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  #1  
Old 04-27-2002, 11:33 AM
Ron G.
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Trans modulator adjustment tool. Is this a hex wrench or a special MB tool??

I cannot find any info here on the adjustment tool. I have the '85 300CDT with the green bodied modualtor valve with the black cap. I have been through the searched posts and even Steve B's MB transmission info at Continental Imports (http://www.continentalimports.com/ser_ic20242.html) ref. figure 5 and it does not mention if this is a special "MB tool". I want to give the adjustments a try but if I do not have the correct tool for the mod valve I do not want to start yet. Any enlightenment appreciated.

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  #2  
Old 04-27-2002, 12:21 PM
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Lightbulb Modulator Adjust!

I'm not sure and undobtedly someone with the factual knowledge will respond eventually but in the meantime:

I believe I have read that the cap covering the adjusting device needs to be pried off and underneath there is a small T- handle that is turned to adjust. There are some threads addressing this issue, as that is where I read it, but I don't know where or which ones they are.

You can also look around here as I believe that there is a detailed procedure somewheres on this site.

http://www.transbuilder.com/store-ie.html

Good Luck, my limited info might help until someone with actual experience responds!
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Old 04-27-2002, 02:09 PM
Ron G.
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I haven't popped the cap off the mod valve yet.......

so I was not sure if this was a separate tool required and did not want to start something that I needed another tool for and could not finish. I guess then it is time to get it up on the ramps and pop the cap off and see what we shall see.

I have printed all of the (searched) long threads and made notes from the short ones compiled notes from them all and have not seen it specifically mentioned that it is there.

Thanks for the reply and the trans link.
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  #4  
Old 04-27-2002, 03:50 PM
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This could possibly help!

Transmission Vacuum Modulator!!

stevebfl
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Before you go to the expense and bother of changing it, try adjusting it. If nothing else it will give you practice as the new one will definitely need adjusting. I would suggest turning the tee handle one turn clockwise.

The modulator uses engine vacuum to sense load and is used to vary shift pressure to suit the load of the moment. It works by placing a vacuum on a diaphram opposed by a spring. The adjustment varies the spring pressure and thus changes the hydraulic pressure in the trans called modulator pressure. This pressure works on the main system pressure regulator as the largest influence.

The proper diagnosis and adjustment of modulators (and thus modulator pressure) would to use a pressure gauge. While driving the pressure would vary greatly with engine load and change smoothly (if not sticking). The proper adjustment is achieved (by the book adjustment that is) by removing the vacuum and holding the rpms to 2000. (It turns out that the pressure is constant just above idle - the 2000 is to be sure the pump has good volume). The measured pressure would then be set to the value from technical data.
We do these measurements every time we rebuild a trans, but seldom afterwards. Afterwards we usually adjust them to feel.

BTW the tee handle is under the rubber cap in the center of the device. It pops off with a small screw driver and the tee handle is locked in the depressions of a castelated rim

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Old 04-29-2002, 09:34 AM
Ron G.
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I wasn't able to get the car up to pop the cap and see but I did come across the same info you posted here before jumping into this. I did have a look at the Boost pressure and vacuum levels at the VAC Amp on the left fender. The boost pressure is a smooth incremental transition as is mentioned in Steve's article referenced above. The modulator valve vacuum output has a range of 17"Hg to 5"Hg vac. The 5" reading was at 60mph as I wasn't able to open up to full throttle. I am going to assume that (according to Steve's article) these are good / normal boost press and modulator vac readings and that the vacuum system is operating properly and hopefully get through the remainder of the Steve's check soon. I did run the car with the vacuum disconnected from the mod valve and I now know what "neck-snapping shifts" are.
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Old 04-29-2002, 11:20 AM
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Can anyone explain exactly how the blue vac amp on the fender works, and what its purpose is? There's nothing in the MB service documentation...


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  #7  
Old 04-29-2002, 11:47 AM
Ron G.
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The best answer I can give is look at the continental imports link that I included in the posting. The author (Steve B.) gives some enlightening info there. From what I gather from it the amp accounts for the powerplant operating status (real time) and compensates (refines) the vacuum output signal to the modulator to provide optimal transmission shift points. From what I saw yesterday the amp seems to be functioning properly and I need to move on to pressure adjustments at the modulator valve. If you connect the vac gauge and do the check as mentioned you can see the interrelation of the vac and pressures. Just reading the info and not doing the check was not enough for me to get it

I have never dealt with these diesel machines before but they are peculiar machines ('85 300CDT).
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Old 04-29-2002, 01:19 PM
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Thanks, Ron... I went to the wrong URL (there are several above). The one you mentioned has some EXCELLENT information. It's getting added to my bookmarks post-haste...



Best regards,

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