1985 "Rack Position Sensor" - can I use this signal? - PeachParts Mercedes-Benz Forum

Secondaries · #1 09-03-2013, 11:28 PM

I'm interested in building a water meth injection system, likely two-stage. However, I only want the setup to spray while I'm giving the engine throttle. Any way to intercept this signal to use to this end? I searched but haven't found anyone using this sensor for extra-OEM purposes.

mach4 · #2 09-09-2013, 04:05 PM

It would be easier to put a micro switch on one of the throttle arms, or possibly use the micro switch already on the bracket on top of the valve cover.

Secondaries · #3 09-09-2013, 04:28 PM

Do you know if the rack sensor is a standard 0-5V output? Because if so pretty much all controllers use that for progressive input. I like that idea better than on-off input if a switch.

Oldbeaver · #4 10-14-2013, 07:18 PM

Quote:

Originally Posted by Secondaries

Do you know if the rack sensor is a standard 0-5V output? Because if so pretty much all controllers use that for progressive input. I like that idea better than on-off input if a switch.

Did you make use of this signal al last? I am trying to do so, so any information about the signal would be useful to me: voltaje range, analog or pulse, etc.

At the same time, I will let you know any discoveries I can make.

Oldbeaver

Secondaries · #5 10-14-2013, 11:34 PM

Oldbeaver (ha!), no, I haven't actually tried to do anything with this sensor. But I still intend to.

That being said, if you look at the wiring diagram in the first post of this thread, you will see the sensor in question. It is labeled #64. I'm no electrical engineer, but it looks like the center wire is 12V in, the one on the left is some fixed voltage out (through a trim pot - reference signal?) and the one in the right it's a variable voltage out (through a rheostat).

Edit: apparently I'm close in my assumptions except that the RPS varies current, not voltage. Still unsure of the total range but EGR function varies from 100% at 530 mA to 0% at 370 mA. Amperage decreases as rack travel increases. Source: FSM section on EGR function.

If that info helps you, please share your findings with me! Otherwise, I intend to eventually put 12V through a spare pump I have laying around and try to get it to tell me something.

bricktron · #6 10-16-2013, 02:17 PM

i like this idea a lot. of course, i cut the wire on mine, i was so offended that mercedes and/or bosch installed it without any removable connector.

would it be any closer to an ideal trigger for the injection to use the EGT measure? i.e. when it gets hot, use the water?

compu_85 · #7 10-16-2013, 04:04 PM

I wanted to hook up an economy gauge to my RPS. Putting a probe / scope on that signal is on my list of things to do... eventually.

-J

Secondaries · #8 10-16-2013, 06:54 PM

Quote:

Originally Posted by bricktron

i like this idea a lot. of course, i cut the wire on mine, i was so offended that mercedes and/or bosch installed it without any removable connector.

would it be any closer to an ideal trigger for the injection to use the EGT measure? i.e. when it gets hot, use the water?

Actually, it does have a removable plug. Or do you mean on the pump end and not the chassis end?

As far as other signals, I do intend for WMI to act as additional fuel, so it needs to come on and go off consistently with throttle.

Additionally, the WMI needs to inject progressively as boost rises. I can use a cheap MAP sensor to this end, but since boost doesn't decrease quickly with decreasing throttle, I need a way to limit WM injection to on-throttle only. The amount of methanol I intend to inject, if allowed to spray while off-throttle, may work against engine braking.

winmutt · #9 11-04-2013, 09:58 AM

You can measure the signal coming out of the cruise control servo...

compu_85 · #10 11-04-2013, 10:17 AM

Only when the cruise is active. With the clutch disengaged a spring winds the mechanism back to 0.

-J

KarTek · #11 11-04-2013, 09:42 PM

I believe the RPS is a variable capacitance type. As for water injection, I would just use a boost sensor switch or some such device.

I remember Edelbrock, years ago made a product called "Varinjection" that used the vacuum and other signals to control a water injection system.

Knuckleballerr · #12 12-14-2015, 02:25 PM

What you can do is put a reed switch & small neodyo magnet on the side of the cruise control or elsewhere on the linkages and use this as your throttle indicator. I'm sure you could also use some of the previously mentioned ideas and get some junkyard sensor off some other vehicle. You may need to run the sensor output through a non-inverting op amp to get useable voltage out of it.

mach4 · #13 12-19-2015, 04:53 PM

Quote:

Originally Posted by KarTek

I believe the RPS is a variable capacitance type.

Can you explain why you think this is the case...and how it would work if it was?

compu_85 · #14 12-20-2015, 05:44 PM

The sensor is variable inductance. A metal core is moved in and out of a coil of wire. This works well because there's nothing touching so there's nothing to wear out.

How do you measure that? I'm not sure.

KarTek · #15 01-06-2016, 01:02 PM

Quote:

Originally Posted by mach4

Can you explain why you think this is the case...and how it would work if it was?

This was purely a guess on my part based on looking at the sensor and then researching different types of sensors by how they worked and what they looked like.

Most of the other sensors my '98 work through a 0-5v signal so I'm not sure how that rack position translates into that.