Hi guys
Just finished major suspension upgrades : coils, shocks, rear sway bar on a 69 Cutlass convertible. Took it out of the driveway for a test drive up and down the street and heard a clunk upon stopping. Thinking it was suspension related I checked things over not finding anything overlooked. Took it out again and noticed that downshifting to a stop from second there is no issue, however downshifting from third to a stop develops a clunk every time. What's up and what did I do to cause it as it did not happen before.

 

Sounds like your transmission modulator hose is leaking or connected to ported vacuum.

 

Thanks, you made me think because another thing I did before the test drive was change the intake manifolds. Could not wait to find out as the sun was setting so went out with a flash light and saw that there are to vacuum lines going to the front of my carb: one for vacuum advance and the other to the tranny. Whether it is attached properly down below will have to wait until tomorrow. Thanks for the thought. I'll let you know.

 

You may have the 2 lines at the front of the carb swapped. Seen it before with the exact symptoms you described.

 

If it was the modulator you would have issues upshifting as the modulator controls line pressure you would have issues across the board. The ported vac. Source might cause an issue but I have never personally ran across that issue with putting the vac. Lines to the carb for the trans ., I usually go from the manifold.

 

There would not be any upshifting problems due to the fact that once your out of the idle circuit you have vacuum.

 

Thanks guys,
Much to consider. Will check ASAP.

 

I have had my line pop off but I never had harsh downshifts just the banzai up shift.

 

Eric,
You were right about the swap of the ported and full time ports. Had them wrong although I haven't taken it for test drive yet. Raining too heavily. Hope that's what it is.

John

 

Let us know

 

With the vac line properly hooked the car now does not shift into third. I ran the history of this situation passed a very reputable local tranny shop and was told it is the modulator. THey sell them in there store front. He claims the one they sell is adjustable via a screw driver so as the proper vacuum can be dialed in. Getting it today and will report ASAP

 

Is the modulator line sucking trans fuild into the carb creating a bunch of smoke out the exhaust? Typical symptom of a bad modulator. Sure hope it is something simple.

 

I've solved the problem by doing the following things.
1. Changed the vacuum hose on the top end of the metal vacuum line to carb port.
2. Swapped out old madulator for a new AC Delco unit and new hosing.
3. Maximized engine vacuum by retune of timing and carb.

Thanks All for your input.

www.oldsmobilesforever.com

John C.

 

Glad it was nothing serious and you got it corrected.