Automatic Transmission Pressure Switch vs. Electric Solution

[Pawel66]

I do not mean it to be a Facebook type of sharing, but I saw some discussions about electric control of Constant Speed Solenoid and what I have noticed this morning was not mentioned.

My pressure switch for reverse stopped working. I did a quick wiring to CSS just to be able to drive with some comfort over the weekend. It works, but the trouble is that when we take the signal from backup light, the CSS is increasing idle immediately when shifter selects R. It takes the tranny about 1 second to engage a gear. Over this one second the revs go up, and when the gear is engaged you get a considerable "thump", which is not nice.

Maybe it is just my car, but when the 4 is engaged and CSS is controlled through the pressure switch, you do not really feel the revs change and gear enagegement is smooth.

Maybe I have something wrong with the set up (but reverse was smooth with pressure switch, pressure is adjusted via 1 and 3/4 turn exercise), but if not - something that delays the signal by about a second would be useful in the electric arrangement.

[stickandrudderman]

Why not just replace the failed hydraulic switch with a new one?

[Pawel66]

Yes, certainly, I wired it up just for the weekend. But I saw threads here where people were thinking of making it electric - so I shared some potential issue related to it.

[FresnoBob]

Do you know where I can buy a new pressure switch?  Bud's doesn't offer them.  My CSS is always activated from the moment I switch on the ignition.  Which of the pressure switches has failed and how can I test this? 

Thanks,

Bob

[Pawel66]

You can buy those switches at mercedes dealers. They are expensive, so indeed it is better to be sure you have a faulty one.

They provide ground to Constant Speed Solenoid. The one on the left is for reverse, the one on the right is for forward gears. When you select Reverse, due to pressure in the gear box, the left switch goes "on" and provides ground to solenoid. And vice-versa for forward gears.

If your solenoid activates on switchin on ignition, it has ground from somewhere.

I think the simplest would be to:
- unscrew wires from switch or both switches
- check if you still have ground on solenoid - if yes, the issue is in wiring; if no proceed:
- take a length of wire(s) and screw it to switch or both switches
- have someone select R or D and see if you get ground on respective wires when gears are selected
- if yes - switches are ok and problem is in wiring - something shorts to ground somewhere
- if one of them provides premanent ground - it may be faulty (the wires from both sides come together on the contact plate on the gear box)

[FresnoBob]

Thank you Pawel,

I will investigate further.  As always, your guidance is much appreciated.

Bob

[Shvegel]

Doesn't sound like much of a job making an "aftermarket" switch.  I deal with small adjustable pressure switches all the time at work.