1985 Mercedes Benz SL500 Please describe this

  • 1985 MERCEDES BENZ SL500
  • V8
  • 2WD
  • 63,000 MILES
What is the porpuse of the engine temp sensor, besides doing what it says.
Do you
have the same problem?
Monday, September 22nd, 2008 AT 10:54 PM

1 Reply


Input from the coolant sensor may be used by the PCM for any or all of the following control functions:

* Start up fuel enrichment on fuel injected engines. When the PCM receives a cold signal from the coolant sensor, it increases injector pulse width (on time) to create a richer fuel mixture. This improves idle quality and prevents hesitation while the cold engine is warming up. As the engine approaches normal operating temperature, the PCM leans out the fuel mixture to reduce emissions and fuel consumption. A faulty coolant sensor that always reads cold may cause the fuel control system to run rich, pollute and waste fuel. A coolant sensor that always reads hot may cause cold driveability problems such as stalling, hesitation and rough idle.

* Spark advance and retard. Spark advance is often limited for emission purposes until the engine reaches normal operating temperature. This also affects engine performance and fuel economy.

* Exhaust gas recirculation (EGR) during warm-up. The PCM will not allow the EGR valve to open until the engine has warmed up to improve driveability. If EGR is allowed while the engine is still cold, it may cause a rough idle, stalling and/or hesitation.

* Evaporative emissions control canister purge. Fuel vapors stored in the charcoal canister are not purged until the engine is warm to prevent driveability problems.

* Open/closed loop feedback control of the air/fuel mixture. The PCM may ignore the oxygen sensor rich/lean feedback signal until the coolant reaches a certain temperature. While the engine is cold, the PCM will remain in "open loop" and keep the fuel mixture rich to improve idle quality and cold driveability. If the PCM fails to go into "closed loop" once the engine is warm, the fuel mixture will be too rich causing the engine to pollute and waste gas. This condition may also lead to spark plug fouling.

* Idle speed during warm-up. The PCM will usually increase idle speed when a cold engine is first started to prevent stalling and improve idle quality.

* Transmission torque converter clutch lockup during warm-up. The PCM may not lockup up the torque converter until the engine has warmed up to improve cold driveability.

* Operation of the electric cooling fan. The PCM will cycle the cooling fan on and off to regulate engine cooling using input from the coolant sensor. This job is extremely important to prevent engine overheating. Note: On some vehicles, a second coolant sensor or switch may be used for the cooling fan circuit only.
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Monday, September 22nd, 2008 AT 11:19 PM

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