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1988 Lesabre with 440T4 Vacuum Modulator Issue

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    1988 Lesabre with 440T4 Vacuum Modulator Issue

    Hello everyone!

    Today I replaced the original Vacuum Modulator in my '88 Lesabre with with a Dual Red Stripe Modulator from Rock Auto. The brand and part number are ROSTRA 5100690000. When I first installed the new modulator I did not install the holding bracket correctly. I had the holding bracket on the transmission side of the Modulator lip, when it is supposed to be on the side of the lip furthest from the transmission. So the hold down bracket was NOT holding the modulator in.

    I was test driving my car around my neighborhood. When I accelerated to ~40mph it felt like I went into neutral and then I saw a lot of smoke come from under the hood. I pulled over immediately and shut off the engine. The modulator had blown out of its hole because the bracket was not holding it in and the fluid shot out everywhere. There was a trail about 30 yards long behind me and a pool of transmission fluid under my car. Luckily I had 3 gallons of AcDelco VI at my house because I planned to replace the fluid and filters 3 times to try and solve some issues I was having with shifting. I re-filled the transmission slowly, and after a gallon and a quart of fluid, the dipstick is reading full again. After a test drive I did not feel any slipping or shift issues at neighborhood speeds or highway speeds. I am hoping I lucked out and did not cause any damage during those few seconds of 40mph acceleration after the modulator had blown out and the fluid had started to leak. Also, the harsh downshift around 15mph I was experiencing is gone and the harsh shift from park to reverse I was having is also gone. So that is promising!

    When I first looked under the hood I found a piece of metal that looks like a tiny barbell on top of my subframe right below the blown out modulator and modulator hole. I tried to see if it would fit back in the modulator hole but it doesn't seem to be where it goes. Does anyone recognize this piece and have any idea where it came from or how I can put it back in its home? I have not noticed any issues with the trans during my test drive after re-filling the lost fluid and installing the modulator and hold down bracket correctly, but I don't want to cause any damage by not having this part put back in its correct place. Here is a photo of the small barbell looking piece I am talking about. Thanks everyone!


    #2
    440T4 Vacuum Modulator Piston

    Hello everyone!

    I replaced the Vacuum Modulator on my 1988 Lesabre. When I installed the replacement the first time I did not place the Hold Down Bracket correctly so the Modulator blew off and the piston blew out of the hole too. Now I can't seem to get the piston in far enough to allow the modulator to seat properly. Does anyone have a tip on how to get the piston in correctly so the Modulator can seat correctly! Thanks everyone

    Comment


      #3
      I have looked at the 1988 Lesabre Service Manual (Page 7A1-9 is the explanation of the Vacuum Modulator System and Page 7A1-12 has a diagram showing each part of the assembly)

      In my first post I called it a "piston" but the proper name is the "modulator valve." The diagram shows the modulator valve going into the transmission first, then the o-ring goes over the vacuum modulator and the vacuum modulator goes into the transmission, and finally the retainer is bolted into the transmission - holding the vacuum modulator in place.

      One idea I had is - maybe I bought the wrong Vacuum Modulator and the one I bought might be longer than what will fit in my transmission. The modulator I bought is a ROSTRA 5100690000 from Rock Auto. I have been searching the internet for information on the problem I'm having and saw that multiple people from other forums bought their Vacuum Modulator from Autozone (Part #MV303). The only problem with that theory is the fact that I can't even get the ORIGINAL Vacuum Modulator to fit into the transmission with the modulator valve in place.

      I'm going to AutoZone tomorrow to pick up an MV303, just to see if maybe it will fit.

      Here is a link to a video I made showing the diagram in the service manual, the Modulator Valve itself, and the transmission without and with the Modulator Valve installed. Any help or tips are appreciated, thanks!

      https://www.youtube.com/watch?v=3EZWLzuUIZU

      Comment


        #4
        I see some variations in the tip of the modulator, so that is probably a bad sign. The errors in parts manuals seem to be spreading, and I don't see any quite like the originals that were made by Holley (which would not normally be a selling point, IMHO). I'm sure at least one is compatible, just haven't had any out or replaced myself.
        Jerry

        Comment


          #5
          Jerry, thank you for the reply. Here is an update - I left a comment on one of David's transmission re-build videos on youtube. He replied back to me and then watched the video I posted and commented on my video with instruction on what to do. The gold pin inside of the bore that I show in my video is actually not supposed to be within the transmission bore. That gold pin is part of the Vacuum Modulator and the pin side goes up into the Vacuum Modulator and the flat side of that gold pin is what the Modulator Valve pushes up against. The Modulator Valve is what seats into the transmission bore, the solid smaller end goes in first, then the end with the cut in it pushes on the flat end of that gold pin.

          What happened was, after I blew my replacement Vacuum Modulator out, I put the original Vacuum Modulator back in without putting in the Modulator Valve. Luckily for me the Modulator Valve did not get blow off into the street somewhere to never be found. It was sitting right on my subframe when I opened the hood. Anyway, I did not put the Modulator Valve back in after the initial blowout because I did not know what it was or where it was supposed to go. So when I put the original Vacuum Modulator back in after the blowout, without the Modulator Valve in place, the gold pin from the Vacuum Modulator fell out of the Vacuum Modulator and into the bore. As David suggested, I used an extendable magnet to retrieve the gold pin and then I was able to seat the Modulator Valve correctly in the transmission bore and install a new, MV303, Vacuum Modulator from AutoZone.

          I feel very grateful that I did not ruin my transmission, lose the Modulator Valve, and for the help of David and this forum. Hopefully if this happens to anyone else, this thread and the video I posted and David's comments on those videos will help relieve some stress and solve the issue for someone else out there.

          Comment


            #6
            Glad to hear that. David is a resource second to none. I had no doubt he would have the answer.
            Jerry

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