the coolant temp sensor in our trucks is a resistor that varies its resistance based on
the temperature of whatever the resistor is placed into. in this case its water. ( this
kind of resistor has special name, its called a "thermistor" )
also note that the PCM is measuring the current that is flowing thru the resistance.
when the resistance varies, so does the current.
2 problems arise:
1. ) there is an tolerance factor for any resistor. i think the temp sensor could be as
large a tolerance as +/- 5% at any given temperature. so at 180* that about +/- 9* !!
2. ) in the PCM there is a conversion "formula" to convert the current flow into a
temperature. this formula was "burned" into the PCM forever. it cant be changed. The
formula was based on a spec for the temp sensor. ( in other words, it was based on
what DC thought the typical temp sensor would give as input to the PCM.) This is
sometimes called a 'calibration curve'. The PCM can not be re-calibrated so we are
stuck with a tiny amount of error in the reading becuase not all temp sensor have the
same curve. I would guess about +/- 1% for this calibration error.
So, when you add up the errors it can become significant.
-dan
Dan K.
'92 Grey Dak CC, 318, 4x2, 46RH, 3.55 rear, 235/75R15, PS, PW, PB, PL, AC.
Mods: Cool air, K&N oil/air, 180* T-Stat, Autolite 3924s, re-located IAT, Trans-Go shift kit.
Future: Swap motor for a slightly modified Magnum 5.9L engine.
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