Cadillac Repair: 1968 coupe de ville transmission shifting, vacuum modulator, trouble shooting guide
QuestionMy 1968 coupe de ville has 38000 original miles. I recently had the trans serviced. The trans is now shifting at 45 mph and then 85 mhp. I do not want to take the car back to the mechanic that serviced the transmission in fear he will cause more trouble. I have the shop manual for the car and am currently going through the trouble shooting guide. I have not check the pressure yet. I have checked and replaced the vacuum modulator and the vacuum lines. I am going to check the downshift solenoid. Do you have any other suggestions where this problem might be coming from.
AnswerHi Jim,
The first thing to check is the fluid level which I'm sure you have done just to make sure it is in the correct level range. Correct as needed.
There are 2 things that allow the trans to shift. The governor will set the shift points when acceleration is lite to moderate so they are always at the same MPH or pressure setting. Then when you are accelerating hard the vacuum modulator takes over the shift points and makes the trans shift later so you build up speed quicker.
Since you already replaced the modulator without a change I would check the vacuum with a gauge at the engine where the trans line hooks up and then test it at the other end with the modulator disconnected. Maybe a line got pinched closed or the vacuum hose is split and not able to provide vacuum. Doublecheck that the replacement modulator isn't the kind with an adjusting screw where the vacuum hose hooks up. You may need to turn that screw to adjust for the correct shift point. I would remove the modulator again and make sure the valve moves freely in the bore.
The governor usually has an outside the trans cover that can be removed to remove the gov. I would suggest taking it out and see if there is any debris that might be causing it to stick and that the weight moves freely.
Hopefully one of those areas will get the shift points back to normal. Bill
