S5 MOP mystery solved? Need a bit o' help...
 

For those running premix and wanting to remove the MOP all together on an S5 I've come close but need some help.

Background:
I've run premix since I put my car together. Removed the MOP, made a blockoff plate, and attached the pump to the fender like alot of people. Over the winter I decided there HAD to be a way to fool the computer electronically. I've found that the actual pump is not required to run the system. There are two connections on the MOP. One is for the stepper motor and the other is for the position sensor. The only thing that is needed is the sensor. The sensor screws into the side of the pump and can be removed. At this point I have removed the pump, retained the sensor and used a simple strap to set the sensor at a predetermined value to operate the ECU.

The ECU must see between 1.0 and 4.2V from the sensor in order to stay out of limp mode. The sensor is provided with about 5V+. There is a plunger on the sensor that rides in and out on a piece inside the pump that varies according to stepper motor position and how much oil is being injected. By pushing this plunger in and out you vary the amount of voltage the ECU sees. By presetting the sensor at a value that stays between 1.0 and 4.2V you can avoid limp mode. The range of voltage provided by the sensor is from a few tenths of a volt to over 4.2V. So it's not as simple as removing the sensor and leaving the plunger all the way out or simply wrapping a zip tie around the plunger and holding it all the way in....I've tried both.:D

Using a small piece of aluminum held to the sensor with a screw and lock nut that is bent in such a way to contact the plunger close to the middle of it's travel range. Using a volt meter to set the bend so that the sensor shows about 3.8V I've managed to avoid limp mode so far. The input voltage will vary slightly according to battery charge, load, etc. So it's a good idea to set the voltage to a value near the middle. Again I've tried 4.2V and hit limp mode as voltage input varied.

So far I've managed to eliminate a couple pounds from the car and unclutter the engine bay somewhat but I'd really like to eliminate the sensor all together and use a set resistor for total simplicity.

The problem:
The sensor is not like any simple variable resistor I've seen. It is supplied by both a 5V+ and a ground. My simple logic would tell me that since the ECU only worrys about seeing 1.0-4.2V+ then the ground is not neccesary and could be removed. With the ground removed a simple constant resistor could be used instead of the variable sensor. But when the ground is removed from the circuit limp mode is the result.

The help part:
There has to be someone with the electronics background to look at the schematics, do some testing and come up with a way to use a constant resistor to do away with the sensor all together. Anyone want to tackle this problem and finish off the MOP/limp mode for good?

Chris

Im sorry I cant help you...But I was wondering if you possibly had a template or something for one of those block off plates? All my injection lines are cracked and leak oil :( I premix anyways, so just blocking it off and letting the pump run would be GREAT :)

just tie the ground into another ground wire or bolt it to the chassis of the car.

Then take a resistor and solder it inbetween your leading going to sensor and ECU.

The ground is probably tehre for ensuring that the whole MOP system is grounded, therefore the motors and everything will work. Once it is not grounded then the MOP won't work? i'm not sure...

This is only an idea/suggestion, i have never played with the OMP before.

Thanks for the suggestion but that won't work. The ground wire already has continuity to ground and the stepper motor functions independently.

Chris

Remove the pump and use it as a template.

C. Ludwig- Hi I don' tknow if you ahve read the sticky I got about going to Pre-mix or not but since you have done this to an S5 and have now (sort of ) figured out how to remove the OMP out of an S5 you could contribute to the "thread" and input your experiences and the overal procedure. I am sure it would not be too much trouble if it will be let me know and I will do it from this post. I think we can all agree that most on this forum would appreciate this information to be included in that thread aswell.


Santiago

Chris,
What you are describing is a voltage divider. This requires two fixed resistors in series from the supply to ground. The center tap between the two is the signal the ECU is looking for.
You can calculate the resistor values using Ohms Law, or experiment a bit. Better still, I would use a potentiometer (variable resistor). That way you could adjust it to the actual voltage you require. The only difficulty here would be the value of the "pot." I hazard a guess that a 500 Ohm pot might do. (24 milliamps constant current draw) Each end of the pot to hot & ground, and the center tap (variable) to the ECU.
Hope this helps, Dave

Don't they sell those things at the "shack" ?

So what you're saying is that I'd have two resistors setup in series and then tap the signal wire to the ECU where the two resistors meet between the hot and ground wires? Easy enough...:)

To use Ohm's Law I'd need to know current to determine reistance value required. Where are you getting 24 milliamps? That kind lost me.

Chris

Ok...So if the ECU doesnt see the pump, itll go into limp mode, right?


Well what if I have a Haltech? Can I just junk the whole pump all together? I was always told you couldnt on S5's...

Sure, it's the stock ECU that will cause problems. Take that out of the loop, and no problem. I assume you'll run premix. ;)

I am pretty sure you can toss it if you go standalone.

Thats GREAT news!

Just don't forget to pour in the majic Juice :D

Santiago

Doesn't need to see the pump. Just a signal from the sensor. That's what we're talking about here.

Based on my knowledge of electroics, I assume that the metering oil pump contains a stepper motor, which is connected to the shaft of a pot so the ECU may monitor it's position.

If that is true, it would be trivial to make a circuit to emulate the pump using a microcontroller.

Sorry...Im not very good with electrical stuff :(






P.S.

I was just informed Im the owner of one of your past vehicles?...

I bought it from Nick Weiss.

No kidding. Small world. I only had it for a month or so. Russ Coleman had it before me. How's it doing for ya?

Pretty good. Just trying to fix up all the jerry-rigging Nick did to it when he had it :)

This is the variable resistor that I use when making FCD's, would this work?

http://www.jameco.com/cgi-bin/ncomme...&search=43P10k

Dunno the specific #s yet but that looks like the trick. I have decided that using something like that will be the equivalent of doing what I've already done. I'm just going to leave the stock sensor in with the mod I've done to it.

Anyway...you no longer have to think you need the whole pump. Just the sensor or equivalent.:D

Chris

What if your sensors bad? :(

From the ECU wiring diagram on page F2-8 of the 89 FSM it appears the position sensor is a simple voltage divider just like the TPS. I can't see why the sensor couldn't be replaced with a 3-pin variable resistor (which acts as a voltage divider) which is then adjusted to suit. No fancy electronic required, just a $1 component.

If you look at page D-18 it shows how to check the position sensor. There are three wires; Vc (5V, brown wire with a white stripe), Vo (position signal, green wire with a black stripe) and E2 (ground, brown wire with a black stripe). Note that Vc and E2 are shared with several other sensors and go back to the ECU, so you can't mess with them.

The resistance between Vc and E2 should be 1-2kohm, so if you where to plug a 3-pin variable resistor of similar value into the sensor plug on the harness you should be able to adjust it to give the 1.0-4.2V signal the ECU wants to see. This would be easily done with a voltmeter.

It may be different is practice, but on paper it all looks very simple. I hope someone tries this and lets us all know whether it works not. :)

Tell me what to do and I'll go try it right now :-D

Do that.