Throttle cable vs vacuum modulator
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From: Comox, BC
Throttle cable vs vacuum modulator
Can someone explain the interaction between the actions of these 2 auto-trans controls. When I got the car a few months ago, the shifts were late and hard, with the shift into OD requiring throttle lift even above 60 mph. Upon checking things, I found that the throttle cable was really tightened up, and also found a vacuum leak from the body of the modulator. I backed off the throttle cable and that lowered the shift points to a suitable RPM and firmness, except for the 3-4 shift, which slips noticable. Any ideas / instructions??
Oh yes........ 1986.5 car 73k miles.
Not being able to get a manual really sucks!! (especially after paying $300.00 for a ghost set!!)
Oh yes........ 1986.5 car 73k miles.
Not being able to get a manual really sucks!! (especially after paying $300.00 for a ghost set!!)
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Joined: Jun 2001
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From: Ball Ground, GA
Unlike an American transmission, the vacuum modulator on a 928 has little or no effect on shift speed. The modulator is adjustable, but affects shift quality only.
The throttle cable affects both the shift speed and quality, but is primarily used to adjust the shift speed.
The full-throttle shift speed is adjusted on the valve body inside the transmission.
The kickdown switch (standard one under the throttle pedal or an added manual switch) affects both the speed and quality of the shifts.
The throttle cable affects both the shift speed and quality, but is primarily used to adjust the shift speed.
The full-throttle shift speed is adjusted on the valve body inside the transmission.
The kickdown switch (standard one under the throttle pedal or an added manual switch) affects both the speed and quality of the shifts.
Three Wheelin'
Joined: Jan 2002
Posts: 1,560
Likes: 2
From: Maryland
Brian,
I'm living this as we speak. I hope to *someday soon* have my problem solved as I have the replacement parts, but am awaiting some R/R guidence and photos from a very kind trans guru.
The cable will determine the "lateness" of the shift. It should be adjusted for no slack, but not to the point where it is pulling.
The modulator regulates control pressure. If you had a vacuum leak to the modulator, that was the cause of the hard shifts.
Now, for the 3/4 slip (flare-up). This is the problem with my car. I have the modulator adjusted for proper working pressure, as indicated on my pressure guage. 1st-2nd and 2nd-3rd shifts are a bit firm and on the "chirpy" side. (my 12 year old daughter thinks the occassional tire bark from 1-2 is cool!) But my 3-4 shift usually has a momentary slip to it.
The anticipated remedy in my case, is to remove the valve body and replace the seals, B2 servo, bushing and install a shift kit. There is an updated B2 servo which may requre the replacement of the associated bushing. I also have a new filter and gasket waiting installation. My guess is that I'm sitting on about $300.00 in parts, but I cannot tell you if this will solve the problem until I get mine done.
When I first noticed the symptom, I replaced the modulator, fluid and filter. No real improvement. It is interesting to note that the condition has not become any worse, even though I drive the car daily and have put maybe 4,000 miles on it since the symptom appeared. It's exactly the same as it was in the beginning. I watch the fluid level carefully.
If your 1st-2nd and 2nd-3rd shifts are good, and only 3-4 has a slip, you may be in the same boat I am. First, make sure the fluid is at the proper level. Second, you may want to bring up the control pressure some by giving the t-handle on the modulator (behind the rubber cap, near the vacuum line, pulls out slightly) 1/2 to 1 turn clockwise. The 1-2 shift should be a little firmer, the 3-4 may or may not show any inprovement, but keeping the pressure up is important until you get the problem solved.
I'm going to post the results on my trans service as soon as I (start, and) complete it. If it's successful, you may need to do the same to yours.
Greg
I'm living this as we speak. I hope to *someday soon* have my problem solved as I have the replacement parts, but am awaiting some R/R guidence and photos from a very kind trans guru.
The cable will determine the "lateness" of the shift. It should be adjusted for no slack, but not to the point where it is pulling.
The modulator regulates control pressure. If you had a vacuum leak to the modulator, that was the cause of the hard shifts.
Now, for the 3/4 slip (flare-up). This is the problem with my car. I have the modulator adjusted for proper working pressure, as indicated on my pressure guage. 1st-2nd and 2nd-3rd shifts are a bit firm and on the "chirpy" side. (my 12 year old daughter thinks the occassional tire bark from 1-2 is cool!) But my 3-4 shift usually has a momentary slip to it.
The anticipated remedy in my case, is to remove the valve body and replace the seals, B2 servo, bushing and install a shift kit. There is an updated B2 servo which may requre the replacement of the associated bushing. I also have a new filter and gasket waiting installation. My guess is that I'm sitting on about $300.00 in parts, but I cannot tell you if this will solve the problem until I get mine done.
When I first noticed the symptom, I replaced the modulator, fluid and filter. No real improvement. It is interesting to note that the condition has not become any worse, even though I drive the car daily and have put maybe 4,000 miles on it since the symptom appeared. It's exactly the same as it was in the beginning. I watch the fluid level carefully.
If your 1st-2nd and 2nd-3rd shifts are good, and only 3-4 has a slip, you may be in the same boat I am. First, make sure the fluid is at the proper level. Second, you may want to bring up the control pressure some by giving the t-handle on the modulator (behind the rubber cap, near the vacuum line, pulls out slightly) 1/2 to 1 turn clockwise. The 1-2 shift should be a little firmer, the 3-4 may or may not show any inprovement, but keeping the pressure up is important until you get the problem solved.
I'm going to post the results on my trans service as soon as I (start, and) complete it. If it's successful, you may need to do the same to yours.
Greg
