REPLICATING THE MAP SENSOR ENHANCER (DEMSE)

STRUCTURE

This device is very simple and basically consists of a plastic box, two variable resistors (potentiometers or “pots), a switch and a few wires. Anyone who had basic training in electronics and can solder parts together, can replicate this device in one afternoon.

WHERE TO GET THE CORRECT PARTS

I found some surplus pots at All Electronics, Van Nuys, CA www.allelectronics.com. But then they ran out of these. There are similar potentiometers at Electronics City at Burbank, CA, near the east end of Burbank Boulevard. But they are very expensive if you ask me.

Online you can order it from one of the following, but there are thousands of other sources around the world. Even a used one from an old instrument will do. All you have to keep you eyes on is that it is “LINEAR” type (marked A) which means the resistance is spread evenly across its movement range. The “B” type is non-linear, and is good for audio applications. It's what you would normally find as the volume control in radios and old TVs. “B” type will work but will be much harder to tune. You want to see “LINEAR” in the catalog, and the part itself will be marked “A” near its printed or embossed value.

Item PC26 from www.action-electronics.com/pots.htm - $3.25
Item 1241 from http://www.cascadesurplus.com - $1.00
Item 271-1722 from http://www.radioshack.com - $2.89

Each of the suppliers should also have a matching knob. If it has a long shaft like the one from Radio Shack then you will have to cut it with a jigsaw which is quite tricky (try to get one with a shorter shaft). All the above are designed for easy panel mounting. Just drill a hole in the dashboard - or add a little panel or box under the dashboard.

I have the Digi-Key and Jameco catalogs, you can ask me for help, or search and order yourself online at http://www.DigiKey.com
or at Mouser Electronics http://www.Mouser.com

In the Mouser Catalog I have isolated the parts that are major but most hard to find – good pots and compatible knobs. Well, I mean hard to find in this quality, price range and compatibility to each other. Both items below should be in-stock. So in the bottom line I recommend you order the following parts, directly from Mouser.com

1. Potentiometer 50K Ohm:
part # 31VF405.
Prices: 1 each $1.25, 10 each $0.96, 100 each $0.87

2. Knob (matches the pot's shaft perfectly as far as I can see in the catalog):
part # 450-3034.
Prices: 1 each $0.54, 10 each $0.36, 100 each $0.33

NOTES:

• Some experimenters I talked to said that a 100K Ohm potentiometer is needed. According to my experiments, measurements and calculations, the 50K does a perfect job. They should cost the same so you're welcome to experiment with a both values, especially if you're about to furnish an entire fleet of similar vehicles with duplicates of this device.
  

• You can replace the fixed resistor with a “trimmer” (small variable resistor) that will reside inside the box. This trimmer, usually adjusted with a small screwdriver, will enable you to find the optimal value of that resistor.
  
  How do you know when it is optimal? You play with the trimmer until you get the widest range of motion on the knob:
  
  This trim is hidden from the final user, and can be eliminated in mass production by duplicating the optimal value found.
  
  

NO FIXED RESISTOR FIXED RESISTOR ADDED

DIAGRAMS OF SEVERAL DESIGNS

Below is a simple design. I had LEDs and stuff but then eliminated them for simplicity. Besides, they were not much help in tuning the device. You'd better do it by simply feeling the car's pull, vibration and behavior (especially uphill). The resistor helps widen the effective rotary range of the knob. NOTE: the resistor that worked best for my sensor was 33K.

The optimal fixed resistor may be 33K Ohm but that depends on the car. The simplest and cheapest possible way of doing this is to find an old potentiometer with a value of 50K to 100K (should be 50 cents in a surplus store), and connect it on the MAP Sensor line, as shown below:

An optional design is depicted in the diagram below. A DPDT switch is used to switch between the original connection and the enhancer (via the pot). The LED will light up when you're in enhanced mode. If you don't want this LED, use an SPDT switch.

The latest version has two potentiometers so one can be set to city conditions while the other optimized for the highway. Wire colors have been chosen randomly, they don't mean anything. Simply connect the corresponding points as shown to the sensor, ECU and ground. Extend the wires to the dashboard or beyond to your convenience.

Leave markings up to the experimenter or the final user of the vehicle. BEFORE TUNING the left side (marked “highway” in the photo) is 100% identical to the right side (marked “city”), and markings should reflect actual use of these sides. For instance you can mark one side as “enhanced” and the other side would be marked “original” (in which case you leave that side at full-rich position) so now you can switch between enhanced mode and original factory setting. Or mark them “hot engine” vs. “cold engine”, or “bypass/uphill” vs. “flat road” - or whatever suits your use and driving conditions.

There diagram means what it says! It has been edited several times until every detail was perfectly workable (other than resistor values that may require changes in some cars). Even the pot side to which you solder the connections was taken into consideration. It has to be replicated exactly as shown for best results and that's why I put pictures instead of electronics symbols.

You're looking at two hookups. There are two parallel pot circuits shown, but you can build three or more identical circuits – and change the switch to one that can select between these three or more inputs.

The signal from the sensor enters at the "higher" end of the pot. It sees the pot in series with a resistor to ground. The ECU sees only PART of this incoming signal, depending on the position of the pot.

The fixed resistor (on each side) is there for a reason. The signal is not full range from 5 volt to zero. It has a more limited range closer to the 5 volt. Which makes the lower end of the pot non usable, since there is no point in sending 1 or 2 volts to the engine, it will kill it. So I added those resistors to enable a much wider rotation of the pot to be usable. It makes tuning easier.

The photograph below shows how the parts are connected inside the box:

The external wires are then connected as shown below. The external plug is useful for maintenance of the device, but is not vital. I mean you can wire the enhancer straight through from the driver's cabin or dashboard to the engine compartment.

GETTING FANCY

If the above is not enough...here's a fancy alternative. Add a second SPDT switch (see def.) to toggle between Enhanced mode (lean mixture with the right knob or the left knob) and Original mode, or factory setting – in which the original (incoming) signal goes right through to the ECU.

The actual enhancer will look something like this:

And the wiring will look as shown below. The output of the upper switch (gray wire), or in other words the selection of that switch, is connected to the lower switch for further selection. The other input of the lower switch (green wire) is the original signal coming from the MAP Sensor. So if “Original” is selected on the lower switch, then the thick red wire, which is output to ECU, will get the full MAP Sensor signal regardless of any knob setting.