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Old 09-19-2014, 02:21 PM
filon102's Avatar
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Join Date: Aug 2009
Posts: 88
Low modulating pressure...

I finally managed to put my auto trans back together after a long year rebuild... but im having a bit of an issue. Car goes forward and backwards, but it requires A LOT of gas. So I checked trans fluid 1st, was a bit low so I added more, dint help.

So I got under the car, and installed oil pressure gauge on the modulating pressure port and pulled off vacuum line to modulator. Started the car up and Im getting 18psi in park on idle, when I put it to drive, I get only 14ish. Specs requires 4 bar or 58psi. I tried adjusting the modulator valve on the side of the trans, pressure did not rise but it fell when I back off of it.

Since I had alldata account, I managed to see a TSB regarding "Slipping or Soft Shifts":

To inspect for this condition, remove the modulator. Start the engine and press in on the modulator valve assembly (figure one). If there is now pressure showing on the pressure gauge, make a shim and install it into the modulator valve (figure two).

I did that and the pressure did not rise when I pressed in, so Im really running of ideas of what it could be. Ill probably check working pressure next, but my guess it is fine since I installed new front pump...
1987 Mercedes Benz 300D Turbo Black/Black
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Old 09-19-2014, 02:49 PM
Diesel911's Avatar
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Join Date: Sep 2007
Location: Long Beach,CA
Posts: 23,333
I am just repeating what I read on My Year and Model Transmission. There is a spec for the Pump Pressure and then there is a spec for th Modulator Pressure.

So I think it is a good guess if the Pump Pressure is too low the Modulator Pressure is also going to be too low.
84 300D, 82 Volvo 244Gl Diesel
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Old 09-19-2014, 03:31 PM
filon102's Avatar
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Join Date: Aug 2009
Posts: 88
Forgot to add, when I step on the gas pedal in park, the pressure rises to ~65psi @ 2000rpm. I'll check the pump/working pressure next, here is a reference I got from all data, I should be getting between 194-223psi

Modulating Pressure 4.0 bar
Working Pressure
14.4 +/- 1 bar

Governor Pressure @ 30 km/h 0.6 bar
@ 90 km/h 2.0 bar

I'll report back with results.
1987 Mercedes Benz 300D Turbo Black/Black
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