Randy mckinney Posted October 27, 2013 Share Posted October 27, 2013 1985 rd688 12 speed maxi torque. What oil is recomended for this tranny? Also what can cause this tranny to lock up when shifting from low to rev. Flip the switck back an forth it comes out of gear and ur good to go till next time u need rev. Any ideas on what can cause this? Mack said to change filter on air going to tranny so i did and no change. Then they told me to bring it to them and they could find the problem for what im sure would be a big bill. Quote Link to comment https://www.bigmacktrucks.com/topic/33275-maxi-torque-questions/ Share on other sites More sharing options...
Challenger Posted October 27, 2013 Share Posted October 27, 2013 80w 90 gear oil is the trans oil.What do you mean by locked up? Does it get stuck in the selected range(Direct, lo, N, reverse) If its getting stuck in selected range and flipping the switch back and forth releases it, maybe you have an oring problem in the rear top cover. As you select the range gear on the shifter do you hear the normal click and clacks? Or does it sound slow? When it locked up do you hear any steady air leaks?Changing the trans air filter is always a good idea but if you go from lo to direct fine, I dont see how the air filter can be causing a problem. Quote Link to comment https://www.bigmacktrucks.com/topic/33275-maxi-torque-questions/#findComment-213463 Share on other sites More sharing options...
Randy mckinney Posted October 27, 2013 Author Share Posted October 27, 2013 When u switch from low to neutral and in to rev. An let the clutch out it stalls the engine will do same thing going from r to n to low but not all the time but pretty often even pausing in neutral before low or rev doesnt make a diferance. Not sure about hearing air will check that out. Quote Link to comment https://www.bigmacktrucks.com/topic/33275-maxi-torque-questions/#findComment-213479 Share on other sites More sharing options...
Challenger Posted October 27, 2013 Share Posted October 27, 2013 All the range shifts are done in the rear case by the rear top cover. Looking at the rear top cover the direct-lo shift is the cylinder, the other shifter is reverse-N. Inbetween the the cylinders is a detain pin that is suppose to prevent forward gears and reverse gears from engaging at the sametime. You probably have a worn shift rail that is sliding pass the detain pin allowing 2 gears to engage. My guess--worn rear topcover shift rails and orings. Orings should always be replaced when the rear top cover is removed. Quote Link to comment https://www.bigmacktrucks.com/topic/33275-maxi-torque-questions/#findComment-213631 Share on other sites More sharing options...
fjh Posted October 28, 2013 Share Posted October 28, 2013 The fixed shift rail can break and also cause this !u need to lift the lid off and look! also a loose lock bolt on the air shift could be an issue Quote Link to comment https://www.bigmacktrucks.com/topic/33275-maxi-torque-questions/#findComment-213681 Share on other sites More sharing options...
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