Buck,
You will have to splice an extra length of wire to get it any further than the base of the airbox. The connectors for the sensor are small and not standard. There should be no chance of a code from extending them, and since the sensor is plastic covered and in a plastic mounting package, polarity should not be a concern.
The service manual specifically states that the sensor is hard-wired to the overhead console.
On the splice, 4 red butt connectors and some 3/8 heatshrink tube would assure a great, trouble-free connection. I would use 18 gauge wire, to match what the stock wiring seems to be.
The sensor is nothing more than a changing resistance that the display sees as a direct co-relation to temperature. The display sees infinite resistance (open circuit) as -49F, and zero resistance as 140F.
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Here are some interesting tidbits from the service manual:
Since the ambient temperature sensor is in close proximity to the radiator, it can pick up engine heat giving a false high outside air temperature reading known as Heat Soak. In order to display a more correct outside temperature, the overhead console will not increase the displayed temperature until it detects movement in the compass over a two-minute period, to insure that there is adequate airflow over the sensor. Movement over a two minute period is best understood as once in the first minute, then some more detected movement in the second minute.
If the display shows an open circuit OC ( or -49° F (-45° C) then there is an OPEN CIRCUIT or a missing sensor and the condition must be corrected. Keep in mind that after a repair is made, the vehicle may need to be driven to update the temperature for the reasons described above. Even if the engine is cool, it will not increase from -49°F without detecting movement.
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The ambient temperature sensor is a variable resistor that operates on a five-volt reference signal. The resistance in the sensor changes as temperature changes, changing the temperature sensor signal circuit voltage to the compass / temperature display unit. Based upon the resistance in the sensor, the compass / temperature display unit senses a specific voltage on the temperature sensor signal circuit, which it is programmed to correspond to a specific temperature.
The compass/temperature display unit then displays the proper ambient temperature.
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The thermometer function is supported by the ambient temperature sensor, a wiring circuit, and the compass / temperature display unit. If the display shows OC (-49° F (-45° C) or SC (140° F (60° C),, there is an OPEN or SHORT CIRCUIT and it must be repaired before the VFD can be tested.
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Measure the resistance of the ambient temperature sensor. At room temperature (approx. 68°F), the sensor resistance should be between 9-11 Kilohms (9000-11000 ohms). The sensor resistance should read between these two values.
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Resuming my comments: If Open Circuit is infinite resistance, display is -49F, and Short Circuit is 0 ohms, display reads 140F. We can guess from there it is a inverse reading comparator circuit; display shows higher numbers as resistance lowers in reaction to heat.
Another hint is the range of 9 to 11K ohms at 68F; aproximately half of the maximum positive limit.
However, looking at the Short Circuit (zero ohms) = 140F, I can only speculate that unless there is a way to change the input conditioning of the circuit and use a wider range temp probe, there may not be a way to read higher temps.
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Strangelove,
We are working with 5 volts here; the first hint that this is a TTL circuit. I will try to take my overhead console down and take a look at the components. If I can identify the comparator IC, I will grab a data sheet and see what could be done to the PCB to extend the reading range.
Theoretically, the display should be able to read up to 999F, using the 3 display digits. The kicke
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