Line pressure issue

[RmvBfrFlght]

OK, the question is: why does the line pressure fall off as the RPM increase. Seems to start at around 2400 RPM.

This is a 200-4R, modified/built, has a stock pan with the CK spacer and the 700R4 filter, .555 main boost and a .284 low/rev boost valve. Trans-go kit with the proper TV line up. Line pressure at 1100 RPM is as follows:

Note that I used the ECM to increase the idle speed, so the idle position is with the TV connected and the throttle body on the idle stop screw. The min is TV disconnected, max with it pulled all the way out.

  Idle  Min  Max
P   89   80  220 psi
R  140  125  210
N   89   80  215
4   89   80  200
3   89   80  200
2  195  190  195
1  195  190  180

However, at a higher RPM that max pressure isn't reached. Here is a graph of a data log showing this:

Note how the line pressure starts to fall off at the RPM climbs to the 2400 RPM range. The trans downshifts on the 100% TPS into 1/2 with the pressure falling even further. Why?

The gear selection stick was in D3 the whole time. I'm rather stumped as I'm no expert on transmissions.

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[No disintegrations]

Strange how the pressure drop mirrors rpm/boost but at frame 70 begins to fall off as expected. I'm no trans expert but it looks like the tv is unhooked in those frames. internal problem with tv?

 

[RmvBfrFlght]

Strange how the pressure drop mirrors rpm/boost but at frame 70 begins to fall off as expected. I'm no trans expert but it looks like the tv is unhooked in those frames. internal problem with tv?

The TV cable is connected and properly adjusted. It is definitely RPM related as I have a few other data grabs that show the same thing without boost. Can see this in the above graph, look at the very beginning as the RPM goes to 2500, the line pressure is dropping off.

The trans then shifts into 3rd bringing the RPM down. With an increase in line pressure.

Is the pump cavitating? Filter too restrictive? I also suspect the pump priming spring (slide spring). Maybe weak or possibly broken.

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[David Husek]

You have way too low pressures to start off with. You have something bad going on in the VB and pump. Stock calibration has more line pressure than you have after all your mods. I would go back to stock. Pressure dropping off could be your fluid level is most likely low and the filter is leaking at the seal along with all the other issues.

 

[bison]

Make sure the line bias valve has a spring and is the correct valve for the bore. I had a trans a few months back that I had taken the line bias spring and used in another trans. Erroneously it wound up in my car and would flat line around 150psi. Adding the spring it was 275+. If dropping at high rpm and everything else is good I'd check the slide spring. The pump is trying to go into decrease as the rpm increases and the spring holds the slide to prevent a drop in output.


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[gunzandgearz]

Check the pressure reg. valvetrain in pump for any valves hanging or being sticky before you pull trans. out and apart, as this might be the problem. If this doesn't pan out you can check valves in valve body for the same issue hangy or sticky valves and if this doesn't pan out its probably an internal hydraulic leak and going to have to go back to your trans. guy

 

[RmvBfrFlght]

You have way too low pressures to start off with. You have something bad going on in the VB and pump. Stock calibration has more line pressure than you have after all your mods. I would go back to stock. Pressure dropping off could be your fluid level is most likely low and the filter is leaking at the seal along with all the other issues.

Appreciate the input. This trans is a hybrid with an AA case and a BQ3 VB. Not sure what the other internals are. It was built many years ago by a pro. I recently swapped the main boost valve for the larger one (from .500 to .555) and corrected the TV line up. This bumped the line pressure up some and provided for the proper pressure rise (at least at 1100 RPM) and TV cable adjustment.

Blocked the line bias valve, 1/8" X .860" steel rod inside of spring.

It had a 700R4 filter in it so I added the CK spacer and a new 700R4 filter. Used two of the ribbed 700 style seals on the filter neck (same as it was). The trans is overfilled to the second bend on the dip-stick.

The pressure drop with increased RPM was there before I made these changes.

> You have something bad going on in the VB and pump.

The pump makes sense, any ideas on what could be wrong in the VB?

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[RmvBfrFlght]

Make sure the line bias valve has a spring and is the correct valve for the bore. I had a trans a few months back that I had taken the line bias spring and used in another trans. Erroneously it wound up in my car and would flat line around 150psi. Adding the spring it was 275+. If dropping at high rpm and everything else is good I'd check the slide spring. The pump is trying to go into decrease as the rpm increases and the spring holds the slide to prevent a drop in output.

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I have the line bias valve blocked, dropped a 1/8" X .860" steel rod inside the spring and reassembled.

As for the slide spring, I too am thinking that and the pump is worn. As you posted the slide spring works against the PR oil to maintain the proper pressure/volume (I've been studying the principle of op book).

If can get this fixed I may get a few more miles out of the trans. At the same time I don't want to freshen it and still have this issue. From time-to-time It does cause a mild slip as I have seen the RPM spike a little.

Thanks for the input.

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[David Husek]

Appreciate the input. This trans is a hybrid with an AA case and a BQ3 VB. Not sure what the other internals are. It was built many years ago by a pro. I recently swapped the main boost valve for the larger one (from .500 to .555) and corrected the TV line up. This bumped the line pressure up some and provided for the proper pressure rise (at least at 1100 RPM) and TV cable adjustment.

Have you tried another test gauge, I had a customer call will a pressure problem complaining and his gauge was off by 100#. Just something simple to start with.

Blocked the line bias valve, 1/8" X .860" steel rod inside of spring.

I do not like to see the line bias circuit blocked. You want it to move it it needs too.

It had a 700R4 filter in it so I added the CK spacer and a new 700R4 filter. Used two of the ribbed 700 style seals on the filter neck (same as it was). The trans is overfilled to the second bend on the dip-stick.

This is good, usually the first bend is where I recommend but if you are not puking fluid, let it ride.

> You have something bad going on in the VB and pump.

The pump makes sense, any ideas on what could be wrong in the VB?

I would check the spring in the TV limit valve. I have seen them wear against the housing and break. You mentioned a few kits installed. I would leave the VB stock except for a helper spring in the line bias. Pull the pump, open it, check the pump halves for wear. I have seen the slide break at the pivot pin and still work with similar issues. The ear on the slide could have broke off from bouncing. You can contact me directly if needed.

 

[RmvBfrFlght]

Have you tried another test gauge, I had a customer call will a pressure problem complaining and his gauge was off by 100#. Just something simple to start with.

I started with an old/new made in USA 3" 0 - 300 psi mechanical gauge. Have since installed a 0 - 300 psi Honeywell industrial pressure sensor. This is what was used for the above graph. The mechanical gauge shows the pressure to be between 5 and 10 psi higher then the sensor.

I do trust the Honeywell sensors, but I don't have any way to test them.

I do not like to see the line bias circuit blocked. You want it to move it it needs too.

Interesting, I'm not sure I can get the pin out without dropping the VB. But I'll see what I can do. If so I'll remove the blocking rod and get a helper spring to install.

I would check the spring in the TV limit valve. I have seen them wear against the housing and break. You mentioned a few kits installed. I would leave the VB stock except for a helper spring in the line bias. Pull the pump, open it, check the pump halves for wear. I have seen the slide break at the pivot pin and still work with similar issues. The ear on the slide could have broke off from bouncing. You can contact me directly if needed.

The TransGo SK200 kit was installed except with a Superior servo at the time it was rebuilt (not by me). Even has the Guppy plate.

It looks like i may be able to get the TV limit retaining sleeve out of the VB without dropping it. If so I can easily check it. But as you mention my bet is that the pump has issues. If the TV limit spring is OK then the trans comes out. Otherwise replace the spring and see if that helps.

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[David Husek]

It looks like i may be able to get the TV limit retaining sleeve out of the VB without dropping it. If so I can easily check it. But as you mention my bet is that the pump has issues. If the TV limit spring is OK then the trans comes out. Otherwise replace the spring and see if that helps.

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The TV limit sleeve can only be removed with VB on the bench. You can check its function while installed as the end of it is visible. Use a pick to see if will move and return.

 

[RmvBfrFlght]

OK, will do, thanks.

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[RmvBfrFlght]

Quick update, the TV limit valve snaps back with force. So unfortunately the spring is likely OK. It will be a bit before I drop the trans as I need to get another car running first. I'll post as I find out what is going on.

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