Transmission question

[Michael_V]

It's been awhile since I have posted, but here goes.  My 89' Reatta with 170,000 miles has developed a strong bump when coming to a stop.  I initially thought it was a sticking caliper, but when I place the car in neutral and stop the issue goes away.  The car also seems to run better in neutral than when engaged.  Then when placing the car back into drive from a stop the car has a hard shift engaging.  The vehicle also seems to be racing at idle, and wanting to lurch forward from a stop.  Are there any ideas on what this could be?

[DAVES89]

Could be the transmission modulator. Or going farther the coil pack, cleaning of the idle air control [both the control and inside where the control goes].

[2seater]

I think Dave is on the right track. If the engine is indeed not idling down properly the transmission will respond incorrectly also. What is meant by racing at idle? Cold idle in park/neutral should be over 1000rpm and slowly decrease as the engine warms, to something like 800-900 rpm and in the 600-700rpm range when in gear.

[Michael_V]

Thanks for the replies.  The car is running a little rough right now though this might be unrelated.  I did purchase a new modulator that arrived tonight in the post, and I have yet to install it.  It looks nothing like the one on the car, being that it is about half the size of the one that is already on there.  Both the IAC and the mas air flow sensor were just taken out and cleaned the other day, with no difference in the way the car runs.  It's to dark outside to replace the part, though I do see oil coming from around the modulator and around the plug going into the transmission.  By the way this fitting was kind of loose when I checked it the other day looking at the modulator.  Also the vacuum line is working going into the modulator. 

Edited December 18, 2018 by Michael_V (see edit history)

[DAVES89]

There is just as much carbon inside the opening of the IAC as there is on the IAC. Be sure to clean inside as well. Also there is a gasket for the IAC be sure you have one.

 The after market modulator you purchased is about half the size of the original. Your comment about the modulator being loose is troublesome. There should be a clamp that holds it tight. Do you have the clamp? If you have oil coming from the modulator I would suspect that the old one is worn out. It is 30 years old...

[Michael_V]

The fitting that is loose is the electrical connection going to the transmission.  The oil that is around the modulator now appears to be just from the engine being dirty which is a good thing.  I did take apart the air intake today along with the sensors just to give a more through cleaning as well as going over the engine looking for anything unusual.  When installing the modulator today I discovered a crack in the hose that I did not see before.  There was also rust on the inside of the hose connections not seen in the photo I'm posting.  This gave a big red flag that the part was probably junk.  After installation I gave a quick drive around the neighborhood finding that the bump when stopping is now gone.  The transmission now shifts VERY hard in reverse as well as engaging to drive.  I think there might be an adjustment on the modulator by turning a fitting one way or the other inside, though I could be wrong.  As for the car running rough, it looks like one of the sparkplug wires in the back was toast as well as the sparkplugs needing to be replaced.  Both the wires and plugs are now on order.  It's probably also time to flush the transmission fluid as well since it's brown and not red like it should be in photo number two.  Thanks everyone for the responses since I didn't know if the modulator was the problem or not.