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Discussion Starter · #1 · (Edited)
I have a 1985 GLH converted to T2 GLHS specs and I have been experiencing a lean condition when I start the car cold. When I start it cold, it will go lean on the wideband, sputter, and eventually die. To troubleshoot, I back probed the coolant signal wire at the LM to get voltage readings. Here is what I found:
  • When the car has been sitting overnight, the coolant signal wire at the LM reads 2.5V with the ignition key on (not running)
  • When I start the car, the voltage on the coolant signal wire will begin to drop. After about 30 seconds or so of running, it will drop below 2V.
  • After the car runs for about 1-2 minutes, the coolant voltage jumps to 3.8V which is the voltage I would expect to see given the temperature of the coolant at the time.
Has anyone experienced this behavior? I did check 2 other different coolant sensors with the same result. If I plug in a new coolant temp sensor without it being installed in the head, shouldn't I be reading 4V? Everyone I have tried thus far has stated 2.5V. What would cause the voltage to jump to the correct level after the car has been running for a period of time?
 

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The CTS uses a dual resistor setup, a "cold step in resistor and a "hot step in resistor".
2.50 volts on a cold engine (cold step in resistor) is 75 degrees F, once the engine warms and the second resistor (hot step in resistor) comes into play 2,50 volts is 192 degrees F.
Voltage on a cold engine starts high, as the engine warms voltage drops, when it gets to 1.4 volts on the cold step in resistor the hot step in resistor comes into play and voltage will increase again to 3.80 volts and then drop down from there until the cooling fan cycles on at 212 degrees F (2.10 volts).

Going by that there is probably nothing wrong with your CTS circuit.
The chart on the yellow paper is live data from my car (2.2L VNT) and my Son's 3.0L using a digital voltmeter for voltage and a scanner for temperature.
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Discussion Starter · #3 ·
Thank you very much for the great explanation. I was under the impression that my lean cold start condition under due to low coolant sensor voltages. I will continue to troubleshoot but at least I know this is not a problem.
 
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