2051 reading car coolant temp sensor?

[SPD010273]

Ok here's the problem I have:

I have an unknown coolant temperature sensor that I want to read.
It's resistance changes with the coolant temperature. High resistance at low temperatures, low resistance at high temperatures.

The setup I want is an AT89C2051 reading the sensor's resistance (either thru analog comparator or preferably through other means which will use less pins), then display that temperature on a seven segment display containing 5 characters, 3 for the digits, one for the degree symbol, and one for the unit.

My question is how can I read the output of the sensor, or what parts can I use to make the signal readable by the 2051? I was also wondering if there was a way to read the temp straight from the ECU or would I have to piggyback the sensor's output.

If anyone can point me in the right direction it would be greatly appreciated.

the vehicle is a 1995 Jeep Cherokee Sport 4.0L EFI

As far as the sensor is concerned I havn't been able to find much info about it. It uses a 5VDC input voltage, and the output leads to the ECU. that's about it.

some more info here, this is a chart i dug up of resistances at certain temps:
**broken link removed**

 

[Oznog]

https://en.wikipedia.org/wiki/On_Board_Diagnostics
I know they used that in 1996, don't know about 1994 though.

Typically a thermistor is read with a fixed dc voltage, then a known value resistor, the thermistor, then ground. It forms a common voltage divider. Math will show you what voltage to expect for each temp.

You must keep the existing source voltage and resistance which are probably with the computer but the resistor could be inside the sensor, if the sensor has 3 connections this is the case. So really you just need to figure out what that resistance is so you can interpret the temp after tapping the voltage.

 

[SPD010273]

the resistor only has two connections, one steady 5VDC, one the signal. That was the method I was going to use, have signal in series w/ another resistor, say 1k, then use an ADC to read voltage at junction, send that to 2051 which relays it to 7-seg display, but so far I havent been able to find anything about the CTS. im gonna look into the ECU some more tomorrow.
I beleive it's OBD II on my jeep I'll have to check.

Oh My God... found an ECU schematic
**broken link removed**

I'll see if I can use maybe a full size 8051 to interface w/ the ECU and get some more data other than coolant temp.

 

[SPD010273]

okay, apperently, the coolant temperature sensor is that circle near the top left corner of the SBEC in the diagram, coming off of pin 2.

here's the diagram
http://jeep.avtograd.ru/Cherokee/injection/mopar/SBEC/index.htm

the site is obviously russian, so I used babel fish to do some very rough translation. the bold black wires connected to A on the sensor is ground.

I beleive this diagram is very accurate as the pin assignments match up with the schematic in my earlier post for items such as ignition coil, compressor, MAP sensor, etc etc.

5VDC comes from the ECU (brown/black wire)
goes to ground thru black wire

now how can i pick up a signal and read it in a digital sense from a microcontroller, without interfereing w/ the operation of the ECU

 

[Oznog]

Is the signal always 0<=x<=5v? Then you can just connect it right to an ADC pin. The ADC is a very high impedance load. It probably won't affect the computer's reading, though there is a chance that the thermistor in parallel with the pullup resistor is too high an impedance for the ADC to measure it with full accuracy. In this case you can buffer it through a unity gain op-amp. The buffer will remove any impedance issues for the ADC and it will be essentially zero loading on the ECU's thermistor pin. Also the op amp will mean if you accidentally have the uC's pin configured as an output it won't affect the ECU operation (though it may damage the op amp).

 

[SPD010273]

ok, one of my last (hopefully) questions it how is the SBEC reading the thermistor if it's supplying 5V to the thermistor, is it measuring current from that 5v pin (I havn't matched the SBEC pin 2 to the schematic, so anything could be between the motorolla MCU and the SBEC's pin 2.

You're saying I should connect the ADC directly to the output of the thermistor?