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????

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.

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?

Is your shift selector on the steering column really in D? If the indicator is out of adjustment you could be in S (second?)

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.