2002 Saturn L200 L61 2.2L field repair with limited tools. I wasn't prepared for this, just happened by and asked to diagnose it by a mechanic friend. Only tools used were a code reader with live data and a meter, along with a handful of resistors.
No codes set, car running pig rich on startup and poor idle. No previous repair history (new owner) other than a new ECT, replacing what looked like an already new ECT. Live data showed ECT at 19F and IAT at -4F even though ambient was close to 80F. All other data PIDs were normal except for fuel trims of course.
Unplugging the ECT resulted in -40F, good. Bypass test by shorting the connector resulted in 303F, also good. Unplugged, both the IAT and ECT showed a stable 5V reference, both to the return and also to battery ground. All other 5V reference circuits I tested were also good, TPS, MAP, and AC pressure switch. Knowing ECT circuit design and operation I concluded there were a few possibilities.
Possible:
Bad sensor (possible but two in a row?)
Resistance in signal or signal return circuits
Bad ground (unlikely in light of other PIDs normal)
Bad PCM (I hate having to call a bad PCM)
Since all other PIDS were normal, I opted to ignore the bad ground possibility for the time being and focus on resistance in the ECT circuit. Using a 2700 Ohm resistor to jump the connector resulted in -3F, which should have been near 77F. To rule out resistance in the return circuit, I used battery ground and got the same result. To rule out resistance in the signal circuit I backprobed the ECM connector on the ECT circuit and got the same result, using both the return circuit and battery ground.
With the ECT reporting low temps, we know that the signal voltage is higher than expected, which rules out shorts to ground, and with a normal 5V reference we can rule out a short to voltage so no need to isolate the circuit from the PCM. Before condemning the PCM I did clean and check all the grounds and splice packs anyways.
The next day a used PCM was sourced and installed (cheap owner) and initial tests showed normal ECT and IAT PIDs. Came prepared the second visit with a scantool, an antitheft relearn was performed and the engine started and ran normally. Performed a CKP relearn for good measure and cleared all generated codes from testing etc.
Just wondering if there was anything else I could have checked with limited tools?
I can see why you were hesitant to make the PCM call. I've never seen one show incorrect thermistor PIDs without an associated wiring or sensor issue. Both reading significantly lower than normal... I'm trying to imagine what could have failed in the PCM that would cause that. :huh:
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