the cruise control to the throttle. Do not see a "modulator" on the side. I hope this helps in identifying the upshift components. I will try and read what looks like an ID plate on the side of the transmission which I noticed in one of the pictures.
Attached Thumbnails
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Name:	Transmission, Rear_DSC00022.jpg
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