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Discussion Starter · #1 ·
Having changed the timing belt, after operating temp my timing light shows six degrees before unplugging the coolant temp sensor and 12 degrees after unplugging coolant temp sensor. I thought the coolant temp sensor would Advance the timing rather than retard it? What am I seeing, and what am I going for here?
 

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1989 Dodge Shadow 2.5 non turbo 3 speed automatic
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my timing light shows six degrees before unplugging the coolant temp sensor and 12 degrees after unplugging coolant temp sensor.
It is advancing the timing. 6 is closer to 0 than 12. It should be at 12 with the sensor unplugged.
 

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It is advancing the timing. 6 is closer to 0 than 12. It should be at 12 with the sensor unplugged.
BTDC means before top dead center. That would mean 12° would be more advanced than 6°. To set timing properly, the coolant temp sensor gets unplugged which sets it in a limp-in or limp home mode. The higher the RPM the more advanced the ignition has to be to complete combustion before the piston travels down ATDC, hence why we need more advanced timing with increased RPM.
Your timing was correct with the initial timing @ 12° with coolant temp unplugged.
 

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Discussion Starter · #4 ·
Right, my timing without the sensor plugged in is 12 degrees. But, the timing goes back down to 6 degrees when the coolant sensor is plugged back in. It seems like the coolant sensor should advance the timing more after it is plugged back in rather than sending it back down?
 

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Discussion Starter · #5 ·
Right, my timing without the sensor plugged in is 12 degrees. But, the timing goes back down to 6 degrees when the coolant sensor is plugged back in. It seems like the coolant sensor should advance the timing more after it is plugged back in rather than sending it back down?
 

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Discussion Starter · #6 ·
Interesting, I guess we get a lot of attention on the 12-degree Baseline reading without the sensor plugged in but very little mention of what the exact timing would be at idle when the coolant sensor is plugged in. Does six degrees sound like a reasonable timing at idle with the sensor plugged in?
 

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The engine controller uses info from ALL sensors (MAP, CTS, TPS, ACT, VSS, Knock) underhood to determine ignition timing under all operating conditions so with all systems functioning properly there is no "real spec" for ignition timing, it is based on sensor input and controller programming.
All changes that the controller makes are based on the base setting which should be 12 Degrees BTDC.

To get to the base setting the system needs to be in failure (limp in mode).
Disconnecting the CTS with the engine running puts the system into failure by setting a hard fault (Code 22 CTS voltage high) which does a couple of things...
Illuminates the power loss/check engine lamp, cycles on the cooling fan, the controller uses a substitute value of 104 degrees on 87 and up vehicles and brings ignition timing to "base".

Remember...
The CTS needs to be unplugged with the engine at normal operating temp and the engine running.
After ignition timing is set you need to shut the engine off before reconnecting the CTS.

IGNITION-2.2-2.5 Ignition Timing.jpg IMAGE-Timing Marks 2.jpg
 

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Discussion Starter · #8 ·
I swapped out my Standard Motor Products brand coolant temp sensor for an NGK. Both sensors have the same timing of 6 BTC at idle when plugged in (12 degrees BTC unplugged). However, the NGK clearly produces more power. I would assume it is advancing the timing more?
 

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You do not understand how the system works and why you are disconnecting the CTS, the CTS does not put out power and has very little to do with actual ignition timing.
It's only job is to Inform the engine controller of engine temp for fuel/spark advance corrections along with other sensors and cooling fan operation, it Does Not Control Anything, it only provides INFO to the engine controller.

The CTS is a "Thermistor", it is the opposite of a "Resistor", as it gets hot it's resistance lowers.
The engine controller sends out a 5 volt reference to the CTS and looks to see how much voltage comes back, it then converts that voltage reading to a temperature.

Ignition Timing/Spark Advance is 100% controlled by the engine controller based on inputs from various sensors...
Map Sensor, Coolant Temp Sensor, Air Charge Temp Sensor, Throttle Position Sensor, Vehicle Speed Sensor, Knock Sensor, O2 Sensor (Closed Loop Only) and HEP.
The Map Sensor is the Dominant Fuel/Spark Advance Sensor as it informs the engine controller of actual load, the CTS, TPS, ACT, VSS, O2 are minor players in the fuel control/spark advance corrections decisions made by the engine controller.
If the controller loses the crank/rpm signal from the HEP the engine will not run.

The engine controller has no idea where base timing is, it makes all of its adjustments to spark advance by advancing or retarding ignition timing based on the assumption that base timing is correct, if it reads sensor values and decides by its programming it needs to advance ignition timing by 5 degrees, (it is looking for 17 degrees BTDC total spark advance), if base is not at 12 degrees BTDC will total spark advance be at 17 degrees BTDC when the controller advances by 5 degrees...NO

When you disconnect the CTS connector with the engine running the engine controller see's the circuit go open, it then turns on the power loss/check engine lamp, cycles on the cooling fan and goes into "Failure Mode" (aka..., Limp In Mode) which is a fixed program which is designed to allow the vehicle to run well enough to get it to a shop, hence the term "Limp In Mode" since that is basically all the early systems would allow.
One of the things that happens in "Failure" is that ignition timing is brought to base with no engine controller control of spark advance.

The only reason for unplugging the CTS is that the CTS is an easily accessible sensor and will put the vehicle in "Failure Mode" when disconnected so you can check/adjust Base Timing.

When you reconnect the CTS and the system is now functioning normally again and the engine controller now takes over all spark advance correction duties as it is programmed to do...
Your reading of 6 degrees BTDC is because the engine controller has decided that is all you need at idle under the conditions you are running at, you cannot change that, the engine controller is doing the job it was programmed to do. (Assuming all of your basics are correct)

If you want to see total spark advance/knock retard under all operating conditions then connect a scanner and go for a ride and watch spark advance under acceleration, no load cruise, decel, WOT, etc.

Here is a much simpler explanation from the Chrysler Fuel Injection Training Manual.
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