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Discussion Starter · #1 ·
I recently performed a service (filters and fluid) on the trans in my truck. Since then I have noticed that the line pressure according to my H&S MM is jumping around all over the place. With normal driving starting from a stop it will jump to 120psi then suddenly drop to 60psi and then right back up. I never noticed it doing this before the service. I've checked fluid levels and all seem to be fine.

Could there be air in lines causing this pressure change or what? I've search and couldn't find anything similar to this so any help would be great. Thanks!
 

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Can you also display the DESIRED line pressure reading? You need to compare the actual line pressure to the Desired Line Pressure (DLP).

The DLP value will change based on input torque (throttle), and also depending on whether the trans is shifting gears, TCC is locked or unlocked, etc. Typically, DLP will drop to 60 psi (or even lower) when cruising steady state. It may be 120 at idle in Park, then after a few seconds drop to 60, then jump back to 120 when you go to shift into Drive, then drop back to 60 again if you sit at a stop in gear for a few seconds, etc.

Line pressure should always match the DLP reading, within a few psi, except sometimes at idle it may be less (due to low pump speed). If you tip gently into the throttle, to raise the engine speed to 1200 RPM or more, it should then match DLP.

The big question is: Is your line pressure merely varying with changes in DLP, or is it really "jumping all over the place" (i.e. unstable)? In most cases, if you are sitting in a steady state (such as idling in Park, or cruising in a set gear at constant throttle), the line pressure should be stable. It shouldn't be erratically (wildly) jumping up and down.

If you have erratic line pressure, most common cause is a bad (misinstalled, or broken) main sump filter (the flat one). The plastic snout on the filter (that sticks up into the pump) should be fully engaged into the seal in the pump housing (not just resting up against it). The seal should be tamped into the bore in the pump (not just stuck onto the filter snout). The seal flange should be FLUSH against the pump casting all the way around (not tilted in the bore). Check that the filter is not split open along the side seam, and is not cracked at the base of the plastic snout.

Also check that the spin-on filter is snugly installed (not loose, or laying in the pan!). This filter should be HAND-TIGHTENED only (just turn it 1/2 to 3/4 turn after the gasket touches the case face), but it should be snug.

So I would check the filters, if your line pressure is erratic under steady-state conditions.
 

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Discussion Starter · #3 ·
As far as I know there is no way to display the DLP on the Mini Maxx, but I could be wrong. How else would I go about figuring out what the DLP would be? It seams to have calmed down considerably in the last day or two. Maybe it was just being difficult for a couple of days.
 

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Can you program your Mini-Maxx to display specific data using a PID (Parameter Identifier) or LID (Local Identifier)?

For example, could you program it to read a 2-byte data value, using PID B04E, factor it by 0.384, and display it as a decimal value? (This is NOT a real example, I'm just wondering if there's anything in the manual that says you could do something like this).
 

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Mine is doing this as well, but stops as the truck warms up.

Did you get anywhere with this?

Was your filter to blame?

I'm changing filters on Monday.
 

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Discussion Starter · #6 ·
The problem fixed itself I guess. I checked the fluid levels again and it was about half a quart low. Don't think that was the issue but haven't noticed any issues since.
 
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