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Temperature sensor and thermo-time switch Test E28 535I

Symptoms of bad E28 temperature sensor and thermo- time switch

  1. High Idle
  2. Poor Fuel economy
  3. Poor cold start

Learning Context

The temperature sensor sends the engine’s temperature information to the ECU to adjust the air-fuel mix ratio while the vehicle is driven. The thermo-time switch allows voltage to the cold start injector allowing extra fuel into the intake when the vehicle is started, after the vehicle starts resistance cuts voltage to the cold start injector stopping it from introducing fuel into the intake.

Why am I testing my E28 temperature sensor and thermo-time switch

I am testing both of these sensors as I am attempting to diagnose an excessive fuel consumption issue with my E28. I am going down the list of what can cause this problem, I start with the easy and get into the more difficult parts.

Summary

  1. Resistance test of temperature sensor
    • Temperature sensor resistance when cold 50°-80°F = 2,100-2,900 Ω
    • Temperature sensor at operating temperature 180°= 270-240 Ω
  2. Voltage reading for the Thermo-Time Switch
    • Check for voltage at the Black/Yellow cable while cranking the car.
    • Continuity should be present when cold across the pins of the thermo-time switch 
    • At operating temperature, No continuity should be present between the pins of the Themo-Time switch

3. Tools/Parts

Procedure

Before starting any job on your vehicle always wear your PPE(Personal Protection Equipment). All it takes is a second for you to get hurt, I advise everybody to wear eye protection and gloves when working on their car. I recommend getting a pair of protective eyeglasses from RaceGerman

Race german protective eyeglases being worn
Protective eyeglasses from Race German with lateral protection
  1. Locate both the temperature sensor and the thermo-time switch sensor. Both of these sensors are going to be found on the thermostat housing, and are right next to each other. Before doing any work on your car always consult your vehicle service manual, if you do not have one you can purchase a service manual for your specific vehicle at any auto parts store.
1987 535I E28 temperature sensor, and themo-time switch location.
The temperature sensor and Thermo-time switch are usually next to each other on the thermostat housing. (E28 535I)
1987 535i E28 thermostat housing diagram.
Location of the sensors on the Thermo state housing of a 1987 535I E28

2. You can distinguish the sensors apart from each other by the color of the connector, the temperature sensor is blue and the thermo-time switch is brown. 

1987 535i E28 thermostat sensor is blue and thermo-time switch is brown
Sensor color can change depending on who manufactures them, Bosch has kept the original color.

3. Switch your voltmeter to Ω and turn the dial to a value high enough to test the sensor’s resistance. I set my Voltmeter to 20K Ω as the cold resistance reading should be between 2,100-2,900 Ω

Voltmeter switched to 20K Ohms
Make sure you set your voltmeter to Ohms

4. Hook up each lead to one prong on the temperature sensor, you should get a resistance between 2,100-2,900 Ω when cold across the sensor. My temperature sensor had a reading of 3,380Ω

5. Next disconnect the Thermo time, take the red lead to the black and yellow connector and the black lead to the ground. You should briefly read the voltage at start up, during start up the black and yellow cable read a high of 9.78 volts.

The voltage reading at the black/yellow wire on thermo time-switch sensor

6. Switch your voltmeter back to the lowest Ω setting and measure continuity across the sensor. My temperature time switch read 33Ω which means that continuity is not present.

1987 535i E28 thermo-time switch Ohm reading when vehicle is cold.
The temperature time switch read 33Ω instead of continuity, and the thermal time switch failed. 1987 535I E28

7. Now start the vehicle and bring it up to operating temperature, for many BMWs of this year it is 180°F or 80°C

1987 535i E28 LCD temp sensor gauge
Digital temp gauge in Celsius not Fahrenheit 1987 535I E28

8. Once at operating temperature turn the vehicle off, take your voltmeter, and check the continuity of the Thermo time switch. At operating temperature continuity should not be present, this means that the sensor is operating as designed. In my case, the thermo-time switch failed as no continuity was present when cold, and resistance only increased as the engine reached operating temperature.

1987 535i E28 tremor-time switch Ohm reading at operating temperature.
My thermo-time sensor showed resistance at the operating temperature 1987 535I E28

9. Check the resistance of the temperature sender, it should read between 270-400Ω. My temperature sensor read 239Ω at operating temperature, the sensor is out of the range required and needs replacement.

conclusion

Both the temperature sender sensor and thermo-time switch failed the resistance and continuity test, I will replace both sensors and perform the test again.

If both sensor readings fall within the ranges mentioned both sensors are working correctly. This would be a place to start if you are experiencing excessive fuel consumption, replacing either of the sensors won’t fix the issue but it is beginning in diagnosing and using the process of elimination to find the problem. My advice if these sensors check out is to check the thermostat, Fuel injectors, Fuel pressure regulator, O2 sensor, and fuel filter.

update 03/22/23

I got and replaced the temperature sensor and performed the test. The sensor passed and I am still searching for a temperature time switch.

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