If the fluid looks good then I would suspect there is something wrong internal to the trans. Most likely you have a cut seal or an issue on the Low Reverse Accumulator. If you look at the fluid diagram for reverse and 1st (assuming you have not made a shift to second before it slips) they have the Low Reverse in common, so your issue is most likely in this circuit.
Also, there is a chance that the unit has low pressure which would mean the pump is failing. This pump is a gear style pump and relies on the clearance between the teeth to generate the pressure. If these gears are worn, then it could be delivering low pressure. There is usually a DTC that goes along with this so the fact that your check engine light is not on may not point to this. I doubt it is a pressure switch because your reverse is run directly off of line pressure controlled by the manual valve (gear selector). So when you select reverse, the manual valve moves to that position and sends line pressure to the reverse clutch to apply it. Low pressure would explain why you don't get reverse but then first engages because it is controlled by the pressure switch. Maybe there is just enough pressure to apply first but once a shift is about to take place, it slips and there is not enough pressure to achieve the next gear.
I attached the pressure chart for each gear if you are able to check the pressure.
Let me know what you find and we can go from there. Thanks
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Sunday, April 21st, 2019 AT 6:51 PM
