Doesn't the ECU also look for changing voltage? I could've sworn that's why there's the 10 minute limp mode, where it acts normal for ten minutes then goes to hell.

 

I wordered about that too. If the ECU's needs to see varying voltage (which would prove to the ECU the stepper motor's not jammed) then the variable resistor trick won't work. I'm sure it would be possible to design a relatively simple circuit that provide a constantly varying resistance, and could be plugged into the harness, but that's well beyond my limited electronics knowledge.

EDIT: Just checked the FSM again. Under Position sensor operation is says "Warm up engine until coolant temp reaches 80degC (176degF) and let it idle for 15 minutes. Check if malfunction code No.20 appears." This would indicate the ECU's looking for varying voltage, indicating correct operation of the stepper motor. The ECU's response to code 20 is "MOP fixed smallest open. Basic fuel injection amount and ignition timing fixed", which I guess is limp mode. Bummer...

 

Damn, I guess no luck for me at the moment, I'm going to go rip the OMP off and see if I can't fix it somehow.

 

It doesn't work. I tried a simple voltage divider across my '89 S5 T2 today with two 1K resistors (this gives half the supply voltage to the ECU's input, about 2.5v). I drove around town without trouble. However as soon as I stuffed my foot into it, the car tried to throw me out the front window. It then set a code and lit the MIL.

Here's what the ECU is expecting:



(I built this table from logged data, it's a little square, but you can get the idea from it!)

 

What about setting up somethign that varries Voltage with RPM?


I really want this to work out for you guys so I can add this to my thread on pre-mixing and S5 guys don't have to leave the pump hianging there.


Santiago

 

I think that perhaps a GM 2 or 3 bar manifold pressure sensor connected to the OMP position sensor connector electrically and connected to the manifold with a check valve (no vacuum to sensor) and a bleed orifice between the check valve and the sensor port (to vent boost once made that the check valve will trap) might just get the job done.

The sensor runs on 5 volts, gives a varying voltage with manifold pressure, and is cheap enough and small enough that perhaps it's a viable option. The goal is to have 1 volt when driving around and no more than 4 volts when at any boost level. IT would probably not be critical to follow the map I posted above, just as long as you didn't go out of those bounds.

On an related side note: When I started pre-mixing I just capped off the OMP oil line ports. About two weeks later the OMP worm pinion siezed and the worm gear tore all the teeth off it. This trashed the entire assembly and jammed the position sensor at 4 volts. I drove around like this for about 10 days before the ECU threw a code and put me into a severe limp home mode.

 

I've run a ton of track sessions with the sensor held in fixed position without a limp mode problem. This would indicate that the ECU does not look for varying voltage.

Chris

 

Ok, I am no expert and I do not have a 89up wiring diagram with me but, if the computer needs a value between .5v - 4.5v or whatever, why not just tap the signal return wire into another sensor. Such as coolant temp or air intake or whatever. The value should always be between that range.

-Jack

 

Well what they are saying is that the voltage that is supposed to be outputted to the ECU should vary with RPM and air pressure/boost so you need something that changes with those factors.

 

I don't know about the rest of you, but my wiring to my stepper motor was screwed and three of the wires were broken off, and my sensor worked fine and I still was tossing codes, even with the sensor unhooked all by itself I still was throwing codes under anything over 25% throttle.

 

So Chris, did you ever find the car unhappy with a particular position on the sensor or were you able to have it any any position?

I wonder if the reason my car went into limp-home mode was because I chose 2.5 volts as a set point. Perhaps I needed to have it at a higher voltage. This is easy enough to do.

If you have a specific position that you set it to for best results, would you take a measurement for me? I would have you check the voltage across the harness side wires of brown/black (ground) and green/black (signal to ECU). Then either a variable pot or a pair of fixed resistors could be speced to have this same voltage very easily.

On the note suggested by jrx13:

unfortuantely I don't see any sensor that will keep a voltage between the threshold values of 1 and 4 volts. The manifold pressure sensor would be the closest bet, but I would worry about 'offsetting' this sensor's output by tying in the OMP feedback signal. Typically these sensors have a pull-up resistor to their supply to alert the ECU to a disconnected or open circuit condition. The current passed by this high value resistor is small but could have the effect of a 'reverse' fuel cut defensor, giving higher boost readings than actual! Futhermore, the stock MAP sensor will definately give you less than 1 volt on decel which may be too low for the OMP system.

 

SonicRaT,

You're right about the codes. The OMP system has four devoted to it:

20 = Position Sensor Fault
26 = OMP Stepper motor
27 = Malfunctioning OMP Control System
37 = OMP / Battery Voltage Drop

I saw #27 when mine shut down. When it was active I got about 25% throttle before the ECU shut the engine down. I am truely surprised that when the pump is disconnected the ECU doesn't immeadiately go into limp-home mode. Strange !

 

You cannot just unscrew the sensor and let it dangle. When the plunger is all the way out it's providing maximum resistance and only showing the ECU a couple 1/10s of a volt. You'll need to position the plunger somewhere in the middle.

Chris

 

I haven't played with it that much to note a difference in performance for varying voltage. I do know that the higher voltage correspondes to higher pump output so I've set mine up on the high end of the scale. Around 3.8V. You want some room for the voltage to go up or down as the battery voltage an load on the alternator changes.

Chris

 

Okay, I'll give the higher voltage a try and report back on it. The actual voltage supply to the sensor isn't supposed to vary much as it's a regulated supply. That said, I see it move a lot more than I'd expect.

How much boost are you running?

 

14.7lbs of NA baby!!!

 

Any luck yet? I am very interested to see how this turns out.