d8pete Posted April 7, 2015 Share Posted April 7, 2015 I have 1984 Mack R model with the TRTXL 1070B tranny. I had the transmission installed a couple of years ago and really love it. My question is when I move the selectair switch the rear compound tries to shift immediately. When I watch others on Youtube go up through the gears they are able to "preselect" the shift of the rear compound and when they move the main transmission lever the rear compound shifts and both the main and rear compound slide into gear easily. It seems with the others something delays the shift of the rear compound until the main transmission shift lever is moved, however it doesn't happen with mine. Any ideas??? 1 Quote Link to comment https://www.bigmacktrucks.com/topic/39613-mack-12-speed-question/ Share on other sites More sharing options...
terry Posted April 7, 2015 Share Posted April 7, 2015 Is there a regulator for the trans air supply, Cuts the working pressure down to about 60 pounds. terry 1 Quote Link to comment https://www.bigmacktrucks.com/topic/39613-mack-12-speed-question/#findComment-285884 Share on other sites More sharing options...
mrsmackpaul Posted April 8, 2015 Share Posted April 8, 2015 yep as terry has said if the air pressure is to high as soon as you flick the switch it tries to shift but the straight cut sliding dogs prevent it from shifting until theres no pressure on the gears ie when you take the load off either backing off the gas going up a gear or when you push the clutch in changing down the only other thing I could think it might be is the the sliding dog or the gears are badly worn but I would lean towards the air pressurePaul Quote Link to comment https://www.bigmacktrucks.com/topic/39613-mack-12-speed-question/#findComment-285929 Share on other sites More sharing options...
d8pete Posted April 9, 2015 Author Share Posted April 9, 2015 Terry & mrsmackpaul - I was wondering about the regulator, but I don't have a service manual and wanted to know what you guys out there with more experience thought before I started to change anything. Thanks for your replies.Pete Quote Link to comment https://www.bigmacktrucks.com/topic/39613-mack-12-speed-question/#findComment-286033 Share on other sites More sharing options...
mrsmackpaul Posted April 9, 2015 Share Posted April 9, 2015 Can you here air leaking past the knob all the time if so it would be a good indication the regulator is set to high and whats the story with the D 8 ????? just wondering Paul Quote Link to comment https://www.bigmacktrucks.com/topic/39613-mack-12-speed-question/#findComment-286062 Share on other sites More sharing options...
d8pete Posted April 9, 2015 Author Share Posted April 9, 2015 Paul - I don't hear air leaks of any kind in the cab either when the engine is running or engine shut down. As for the "D8", Caterpillar is my first love and I have always had a great appreciation for the model D8, 1938 to the present model D8T. I have a 1957 D8 14A, its a real work horse and one of my favorite machines to operate.Pete Quote Link to comment https://www.bigmacktrucks.com/topic/39613-mack-12-speed-question/#findComment-286132 Share on other sites More sharing options...
steve p Posted April 10, 2015 Share Posted April 10, 2015 there are two air pressure settings 60 psi and 70 psi depending on year and series Quote Link to comment https://www.bigmacktrucks.com/topic/39613-mack-12-speed-question/#findComment-286156 Share on other sites More sharing options...
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