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| Transmission Pressure [message #305260] |
Wed, 10 August 2016 21:48  |
George Beckman
Messages: 1085
Registered: October 2008
Location: Colfax, CA
Karma: 11
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Senior Member |
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Does anyone have a transmission pressure gauge on their GMC? I am looking for someone to compare notes with, in actual driving. I am not so much interested in on the stand type tests, or front wheels jacked up sorts of tests, in the GM manual. I want to compare actual driving pressures. The things I am seeing cannot be seen in GM Shop Manual tests or on the test stand.
The one instance I am particularly interested in is the pressure in Drive when under full deceleration. I have textbook pressures when driving except when I let off the gas. When I let off the gas, at any speed from 80 to 15 mph, my pressure suddenly drops to about 52 lbs.(Please note that at times, the RPMs are 3000 and pressure drops to 52.) It remains there until about 15 mph when it suddenly jumps back to about 65 or 70, depending on the heat of the oil. I assume the transmission has downshifted.
My gauge has a red light at 50, and there have been times deceleration has caused the light to come on. My concern is the rear (first out) band in my transmission has slipped and burned out twice. When it does, there is no engine braking. In other words, when you let off the gas at 65, the RPMs go to idle. My pea brain tells me this band is keeping the engine and wheels at speed when decelerating. I wish I had had a gauge on it when that band gave out. I also fear 52 lbs is not enough to keep the band tightly engaged.
I have also noted that in neutral, pressure is normal 65-75 depending on oil temp, and when I put it in drive it takes a sudden drop that turns on the light. I assume this is torque converter, pistons and clutches sucking up oil for a moment and the accumulator cannot keep up.
And yes, I asked Manny about the 52 lbs. He says, "That's too low." I agree.
'74 Eleganza, SE, Howell + EBL
Best Wishes,
George
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| Re: [GMCnet] Transmission Pressure [message #305289 is a reply to message #305260] |
Thu, 11 August 2016 12:55  |
powerjon
Messages: 2446
Registered: January 2004
Karma: 5
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Senior Member |
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George,
Use the factory TH425 manual, link below. It has all the necessary pressure information for the transmission. It provides a systematic approach to diagnosing the TH425 problems.
http://www.bdub.net/manuals/THM_425_Trouble_Shooting.pdf
JR Wright
GMC Great Laker MHC
GMC Eastern States Charter Member
GMCGL Tech Editor
GMCMI
78 GMC Buskirk 30’ Stretch
1975 GMC Avion (Under Reconstruction)
Michigan
> On Aug 10, 2016, at 10:48 PM, George Beckman wrote:
>
> Does anyone have a transmission pressure gauge on their GMC? I am looking for someone to compare notes with, in actual driving. I am not so much
> interested in on the stand type tests, or front wheels jacked up sorts of tests, in the GM manual. I want to compare actual driving pressures. The
> things I am seeing cannot be seen in GM Shop Manual tests or on the test stand.
>
> The one instance I am particularly interested in is the pressure in Drive when under full deceleration. I have textbook pressures when driving except
> when I let off the gas. When I let off the gas, at any speed from 80 to 15 mph, my pressure suddenly drops to about 52 lbs.(Please note that at times,
> the RPMs are 3000 and pressure drops to 52.) It remains there until about 15 mph when it suddenly jumps back to about 65 or 70, depending on the heat
> of the oil. I assume the transmission has downshifted.
>
> My gauge has a red light at 50, and there have been times deceleration has caused the light to come on. My concern is the rear (first out) band in my
> transmission has slipped and burned out twice. When it does, there is no engine braking. In other words, when you let off the gas at 65, the RPMs go
> to idle. My pea brain tells me this band is keeping the engine and wheels at speed when decelerating. I wish I had had a gauge on it when that band
> gave out. I also fear 52 lbs is not enough to keep the band tightly engaged.
>
> I have also noted that in neutral, pressure is normal 65-75 depending on oil temp, and when I put it in drive it takes a sudden drop that turns on the
> light. I assume this is torque converter, pistons and clutches sucking up oil for a moment and the accumulator cannot keep up.
>
> And yes, I asked Manny about the 52 lbs. He says, "That's too low." I agree.
> --
> '74 Eleganza, SE, Howell + EBL
> Best Wishes,
> George
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J.R. Wright
GMC GreatLaker
GMC Eastern States
GMCMI
78 30' Buskirk Stretch
75 Avion Under Reconstruction
Michigan
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| Re: [GMCnet] Transmission Pressure [message #305303 is a reply to message #305289] |
Fri, 12 August 2016 01:17 |
George Beckman
Messages: 1085
Registered: October 2008
Location: Colfax, CA
Karma: 11
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Senior Member |
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powerjon wrote on Thu, 11 August 2016 10:55George,
Use the factory TH425 manual, link below. It has all the necessary pressure information for the transmission. It provides a systematic approach to diagnosing the TH425 problems.
Thanks.
Unfortunately, I don't have any symptoms... I just know what has happened "later", two times, and trying to figure out why. The best are puzzled, so I put in a gauge and was hoping someone had actual driving pressures rather than wheels in the air pressures. The one 30 mph (page 2) in Drive indicates a minimum of 55 which is why I am concerned by 52. Logic tells me that that 52 at 30 shouldn't be 52 at 70.
Throttle (vacuum) and modulator (adjustable) change pressures tremendously but nothing changes the 52 when the throttle is closed in my case.
'74 Eleganza, SE, Howell + EBL
Best Wishes,
George
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