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I had seen most of that video though I probably didn't understand what he was saying about the pump when I first watched it; it makes more sense now. Interestingly, he called the oil pump cover (or maybe just part of it) a stator also. I don't see how a burr would develop on the control piston on the contact side (rather the leading/trailing edges) but I trust his experience. It makes sense that if the burr causes the piston to stick, the pump would be left in the low pressure position.

User "Solder" in the R31 forum gives another reason for decreased pressure, the line pressure valve/solenoid. He seemed to have a lot of information about how the pulses control the solenoid position. When I was brainstorming how to artificially increase the pressure to counteract a weak pump, I wondered if a "tune" can be installed that would control the line pressure solenoid allowing it to be more open (or closed, whichever causes more pressure). I didn't see anything related to PWM in the transmission FSM. Is "on-off duty signal" their way of saying PWM? Probably. Too bad there isn't a pressure sensor that can be read via ODBII to help adjust something like this.

Interestingly, he mentions some kits that physically modify the valve body (one of my ideas for increasing pressure).

It seems there are many hydraulic pressure vs throttle position curves in the manual; I also wondered if the throttle position sensor (TPS) can be used to increase the line pressure artificially; Solder mentioned this in a later post (same thread) too. Though just plainly telling the TCM that the throttle is depressed more than it really is would cause hard and high RPM shifts (hopefully along with more oil pressure).

I only read the first few pages of that thread. That Solder guy is providing a lot of information; too much for me to weed through and understand. There are 32 pages and he is still adding to it in 2020.
 

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It seems there are many hydraulic pressure vs throttle position curves in the manual; I also wondered if the throttle position sensor (TPS) can be used to increase the line pressure artificially; Solder mentioned this in a later post (same thread) too. Though just plainly telling the TCM that the throttle is depressed more than it really is would cause hard and high RPM shifts (hopefully along with more oil pressure).
Maybe not hard shifts if the line pressure is brought up to normal. Also, not high RMP shifts but maybe early shifts? I'm not sure. I'd be interested to hear what effects people think maladjusting the TPS would cause in this scenario.
 

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On the topic of artificially increasing the line pressure to compensate for deficient pumping:

Reading more of Solder's posts on R31 and following links from that thread and other searches, I came across a thread on patrol4x4.com (Line Pressure Solenoid Dropping Resistor) which talks about the dropping resistor and how it affects the line pressure solenoid. If the throttle is not depressed, the voltage across the solenoid is battery voltage which opens the solenoid to max (low pressure). If the throttle is depressed, a different line to the solenoid is activated (the one with the dropping resistor). The more the throttle is depressed, the lower this line voltage is set thus the more closed the solenoid is (higher pressure). If you disconnect the resistor (open circuit), you'll get maximum pressure at any throttle position greater than idle.

So, the idea is to put a larger resistor here thus causing generally increased line pressure. They do it to overcome a soft 2->3 shift. I would do it to try to regain pressure overall. The dropping resistor on my 2004 Xterra SC 4x4 is bolted to the body next to the battery (long slender metal case with a connector that feeds into the nearby wiring harness).

Plan:
Check shift lever and make sure my TPS is adjusted properly (I messed with it 9 months ago).
Pressure test at stall (still waiting on the test kit) though I expect a low pressure.
Pull the dropping resistor and test pressures to see if there is sufficient improvement.
If so, replace the resistor with a large power resistor. Maybe 4K Ohms, 1 W.

If this works as planned, there is still no doubt that the transmission has a problem and should be replaced/rebuilt. Hopefully this bandaid is something that could pull more mileage out of the vehicle which is hardly worth the cost of transmission repair.
 

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Discussion Starter #24
Interesting ,,, looks like you have been doing your homework !! If it works out for you im going to make sure i keep this link incase i need to try the same ,, but for me and my situation , well ,, i just gave in to the inevtiable and swapped out the trans .
312841
 

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That's awesome, good job. I wish I could if it weren't for the work/cost.

If my plan works though I'm going to have to ignore the rear main engine oil seal leak that I found while crawling around under there.
:cautious:
 

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More journaling:

I got the pressure tester. I actually previously tried a compression tester that had an M10 adapter (Compression Test Kit 8 Pc). I only got it in maybe 2 turns at most so I never trusted it and decided that maybe the transmission pressure test port was NPT instead of metric so I got this one: Advanced Tool Design Model ATD-5550 Automatic Transmission and Engine Oil Pressure Gauge Kit.

I tried to put the NPT fittings in and they wouldn't go in at all. Luckily, it has an M10-1.0 to 1/8 NPT adapter, I tried that also and it only went in a couple turns again. Not sure what to do. According to this (Update: Transmission Temperature Gauge Tech Info) thread, it seems like this is supposed to be the correct size though I don't know what year/transmission he has. I couldn't find the service manual suggested part (except maybe on an obscure Russian website). Maybe I'll just force it .... If anyone knows for sure of the thread size on this port on the RE4R01a, it would be nice to know.

While I was under there, I remembered some allen bolts, one is right above the reverse test port, I wonder what it is. The service manual lists some of the other ports as "chamber pressure" but no lable for this one. Any ideas would be helpful especially if my current plan doesn't work.

312891



312892
 

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Line Pressure Test Results (with and without dropping resistor):

I had an issue taking off the shift cable bracket to get at the forward gears test port (see here: Can't remove AT shift cable bracket) but finally was able to do a line pressure test. The results weren't what I expected in that the D, 2, 1 test port pressures were normal! I expected them to be low also.

REVERSE:
In reverse, at the "Test port for the R position" (aka "3 -> 4 & N -> R accumulator back pressure", see above), I found that when cold, the pressure could go up to 80 PSI idle and when giving gas it would bounce between 25 and 80 PSI. I could get movement on flat ground if over 25 PSI.

When warm, the R pressure was usually 0 to 10 PSI at idle, 0 to 25 PSI when giving gas and 65 PSI during stall test (I didn't do this but twice because I knew the clutches were slipping.)

When I unplugged the dropping resistor (which should give maximum line pressure), there was virtually no change. Sometimes when giving gas, I'd get between 0 and 25 and sometimes up to 50. I didn't do a stall test with the resistor unplugged (didn't want to burn the clutches).

FORWARD:
Very normal. I only tested D selection. I had 70 PSI at idle, between 75 - 125 PSI when driving normal and 150 PSI at stall. With the dropping resistor removed, the shifts were crisp but not hard (only tested when warm). The pressure hung around 125 PSI and went to 150 PSI at stall.


Discussion:
I'm a little upset that pulling the resistor didn't increase pressure in the reverse line. It would be nice to have a temporary solution to this. It seems the pump is cranking enough to get up to 150 PSI so I would assume that we would see at least that pressure in the R line. I don't know that this rules out a problem with the pump but I'm less suspicious of it.

So, there is loss of pressure in the R line. What could cause this? Of all the solenoids, gaskets, valves, springs, steel balls, accumulators, orifices and tubes, I suppose it could be anything. I can focus on those things related to the R line though. One place to start may come from the manual where it calls the R test port "3 -> 4 & N -> R accumulator back pressure". I have not seen a problem with 3 -> 4 shift. Another aspect to investigate is why can I achieve higher pressure when the oil is cold?

Plan:
Now that I have a base line, I'm going to add K&W TransX (75K) and see if anything changes.
Study the R line hydraulic lines and components.
Pull the valve body and fish around for problems.
 

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Discussion Starter #30
Thanks for the update`s ! I`m thinking ( I could be wrong on all point`s ) that since your seeing pressure even though it`s low for the reverse circut , that atleast there is fluid flow , which might mean that the solinoids , accumulator piston , and valves , ect , for the reverse circut are doing their part , where the pressure goes when it warms up seems to be where the problem lies which could possibly mean a hard parts failure from this point , being the high clutch and the reverse friction & steel plates , seals maybe ??
 

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Yeah, a leak which wouldn't allow viscous oil through but once warm, more oil can get through causing even lower pressure. There just seems to be a hundred areas of failure. I don't have the knowledge to understand all the components (especially the pressure regulator valve, pressure modifier valve and the pilot valve) and how they respond to various inputs.

I'll have to focus on what I can fix myself if needed which is the valve body. If the problem is up in the gears, it'll be out of my hands. As someone's problem was fixed during a rebuild, I wondered if a valve body separator plate gasket is commonly included in those rebuild kits. That could still implicate the VB.
 

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Nice job on documenting everything so far it really helps!

I have a '02 xterra 2wd with 250k miles that was starting to slip. I decided to drop it and rebuild and actually just got it back in the truck yesterday with no issues. The rebuild kit I got is the ATP NM-20 found in rock auto. That kit does include the gasket for the valve body, but it is a paper gasket. The one in my valve body was metal so I just reused it. As far as the rebuild goes, it isn't that bad honestly the worst part of the whole job was removing it from the vehicle and then reinstalling it. BTW you can check the seals on the clutches by using compressed air once you remove the valve body. This is something you could potentially do while it is still in the car, just need to know which ports actuate the clutches you are testing and then listen for the movement of the clutch pistons.
 

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Thanks. It looks like replace/rebuild may be inevitable but of course I want to be sure that it isn't something that can be fixed otherwise. I was about to head out to the garage to drop the VB and test the reverse ports with compressed air when I saw your comment. When looking at the schematics, I don't know if these ports come FROM the pump or if they go TO the clutch. Any idea? As such, I'm not sure how to interpret the results; is air going into the clutch or into the pump?

I've been confused that there isn't just one pressure line coming from the pump that the valve body then sends where needed. Instead, there are all these pressure lines from the pump but the schematic doesn't say how they get to the valve body or alternatively, how the pressure gets to the clutch.

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Yes I would agree with that. As I understand, the oil pump will supply the VB with oil pressure and then the VB will distribute that pressure to the desired clutches through the channels on the housing. That would mean that the ports you see on the housing underneath the VB will lead to the clutches. There are both pressure channels and lubrication channels. There is a pressure regulator and multiple valves within the VB that will distribute that oil where it needs to go. It's very possible that if you are not loosing any pressure through any of the clutch seals, that the problem may lie within your valve body. Have you checked any junkyards local to you? If you suspect the VB is the problem, you could pull one from a car at the junk yard and swap that in to see if the problem is still present.
 

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Out of frustration, I stepped away for a while. When I went out to pull the valve body and test the clutches with air pressure, I realized I couldn't drop the oil pan because the transmission mount was covering 2 of the bolts. Remember, I'm on a 2004 Xterra 4x4; I don't know if this is different on your Frontiers. I would have to support the transmission, pull the torsion bars and pull the cross-member just to get to the valve body. I feel like if I do that much, I might as well just drop the whole thing.

I decided to blow some air in the external R pressure test port. With the engine off, I shifted into R. When I applied air, I heard fluid being squirted into the pan. I did it again and the dip stick popped off and ATF squirted out all over that side of the engine bay. I have no idea if this is meaningful.

It'll need to be rebuilt. I'm still not sure if I'll do it myself or have a shop do it. If it wasn't my only vehicle, I would be more inclined to do it myself. Regardless, when it is rebuilt, I will or I'll have someone do the reverse clutch pressure test shown in the manual before and after being rebuilt. This should help be sure of the problem.
 

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Discussion Starter #36
Oh yeah , you can get the pan off without all that work , the pic`s in my 2nd post on the first page show the pan off and valve body fully exposed with the underside of the truck and front drive shaft in view ,, i " think " i just loosend the trans mount enough to pry it back so i could get the socket on the bolts and i might of used the floor jack to raise it a bit ,, totally do-able !!
 
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