Why don't thermostats have a sensor?

Question

One of the more common reasons of overheating is a stuck thermostat. One of the most common reasons for blown head gaskets is overheating. One of the most expensive vehicle repair procedures is head gasket replacement. Most thermostats, on the other hand, cost around $20 and the replacement is well within the DIY domain.

Why isn't there a sensor on the thermostat to detect opening and closing and notify the ECU, which, in the absence of an expected pattern in thermostat activity logs, relative to other related factors, would raise a red flag in the form of an engine code to alert the driver to stop driving until the thermostat is fixed?

I imaging something like that should be relatively easy to record and preventively alert the driver.

Answer 1

It might seem easy to integrate a sensor that detects the opening/closing of a mechanical thermostat's pintle (LVDT, for example) but here are some challenges which would need to be addressed:

• Cost vs Benefit

  Is a "smart" thermostat really worth the extra hassle and money when its dumb counterpart has the following to offer? :

  • it's already cost-effective
  • it isn't usually failure-prone
  • you can tell if it fails stuck-closed because coolant temperature will climb
  • you don't worry too much if it fails stuck-open as there isn't any serious risk of short-term, catastrophic engine damage (engine runs cooler than it should, HC emissions will worsen, maybe clogged secondary air tracts and cats in case of serious neglect over a sustained period)

• Operational challenges

  Remember the thermostat is deployed in a relatively hot and wet environment, which isn't really ideal for electrical systems due to material and hazard-related challenges.

  It would be difficult to justify the introduction of an early-warning sensor at the expense of reliability and part longevity given the application's criticality.

• KISS principle

  Beyond serving as a diagnostic aide, I cannot think of any additional benefit that modern-day engine management could reap from knowing the thermostat pintle's position. One could correlate it to coolant flow rate but we don't really care about that¹; its temperature is sufficient and we already have a sensor present for that purpose.

  Given this and the thermostat's reliability, design engineers would be more than happy to delegate the responsibility of coolant temp regulation to a mechanical system and take its operation for granted.

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¹ - some people do, but this is more of a nice-to-have than something mission-critical

Answer 2

As per your question the issue is about overheating. For that we already have a coolant temperature sensor, which can alert the driver when there is over heating. For your kind information there are electrically controlled thermostats in some modern engines of Mercedes and maybe in some others too!!!