I have a 01 3800 Open with Cummins 8.3 / 450 Diamonds and ZF280 transmission. Recently my port transmission is not returning to its normal low pressure when placed in neutral. It returns only halfway between the 300 plus psi in gear and where it traditionally has returned to out of gear (I don’t recall this exact psi on the analog gauge). The sensor on the transmission is maybe 3 years old. Any advise on the issue or cause is appreciated.
The electrical sender/analog gauge system is prone to false readings, especially as things age (like the wiring/connections). Step 1 is to get a reading with a fluid filled mechanical gauge. Temporarily remove the sender and replace it with a mechanical gauge and get readings in F/N/R. Get a gauge good for about 600psi, less than $20 on Amazon. Use a thread sealant. The connection for the sender is 1/8 NPT. Normal pressure in N is about 70psi.
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