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automatic variable resistor?

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fiveten

Member
I am building a project and I am trying to simulate a signal to an ecu. The ecu is looking for a resistance value that changes over time. The change has to be smooth an continuous up to a determined value. Besides using a manual pot is there a way to make it automatic?

Thanks
fiveten
 

audioguru

Well-Known Member
Most Helpful Member
Duplicate what it normally sees. A thermistor or light-dependent-resistor.
 

bmcculla

New Member
Various companies make digital pots. It works like a normal pot but will have say 256 "taps" that you can select digitally. Various interfaces are used: simple up/down, I2C, SPI.

Look on digikey.com.
 

fiveten

Member
I want it to act as a thermister in an automobile. The ecu wants to see about 80 deg. f at startup which is 1.5k and slowly rise to 180 deg. F which is 500 ohms it uses a negative coefficient thermister.
 

Juglenaut

New Member
If you are attempting to fog the ECU or ECM you must follow its peripheral tree, other sensors will have to be manipulated as well. If the car is new to being tweaked with the MEM cal might throw codes all the time, but if you can get it close to normal parameters it will not.

Another thing is if the ecu sees the engine cooler other componets will react differently on the set in the bin file, so in other words other sensors must be manipulated as well.

Small programmable logic controllers worked for my project. I also used several servos and other stuff. I still have 2 of them still performing there functions on my engine.
 

fiveten

Member
Thanks for everyones help.

I am building this because I am an automotive technician. I need this signal generator to fool the ecu that the engine is cold. I need to fool it because emission monitors will not run unless the ecu sees a cold start and engine warn up.

Juglenaut,

I am just trying to simulate a coolant temp and intake temp signal. It should not set a code if the ecu sees both signals showing temp. rise. The ecu focuses on these two signals until a certain temp or time period. Then other sensor parameters kick in. The old programing allowed you to just kick from a cold signal to a hot signal. The new instructions want to see a smooth transition. This is why I'm looking for that logic controller you mentioned.

I would use a thermister and heater but I'm trying to get it into a small package that is very portable and easy to tap into the vehicle wiring.


fiveten
 

audioguru

Well-Known Member
Most Helpful Member
Hi Fiveten,
You want to do the opposite to what I thought you would do.
Many mechanics just copy an emissions test of a car that passed, and for a few extra bucks will pass that good result somehow to your faling car's record so it looks like it passed.

You want to fool a car's computer so it makes a very rich mixture like it is cold, so that it fails the emissions test and you get a nice expensive "fix" to do on the car, then re-test.

I am sorry if I am mistaken but are you a crook?
 

fiveten

Member
I think you are misunderstanding audioguru. I am a technician, so I fix vehicles. I know many people are affraid of mechanics but this is another thread.

Here is a little background on vehicle engine management. On 1996 and newer vehicles auto manufacturers have to use a standardised communication system for interfacing with the vehicles ecu. It is called OBD2. As part of the programing the ecu need to monitor different emission control systems. I.E. Fuel EVAP system, Ign. timing, Fuel control, and exhaust emission system, to name a few. The ecu has to make sure these systems are functioning properly. In order to do this the ecu have to perform functional tests. These tests only execute when certain enable criteria has been met. The vehicles that I service have a very sensitive Evap system. The functional test also called a "monitor" will only run when the ecu sees a cold engine, (below 80 F. ) first start after cold soak. When I repair the vehicle I need to test the system by running the monitor. The monitor will not run until the ecu sees the cold engine. In stead of letting the vehicle sit overnight I will be able to hook this automatic resistor to it and the ecu will think that the vehicle has been sitting for a while. I am not a crook, I am building tools that are not available to make my job easier. So I can fix vehicles faster.
 

audioguru

Well-Known Member
Most Helpful Member
Sorry Fiveten,
I see that you need to fool the ECU to do a real repair quickly.

I think the whole idea of emissions testing is a joke. By far, most of the newer car owners are paying for the test, over and over to see that the air going into the engine is dirtier than its exhaust. My car's printout shows almost zero emissions, way lower than the limit, test after test. But trucks, older cars and boy racers with their emission control devices removed are polluting the air emmencely. Maybe the test is designed to catch the few people who never change their air-filter, but many of them just throw it away when the "service engine" light comes on!
I had a car that passed with flying colours. When I drove away I left the testing place in a cloud of black smoke (no, not a diesel). :lol:
 

jkmadsci

New Member
Depending on the control unit circuitry, you might be able to use an (optocoupler or photocoupler) to give variable resistence proportional to voltage. If this
idea works i could program a pic to ramp up a voltage over a 5 minute time period. As russik mentioned you could use a thermistor attached to a
small 2 watt resistor controlled by a pic

Worst case senerio would be to use a motorized pot or a servo controlled by a pic
 

bmcculla

New Member
something like:
http://www.electro-tech-online.com/custompdfs/2005/03/DS1804.pdf

Is a simple digital pot with 100 taps (just like a regular pot but with only 100 wiper positions). You can program the nonvolatile memory to load the initial resistance to be your cold engine temp. A 555 timer and counter (to devide the frequency down) can be set up to generate a regular increment/decrement pulse. If you set the time between pulses right you will have a nice automatic resistance ramp.

The part is ~$3 at digikey in an 8 pin DIP package. You might want to try to find the same part with 256 taps for a bit more accuracy. Any digital up/down pot with nonvolatile memory for the startup position should work.
 

audioguru

Well-Known Member
Most Helpful Member
The nonvolatile memory in most digital pots has a fairly low limit on how many times they are set. The limit is probablly OK for this application but for a volume control on a car radio or home stereo if it is used more than a few times per day then in a few years it will get stuck, hopefully not too loud.
 

bmcculla

New Member
This digital pot lets you modify the wiper register without changing the EEPROM contents. You would only need to write the NV memory once to set the "cold" resistance.
 

audioguru

Well-Known Member
Most Helpful Member
Writing to the NV memory of a digital pot has a limit of a fairly low number of times before it is "worn out" and the memory can't be changed anymore. A writing cycle occurs each time the pot's setting is changed a certain amount and during power down. 50,000 or 80,000 times? Even the manufacturer doesn't know. It sounds like a lot, but see what happens in a few years if you use it a lot.
Ouch! Why was my radio so loud?
 
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