Author Topic: Modulator pressures?  (Read 2210 times)

twistedtree

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Modulator pressures?
« on: December 08, 2011, 20:03:24 »
I'm trying to sort through a couple of things on one of my transmissions and have a few questions about the modulator pressure.  I've got a gauge hooked up and am taking readings at a fast idle, trans in park, vacuum line disconnected.  For additional control, I have also disabled the 3 position solenoid by disconnecting one of the throttle switch wires.  That way I can move it manually for better control.  I'm trying to understand what pressure I should be getting with the 3 position solenoid linkage in each of it's three positions.

If I follow the BBB correctly, it should be as follows:

- Linkage in aft position, "Basic pressure", 8.7 psi
- Linkage in center position, "Max pressure", 40 psi
- Linkage in forward position, "Kickdown pressure", 67 psi

Does that sound right?  The Wiki and various discussions talk about 40 psi, but it's not clear which position the linkage should be in.  From the BBB, I'm concluding it's the center position?

And now the issue:  When I move the linkage forward I get no change is pressure at all.  It just stays at 40 PSI.  Any ideas?  Should it move up gradually as I move the linkage forward, or is there some point where it abruptly kicks up in pressure?  I'm thinking I might need to adjust my linkage a bit, but from what I've read I think I should start to see an increase in pressure as soon as I start moving the linkage.

Thanks in advance,

Peter
Peter Hayden
1964 MB 230SL
1970 MB 280SL
2011 BMW 550xi

twistedtree

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Re: Modulator pressures?
« Reply #1 on: December 10, 2011, 00:02:12 »
Problem solved.  There are two morals to the story:

1) With the power to the 3 position solenoid removed, if you don't get the correct pressure readings in each of the three positions, try adjusting the modulator linkage.  With a little adjustment I was able to get correct pressure in all three positions.  10 psi in the aft position, 40 psi in the center position, and 67 psi in the forward position.

2) Just because you can get the correct pressures manually moving the linkage, re-power the solenoid and confirm it can do it too.  I found I needed another slight adjustment to reliably get the low pressure setting.

For the first time the transmission shifts solidly into forward and reverse.  It used to slip horribly before.  Now I can cross "new transmission" off the list.
Peter Hayden
1964 MB 230SL
1970 MB 280SL
2011 BMW 550xi