heygibb

Drive train hiccup...does AT need work?

Recommended Posts

1 hour ago, heygibb said:

It's funny I've never been aware of that part, considering so many other parts I"ve replaced,

 

The transmission vacuum modulator has been discussed here many times. You must have somehow missed it.  :)  I doubt the modulator was the reason you felt the car was coasting better before you changed it. The engine RPM could have been staying higher than normal when you let off the gas due to raw air and transmission fluid being pulled into the engine due to the diaphragm inside the modulator being broken. The function of the modulator is to make transmission shifts firmer or softer depending on the load on the engine (based on engine vacuum).

Share this post


Link to post
Share on other sites
21 hours ago, Ronnie said:

 

The transmission vacuum modulator has been discussed here many times. You must have somehow missed it.  :)  I doubt the modulator was the reason you felt the car was coasting better before you changed it. The engine RPM could have been staying higher than normal when you let off the gas due to raw air and transmission fluid being pulled into the engine due to the diaphragm inside the modulator being broken. The function of the modulator is to make transmission shifts firmer or softer depending on the load on the engine (based on engine vacuum).

Yes, stupid comment by me. Spending hours and hours on this (and your) site doesn't translate into having picked up on every repair.

Shifts are definitely firmer. There has to be a connection to the mod change though, otherwise, why the noticeable difference in deceleration now? Your explanation covers the connection.

Share this post


Link to post
Share on other sites
11 hours ago, Asa Bergman said:

 

Do you notice the RPM's fluctuating when this happens? I'm aware of what you mean by the Reatta seeming to coast for awhile, the ECM opens the idle air valve to crudely maintain your speed if it doesn't receive a signal from the brake pedal switch. Not the same as cruise but the car doesn't immediately start to slow like a carbureted engine at idle. If the brake switch is faulty and the circuit is stuck open it'll constantly enable DFCO in the ECM and as well your TCC circuit will never close(thus no lockup). Likely your cruise control won't engage either if this is the case.

I did not check RPMs during decel but I will.

You hit the nail on the head. I tried my cruise control on my trip and it did not engage. I need more training to understand some of your acronyms. No clue what DFCO is,  but from your diagnosis, I must have a faulty brake switch? If so, is this it? Kudos to ROJ.

http://www.reattaowner.com/roj/component/content/article/index.php?option=com_content&view=article&catid=84:brakes-electrical&id=163:abs-pressure-switch-replacement

http://oemcats.com/oem-parts/25533700.html

Edited by heygibb
more info (see edit history)

Share this post


Link to post
Share on other sites
3 hours ago, Asa Bergman said:

Modulator pressure connects to the internal pressure regulator to modify main line pressure. TH350 also does this, it's pretty much standard auto trans design. Incorrect line pressure from a faulty modulator causes the entire trans to not work as intended because these are 'dumb' hydraulic valve bodies.

 

In other words the modulator does what I said. It controls the firmness of the shift depending on load (torque demand) of the engine.

Share this post


Link to post
Share on other sites
18 hours ago, Asa Bergman said:

 

Yikes, not that brake switch! That's the brake PRESSURE switch for the Teves unit, don't order that one. Lol. I forgot the Reatta is an overly complex car.... The brake PEDAL switch, the one that controls your brake lights. It's attached to the brake pedal. There's two. The bottom one is for the brake lights, and the top one  controls the TCC and ECM circuit. Watch the RPMs next time. Usually if I'm going ~60 and I tap the pedal to turn on DFCO the RPM's will drop down to around 1000 while if I just take my foot off the pedal they'll sit around 1700.

 

The brake pedal switches adjust by pushing in the brake pedal by hand and then gently pushing the switch in until you hear a few clicks. The switches are threaded but don't thread into it, there's a little tab that just holds onto the threading. After you push the switch in a few clicks you just let the pedal back down and it'll adjust itself. I'm not sure how the 90-91 diagnostic mode works but if you can watch ECM input EI71 it should read High when the pedal isn't pressed and low when it's pressed. Could also switch the wire from the other switch temporarily(if it fits) and watch the brake light. Anyways, if the adjustment makes no change you could try replacing it, the switch IS part of the TCC circuit so if it's worn out it'll cause funny problems. I'm not saying this is your problem but brake switches are overlooked and much much cheaper (~$8)to replace in hopes rather than taking apart your trans in despair.

 

DFCO is Deceleration Fuel Cut Off. When you are fully off the throttle the car normally keeps the engine going at a steady pace and uses the IAC(Idle Air Control) valve to try to crudely maintain your speed or at least slow your deceleration. IF you however are off the throttle and you push the pedal the top brake PEDAL switch triggers an input to the ECM(engine control module) that says "Hey! I wanna slow down!". What the engine then does is limits the firing of the injectors or completely turns them off. This helps slows the vehicle on flat ground or can help maintain speed going downhill. You can take advantage of this feature on mountains. On the highway I take there's a three mile stretch going down a mountain, I tap the brake pedal to turn on DFCO and the car maintains 50-55 MPH and uses only trace amounts of fuel for that three miles and not using the brakes. If I don't tap the pedal the Reatta will barrel down that mountain at 70mph. You can watch this happen by watching the injector pulse width data on the 88-89 Reatta CRT, I don't know if your 1990 one can view this data as well but it's not important.

Just wanted to acknowledge this post. Several things there for me to do, that I have not had the time to do.

re the switch... I'm glad you cleared up the switch(es) we are talking about. ? I did a quick search for brake switch and that is what came up, so thanks for the followup. (Of course, that lead to another issue of acquiring a discontinued part for the time when mine eventually fails.) Anyway, I removed the plastic shroud covering the electronics under the driver's side to take a look at them. Not easy to work on but I see their position and how they are installed, if I have to pull one or both.

The rain is keeping me from roadtesting for the time being, but I'll check the RPMs and probably the ECM diagnostic readout when I can.

brakeswitches1.jpg

bs2.jpg

bs3.jpg

bs4.jpg

Edited by heygibb
add pics (see edit history)

Share this post


Link to post
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now