1985, '86 & '87 Coolant Sensor and Circuit
Internally, the 1985, '86 & '87 coolant sensor also has NTC thermistor, however; there is no fixed resistor in parallel with the NTC thermistor. See Fig. 15. This makes the coolant sensor's resistance higher. It is rated at 9,120 ohms - 10,880 ohms at 77F. Different coolant sensor circuits inside the logic module are used depending on coolant sensor temperature. The following sequence of events is what happens on a cold engine (32F) that is warming up to normal operating temperature. Inside the logic module there are 2 pull-up resistors. When the ignition key is first turned "on", the microprocessor will remove the ground on the base circuit of the PNP transistor. See Fig. 15. This will turn "off" the PNP transistor. With the PNP transistor turned "off", the emitter is no longer connected to the collector. This shuts off the 5 volts to the 1,000 ohm pull-up resistor. This is called switching out the 1,000 ohm pull-up resistor. There is a 2,700 ohm resistor that is connected from the emitter to the base of the PNP transistor. The resistor is there to make sure the transistor turns "off" when the microprocessor removes the ground from the base circuit. This will put 5 volts on the base circuit and the transistor needs a grounded base circuit to be "on".
The 1,000 ohm pull-up resistor is switched out when the ignition key is first turned "on". Then the logic module sends a regulated 5 volts to the 10,000 ohm pull-up resistor. See Fig. 15. After the 10,000 ohm pull-up resistor, the 5 volts are dropped to some lower voltage (depending on the coolant sensor resistance). This voltage leaves the logic module on the TN/WT wire and goes to the coolant sensor and then to signal ground at the fuel rail mounting bolt. This voltage is also present at the input to the AID converter. The resistor and capacitor at the input to the AID converter are for RFI or noise suppression.
The coolant sensor signal voltage is taken after the pull-up resistors. The coolant sensor signal voltage is, with the 1,000 ohm pull-up resistor switched out, actually the voltage remaining between the 10,000 ohm pull-up resistor and the coolant sensor.
To calculate the coolant sensor signal voltage, subtract the voltage drop across the 10,000 ohm pull-up resistor from the 5 volt source. For example, a cold (32F) engine may have 3.89 volts for a coolant sensor signal voltage. As the engine warms up, the coolant sensor resistance decreases. This causes the voltage drop across the 10,000 ohm pull-up resistor to increase with engine temperature. As the voltage drop increases, the coolant sensor signal voltage will decrease with engine temperature. For example, the cold (32'F) engine is now at 77F. This will decrease the coolant sensor resistance to 10,000 ohms (approximately). Now there are 2 10,000 ohm resistors in series (the pull-up resistor in the logic module and the coolant sensor thermistor). They will each use 1/2 of the 5 volt source. The coolant sensor signal voltage has decreased from 3.89 volts at 32F to 2.5 volts at 77F. The coolant sensor signal voltage will continue to drop as the engine warms up. At approximately 125F, the coolant sensor signal voltage is 1.25 volts. At this point the engine temperature may increase by a 100F or more, but there is only 1.25 volts left after the 10,000 ohm pull-up resistor. This is true because the 10,000 ohm pull-up resistor is using up most of the 5 volt source. The coolant sensor resistance has decreased so much (from 35,000 ohms at 32F to 3,400 ohms at 125F) that it would take a large temperature change to affect the coolant sensor signal voltage. This would make the coolant sensor signal voltage inaccurate at higher engine temperatures. Hot engine temperature accuracy is needed to determine when to turn the cooling fan on and off.
At approximately 125F, inside the logic module, the microprocessor will ground the base circuit of the PNP transistor. See Fig. 15. This will turn "on" the PNP transistor. With the PNP transistor turned "on", the S volts on the emitter will be connected across the collector to the 1,000 ohm pull-up resistor. This is called switching in the 1,000 ohm pull-up resistor. There is a 3,300 ohm resistor in the base circuit of the PNP transistor. This resistor is there to limit current flow and thus reduce heat. Now, in the logic module there is, the 10,000 ohm pull-up resistor in parallel with the 1,000 ohm pull-up resistor. The calculated or effective resistance of the pull-up resistors is 909 ohms. This will decrease the voltage drop because the coolant sensor is the larger resistor again. For example, with the 1,000 ohm pull-up resistor switched in, the calculated or effective resistance of the pull-up resistors is 909 ohms, this is in series with a 3,400 ohm resistor (the coolant sensor). The voltage drop will decrease dramatically (when the 1,000 ohm pull-up resistor is switched in) to 1.05 volts. This will increase the coolant sensor voltage to 3.95 volts. From this point, the coolant sensor signal voltage will continue to drop again as the engine temperature increases. The coolant sensor signal voltage is now very sensitive to engine temperatures that are on the hot end of the temperature scale.
In summary, the 10,000 ohm pull-up resistor is for cold temperature accuracy while the 1,000 ohm pull-up resistor is for hot temperature accuracy.
When checking the coolant sensor signal voltage, one must know if the 1,000 ohm pull-up resistor is switched in or out. Then it will be known which temperature vs. voltage curve to use to translate a voltage reading into a temperature. For example, a coolant sensor signal voltage may read 3.6 volts. This could mean the engine temperature is 40F or 140F. Use the cold curve when the engine temperature is below l00F. See Fig. 18. Use the hot curve when the engine temperature is above l25F. There is an overlap period of 25'F where the logic module could have the 1,000 ohm pull-up resistor switched in or out. The reason for the overlap period is to stabilize the coolant sensor signal voltage during the switch over point. If there were no overlap period and the engine temperature was right at the switch point, the logic module may switch the 1,000 ohm pull-up resistor in and out with only a slight change in engine temperature. This would change the coolant sensor signal voltage back and forth from cold to the hot curve with a large change in voltage. With the 25F overlap period, once the logic module has switched in the 1,000 ohm pull-up resistor (hot curve), the engine would have to cool down 25F before the logic module would switch out (cold curve) the 1,000 ohm pull-up resistor. The overlap period stabilizes the coolant sensor voltage during the switch, from cold to hot or hot to cold curve.
For the overlap period between l00F and 125F, if the engine is above 100F and the temperature is rising, use the cold curve. If the engine was above 125F and is cooling down into the overlap period, use the hot curve till the engine temperature is below l00F.
