Line pressure drops with increased throttle
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Joined: Mar 2023
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From: BERWICK PA
Line pressure drops with increased throttle I am working with a guy who has a trans from me, He has had three previous ones from others all lost the 3-4 clutch, I know at least one as I talked to the builder was a pump issue.
But here is the issue with mine, Because he has had repeated failures some of the builds from well known builders, I elected to use the transgo 46MOD with his, Now he has installed and at light to normal throttle the shifts are good , But at wide open slips cannot make the shift. I had him check the line pressure and oddly it it around 170 psi at idle but as you give throttle it actually drops to as low as 90 psi at WOT. Completely reversed to what it should do.
He has the vacuum teed into the brake booster line.
He is not blown or boosted.
He does have a pretty choppy cam.
The reaction to the throttle is as if the vacuum is increasing with throttle rather than dropping as it should.
Honestly it is rather baffling to me . He does not have a vacuum gage. While I would expect semi low vacuum at idle with large cam, I cannot explain the increase (I think in vacuum with throttle that I can only imagine is happening.)
Today he is going to retest with the vac line unhooked . I will update once I have those numbers, Just what I am seeing is a bit baffling.His line reading are literally what I would expect at WOT at idle and decrease to what i would expect to see at idle at with the vac mod setup,
Open to thoughts and ideas ?
NOTE I AM NOT A HUGE FAN OF THE 46MOD but with his several previous issues with other builders trans it seemed like the best option but this outcome was unexpected. . I am thinking of having him move the vac to a port directly on the intake incase maybe he has some sort of one way valve installed in the vac for the brake booster due to the cam ?
It is possible trans is already damaged if so i will fix it for free but do need to solve this else it will just repeat before doing that.
PS SORRY for misspelled on title but wont let me change it so it is what it is.
FRANK : I updated your title. After selecting "Edit", select "Go Advanced" and you can then edit the title too.
But here is the issue with mine, Because he has had repeated failures some of the builds from well known builders, I elected to use the transgo 46MOD with his, Now he has installed and at light to normal throttle the shifts are good , But at wide open slips cannot make the shift. I had him check the line pressure and oddly it it around 170 psi at idle but as you give throttle it actually drops to as low as 90 psi at WOT. Completely reversed to what it should do.
He has the vacuum teed into the brake booster line.
He is not blown or boosted.
He does have a pretty choppy cam.
The reaction to the throttle is as if the vacuum is increasing with throttle rather than dropping as it should.
Honestly it is rather baffling to me . He does not have a vacuum gage. While I would expect semi low vacuum at idle with large cam, I cannot explain the increase (I think in vacuum with throttle that I can only imagine is happening.)
Today he is going to retest with the vac line unhooked . I will update once I have those numbers, Just what I am seeing is a bit baffling.His line reading are literally what I would expect at WOT at idle and decrease to what i would expect to see at idle at with the vac mod setup,
Open to thoughts and ideas ?
NOTE I AM NOT A HUGE FAN OF THE 46MOD but with his several previous issues with other builders trans it seemed like the best option but this outcome was unexpected. . I am thinking of having him move the vac to a port directly on the intake incase maybe he has some sort of one way valve installed in the vac for the brake booster due to the cam ?
It is possible trans is already damaged if so i will fix it for free but do need to solve this else it will just repeat before doing that.
PS SORRY for misspelled on title but wont let me change it so it is what it is.
FRANK : I updated your title. After selecting "Edit", select "Go Advanced" and you can then edit the title too.
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Frank formerly of Performabuilt, Now just me, What can I build for you today? Call or message me. Click sig pic for my facebook
Frank formerly of Performabuilt, Now just me, What can I build for you today? Call or message me. Click sig pic for my facebook
Last edited by mrvedit; Jan 8, 2025 at 04:58 PM. Reason: Update title
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Joined: Jan 2019
Posts: 677
Likes: 490
From: Roxana, IL
You don't say what engine this transmission is attached to. Is it possible for it to be hooked to ported vacuum? It must have 15"hg minimum at the modulator. "0" would be high line pressure and the 15"hg should be 70-90psi that would still allow a minimum throttle 1-2 shift.
This theory is why I get so mixed up over map readings...
This theory is why I get so mixed up over map readings...
Thread Starter
LS1Tech Sponsor
Joined: Mar 2023
Posts: 1,821
Likes: 1,156
From: BERWICK PA
You don't say what engine this transmission is attached to. Is it possible for it to be hooked to ported vacuum? It must have 15"hg minimum at the modulator. "0" would be high line pressure and the 15"hg should be 70-90psi that would still allow a minimum throttle 1-2 shift.
This theory is why I get so mixed up over map readings...
This theory is why I get so mixed up over map readings...
__________________
Frank formerly of Performabuilt, Now just me, What can I build for you today? Call or message me. Click sig pic for my facebook
Frank formerly of Performabuilt, Now just me, What can I build for you today? Call or message me. Click sig pic for my facebook
Last edited by FranksCustomTrans; Jan 3, 2025 at 05:51 PM.
