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Discussion Starter · #1 ·
Hello All:


Re: our 1967 Silver Shadow V8
  • we have had this vehicle for about three months, and it has run satisfactorily within the tranny context
  • two weeks ago, it started giving problems, the engine would not transition into "Drive" and would rev high until it "kicks" into Drive with a wham! Very disturbing...
  • I had the fluid checked and it was very low about 2 litres needed to be added, however, this does not resolve the problem.
  • Under the vehicle, I tightened all the bolts on the trans pan. ..... no improvement.
  • the vacuum modulator is intact.
Can anyone advise me if any of the above tips could be of help. OR, can add other advice or suggestions that could help.

Thanks so much for the contributions of the experienced members....there is so much knowledge in this community.

GAM
67 SS
79 Wraith
 

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Personally, I'd take it to your local transmission shop of choice. One thing that's not special or mystical in any way on these cars is the transmission. They were GM transmissions and any shop that is familiar with "old school" GM transmissions is perfectly capable of checking out and/or repairing the transmission in your car.
 

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What's the idle revs of the engine? Is pushing the gear selector to D causing the engine to rev high or are the engine revs high in P, before you move the selector to D?
 

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Have you rechecked the fluid level again after all the testing? If the torque-converter was empty it will have taken up lots of fluid from the gearbox. Just a thought. I've seen it before.

Alan D.
 

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Very important to check the trans fluid while in idle, at PARK after a run. If it revving, it is because it needs pressure build up to enable to valve body to work.
You may also have a vacuum leak at the vacuum line going to the modulator, but usually the cause is lo fluid. Overfilling can be a disaster.
 

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Personally, I'd take it to your local transmission shop of choice. One thing that's not special or mystical in any way on these cars is the transmission. They were GM transmissions and any shop that is familiar with "old school" GM transmissions is perfectly capable of checking out and/or repairing the transmission in your car.
I fully endorse this comment with one additional piece of information, the GM T400 transmissions fitted to the Shadows for certain and most likely the SZ series as well were specially modified with the fitment of a "wave" profile clutch disc plate as the first plate in the 1st gear clutch pack presumably to give a smoother take-up when starting from rest. I was advised by the NSW R-R/B Service manager that the clutch pack usually required servicing around 95,000 miles for cars mainly subject to stop/start city driving as the transmission was set-up to allow this pack to "slip" when the car started from rest to give imperceptible starts.
 

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Discussion Starter · #7 ·
What's the idle revs of the engine? Is pushing the gear selector to D causing the engine to rev high or are the engine revs high in P, before you move the selector to D?
Pushing to D does not cause the engine to rev high. Its revving from acceleration but not smoothly moving into upper D etc. Revs increase steadily until it slams into D gear.
Thanks for your question.
 

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Discussion Starter · #8 ·
Very important to check the trans fluid while in idle, at PARK after a run. If it revving, it is because it needs pressure build up to enable to valve body to work.
You may also have a vacuum leak at the vacuum line going to the modulator, but usually the cause is lo fluid. Overfilling can be a disaster.
Thanks Wraithman. I will check the levels again as you suggested. and also the vacuum modulator.
cheers
 

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Discussion Starter · #9 ·
I fully endorse this comment with one additional piece of information, the GM T400 transmissions fitted to the Shadows for certain and most likely the SZ series as well were specially modified with the fitment of a "wave" profile clutch disc plate as the first plate in the 1st gear clutch pack presumably to give a smoother take-up when starting from rest. I was advised by the NSW R-R/B Service manager that the clutch pack usually required servicing around 95,000 miles for cars mainly subject to stop/start city driving as the transmission was set-up to allow this pack to "slip" when the car started from rest to give imperceptible starts.
Thank you for this. However, your comment " the transmission was set-up to allow this pack to "slip" when the car started from rest to give imperceptible starts. " is a bit confusing........"allow this pack to slip" ?? Could you explain in more detail?
 

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Thank you for this. However, your comment " the transmission was set-up to allow this pack to "slip" when the car started from rest to give imperceptible starts. " is a bit confusing........"allow this pack to slip" ?? Could you explain in more detail?
The following basic explanation may help explain the role of clutch plate packs in automatic transmissions to replace the manual clutch used for vehicles fitted with conventional manual transmissions:

When first engaged, the wave plate has less initial contact area than a standard flat clutch plate when in contact with the adjacent flat plate in the "pack" which contains a number of separate plates which I have forgotten [a number between 10 and 15 plates keeps coming to mind]. The second gear clutch pack is similar but comprises flat plates only. These clutch packs are engaged and disengaged through the transmission hydraulic valve body in response to the driver's transmission gear selector setting thus allowing the automatic gear change without driver involvement.

The apparent reasoning behind RRMC's modification was the take up of a wave plate against the adjacent flat plate would take slightly longer than a conventional flat plate/flat plate arrangement thus reducing the shock pulse when the plates closing in response to the applied hydraulic pressure made contact. This reduced shock pulse would then be less perceptible to the driver and vehicle occupants each time the car moved from rest.

Keep in mind the Shadow was designed with the US market in mind with its major competitors being Cadillac and Lincoln, it is my understanding a slightly modified T400 transmission was fitted to the Cadillac to give smooth starts from rest and RRMC possibly had the idea of "out-Cadillacing Cadillac" with the Shadow in the lucrative and essential US luxury car market. Note all LHD Shadows destined for the targeted lucrative US Market were fitted with T400's for this reason whereas early RHD drive vehicles were fitted with the old GM 4 speed Hydramatic fluid coupling transmissions which carried over from the Silver Cloud series and which were noticeably less smooth and had more perceptible gear changes in operation than the newer torque convertor transmissions.
 

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Further to AlanD and Wraithman's comments above, further meditation on possible causes of the harsh gear change reminded me of a similar problem caused by a damaged/missing O-ring on the top of the transmission fluid pick-up pipe from the sump to the valve body.

If this O-ring cracks/splits or missing, suction from the transmission fluid pump can draw air into the transmission proper causing hard shifting/fluid hammer problems. Fixing this is a simple matter of removing the oil pan to access the fluid pick-up pipe and pulling it down to release it from the valve body then checking for a missing, cracked O-ring on the end of the pipe. If the O-ring is missing, check for a jammed O-ring in the valve body before replacing the O-ring and replacing the pick-up tube in the valve body.

Be careful to retorque the fluid pan retaining fasteners as specified in TSD 2476 Section T when replacing the pan - do not over-tighten.
 

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In addition to all this great information, Ronnie Shaver still has his 5 part transmission videos available for free on youtube.

 

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Hold it everyone. Are we talking about the early 4-speed or the later 3-speed transmission. 1967 is about the time of the switchover. What is the chassis number of the subject car? The gearbox fitted should be detectable from the parts catalogue.

Alan D. (Or am I talking rot?)
 

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Hold it everyone. Are we talking about the early 4-speed or the later 3-speed transmission. 1967 is about the time of the switchover. What is the chassis number of the subject car? The gearbox fitted should be detectable from the parts catalogue.

Alan D. (Or am I talking rot?)
Hi Alan,

All US-delivered LHD Shadows had the GM T400 3 speed torque convertor automatic transmission as a standard fitting.

We lesser mortals mainly in the British Commonwealth who drove on the left side of the road had to wait until the RRMC stock of the superseded GM Hydramatic fluid coupling auto transmission from the Cloud era were used up on the first RHD Shadows.

Regards David
 

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"the vacuum modulator is intact". Does that mean you simply noticed it was connected? or..was it removed and it's diaphragm tested for leakage?
 

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Discussion Starter · #17 ·
The following basic explanation may help explain the role of clutch plate packs in automatic transmissions to replace the manual clutch used for vehicles fitted with conventional manual transmissions:

When first engaged, the wave plate has less initial contact area than a standard flat clutch plate when in contact with the adjacent flat plate in the "pack" which contains a number of separate plates which I have forgotten [a number between 10 and 15 plates keeps coming to mind]. The second gear clutch pack is similar but comprises flat plates only. These clutch packs are engaged and disengaged through the transmission hydraulic valve body in response to the driver's transmission gear selector setting thus allowing the automatic gear change without driver involvement.

The apparent reasoning behind RRMC's modification was the take up of a wave plate against the adjacent flat plate would take slightly longer than a conventional flat plate/flat plate arrangement thus reducing the shock pulse when the plates closing in response to the applied hydraulic pressure made contact. This reduced shock pulse would then be less perceptible to the driver and vehicle occupants each time the car moved from rest.

Keep in mind the Shadow was designed with the US market in mind with its major competitors being Cadillac and Lincoln, it is my understanding a slightly modified T400 transmission was fitted to the Cadillac to give smooth starts from rest and RRMC possibly had the idea of "out-Cadillacing Cadillac" with the Shadow in the lucrative and essential US luxury car market. Note all LHD Shadows destined for the targeted lucrative US Market were fitted with T400's for this reason whereas early RHD drive vehicles were fitted with the old GM 4 speed Hydramatic fluid coupling transmissions which carried over from the Silver Cloud series and which were noticeably less smooth and had more perceptible gear changes in operation than the newer torque convertor transmissions.
Thank you again for all the detail in your explanation.
"the vacuum modulator is intact". Does that mean you simply noticed it was connected? or..was it removed and it's diaphragm tested for leakage?
Hello Wraithman
"the vacuum modulator is intact". Does that mean you simply noticed it was connected? or..was it removed and it's diaphragm tested for leakage?
Re: Replacing Vacuum Modulator

Hello Wraithman

First of all, a big thank you for all your time and feedback. You are a dedicated contributor to helping us fledglings in these forums.

I plan to replace the vacuum modulator on the transmission as one step to resolving my slipping issue (67 Silver Shadow).
Q. Will I need to drain the entire transmission fluid from the system? I am expecting that when I remove the vacuum mod., that there would naturally be a lot of fluid escaping.
Q. If draining is the first step, is there a drain plug specifically for the tranny and I assume I would locate it in the workshop manual.?

Anyway, that's my plan so if you have any other opinions/suggestions please let me know

Regards

GAM
 

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There should be no fluid upon removal. There is an O ring and bolt. Make sure there is a new elbow and clamp of some kind, either pinch or hose clamp.
Draining is removal of the pan and plan a small mess with absorbent mats or like. Best to loosen bolts on one end and tilt to drain a little. You will need the required amount of fluid as per the owners manual or workshop manual. This is a straight forward drain, replace filter and pickup O-ring, pan gasket and re-fill. Look up GM TH-400 re-fill on YTube.
 

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Thank you again for all the detail in your explanation.

Hello Wraithman


Re: Replacing Vacuum Modulator

Hello Wraithman

First of all, a big thank you for all your time and feedback. You are a dedicated contributor to helping us fledglings in these forums.

I plan to replace the vacuum modulator on the transmission as one step to resolving my slipping issue (67 Silver Shadow).
Q. Will I need to drain the entire transmission fluid from the system? I am expecting that when I remove the vacuum mod., that there would naturally be a lot of fluid escaping.
Q. If draining is the first step, is there a drain plug specifically for the tranny and I assume I would locate it in the workshop manual.?

Anyway, that's my plan so if you have any other opinions/suggestions please let me know

Regards

GAM
Most if not all US after-market auto parts outlets would have a sump drain conversion kit for the GM T400 auto transmission, I bought one from Summit Racing in the 1990's when I rebuilt the transmission on DRH14434 and fitted it to the standard oil pan. It was a simple exercise with the pan off the transmission just finding a clear space, drilling a hole and fitting the assembly.

I am a great believer in replacing all fluids including the auto transmission fluid on a regular basis.
 

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I think the idea behind no drain plug was to motivate you to replace the filter which is important and cleaning the magnet that is usually found within the pan. Notice the grey sludge...that's from the multitude of gears, plates, bands, etc. The drain plug makes draining so much easier and mess-free. I have seen cast pans with drains for TH400's.
 
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