#1  
Old 10-10-2022, 05:22 AM
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A390GT A390GT is offline
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Default Noticeable deceleration at highway speed

I replaced the vacuum modulator on the 68 Catalina’s turbo 400 because it wouldn’t shift out of 1st gear.
Now it shifts fine through all the gears.
I took it yesterday to go apple picking and took the highway for the first time. At highway speed when Id take my foot off the gas the car would decelerate instead of coasting along.

It’s sorta like engine braking… the engine rpm goes up… I had to keep my foot on the gas all the time and I never had to do that before.

Is this the replacement vacuum modulator or something else????
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Old 10-12-2022, 01:30 PM
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Also, if you shift into neutral at highway speeds does the car coast along normally? You want to eliminate other possibilities like dragging brake, wheel bearing or diff issue. if those check out then it could be a modulator issue as you suspect.

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  #3  
Old 10-12-2022, 05:44 PM
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A390GT A390GT is offline
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North,
No brake issues or stuck wheels…
It coasts fine when I put it in neutral.
I thought the vacuum modulator was just for up shifts. Why is it hanging? Is it the torque converter staying locked in 3rd or something?

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Old 10-12-2022, 09:43 PM
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Is your shift selector on the steering column really in D? If the indicator is out of adjustment you could be in S (second?)

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My Break Away Squad
1969 Fbird (Base, 350 & Sprint Cvt’s - 400HO & TA Hardtops)
1969 LeMans (2dr & 4dr Hardtop and a Cvt)
1969 LeMans Safari 2 seat Wagon
1969 GTO (2 Cvt, 2 Hardtops & Judge Hardtop)
1969 Catalina (3 Cvt’s & a 2dr hardtop)
1969 Ventura 2 Seat Wagon
1969 Executive 4dr Sedan
1969 Bonnie Cvt
1969 Bonnie 3 Seat Wagon (2 of them)
1969 Bonnie Brougham (4dr Hardtop & Cvt)
1969 Grand Prix SJ (2 of them)
1969 2+2 2dr Hardtop (Canadian model)
  #5  
Old 10-12-2022, 10:02 PM
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I just looked in the 68-69 factory diagnosis manual and they have your issue…

The easiest thing to check is B, which is on the gas pedal area, make sure the plunger pops out freely.

If that is good then test the lockdown solenoid (engine off but key in run and shifter in D. Giving 12V to the solenoid connector should create a clunk noise from the bottom of the trans you can hear if you are under the car.

If these two check out and another modulator doesn’t fix it (maybe your new one is defective or incorrect) trip to a tranny shop may be your next step.
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__________________
My Break Away Squad
1969 Fbird (Base, 350 & Sprint Cvt’s - 400HO & TA Hardtops)
1969 LeMans (2dr & 4dr Hardtop and a Cvt)
1969 LeMans Safari 2 seat Wagon
1969 GTO (2 Cvt, 2 Hardtops & Judge Hardtop)
1969 Catalina (3 Cvt’s & a 2dr hardtop)
1969 Ventura 2 Seat Wagon
1969 Executive 4dr Sedan
1969 Bonnie Cvt
1969 Bonnie 3 Seat Wagon (2 of them)
1969 Bonnie Brougham (4dr Hardtop & Cvt)
1969 Grand Prix SJ (2 of them)
1969 2+2 2dr Hardtop (Canadian model)
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