Anyone have any idea where a person could find a schematic or listing of some kind that shows what the acceptable voltages are from a particular sensor?

Specifically, I'm looking to get the information on what the range of acceptable voltages going from a the sensors on an engine to the ECU.

My theory is that the ECU looks for a particular voltage from the sensor. If that voltage exceeds or drops below the acceptable value, the ECU logs an error and triggers the CEL.

Any thoughts on this?

 

Here you go. I hope you have an OBD2 car or this might a lil confusing unless you wanna convert the pinouts with an OBD1 ecu.

https://honda-tech.com/zerothread?id=1284177

 

Awesome...

I do have an OBD1 ECU that I'm working with... Does anyone know if the voltages for the various sensors would be about the same for OBD1? I'm thinking that OBD2 would probably be higher, but I would really like to see a chart for an OBD1 to confirm.

I checked (quickly) a pdf version of a '94 teg service manual I have this morning, but didnt' see a chart like that in there...

Anyone else?

 

The manual will not have a single chart. but if you follow their trouble shooting guides and flow charts, they will tell you what voltages to expect. Is there a specific sensor you are testing?

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by B18C_EJ8 &raquo;</TD></TR><TR><TD CLASS="quote">My theory is that the ECU looks for a particular voltage from the sensor. If that voltage exceeds or drops below the acceptable value, the ECU logs an error and triggers the CEL.</TD></TR></TABLE>

Your theory is correct, but only covers part of the story. The ecu also looks for ineteractions between sensors and the expected behavior. This double checking gets more complex in higher OBDs, OBD2 being the most complex due to the emmisions.

For example, if the ecu sees the map signal changing with the TPS signal staying constant, it will trigger the check engine light and store a code for the TPS sensor. OBD2 cars also monitor long term and short term fuel trim, and set a check engine light and store a code if they exceed certain values. OBD2 cars also monitor the crank fluctuation sensor and compare it with expected values to determine a misfire, and even which cylinder is causing it.

 

Hmmm...

So what you're saying is that if I just get an ECU and feed the necessary voltages to the appropriate pins, might not be enough to stop that damn CEL from coming on???

I'm not working on this to circumvent a stubborn CEL... This is rather a "science fair" project of mine...

 

Correct. Just because the sensor value is between 0-5v or whatever it's "allowed" to be, doesn't mean the ecu is going to be happy with it. For instance, if you kept the O2 sensor signal at 0.45v, you would have an angry ecu. Being a closed loop system, it expects certain behavior based on the commands it gives to various acuators, like fuel injectors, egr, evap system, etc.