How To Test The ECT Sensor (1988-1993 2.8L Chevrolet S10 Pickup, GMC S15 Pickup, GMC Sonoma)

The engine coolant temperature (ECT) sensor might look tough to test, especially since the repair manual calls for a scan tool with live data capability to test it, but it's actually quite simple to test without a scan tool.

In this tutorial, you'll find the step-by-step instructions on how to test the ECT sensor with a multimeter. The results of your tests will let you know if the ECT sensor is good or bad.

APPLIES TO: This tutorial applies to the following vehicles:

• 2.8L Chevrolet S10 Pickup: 1988, 1989, 1990, 1991, 1992, 1993.
• 2.8L GMC S15 Pickup: 1988, 1989, 1990.
• 2.8L GMC Sonoma: 1991, 1992, 1993.

This tutorial also applies to the 2.8L Chevy S10 Blazer, 2.8L GMC S15 Jimmy. See the 'Applies To' box on the left column (desktop) or at the bottom of the page (mobile device) for more info.

Symptoms Of A Bad Engine Coolant Temperature (ECT) Sensor

The ECT sensor provides critical engine temperature data to your vehicle's onboard computer, which is responsible for monitoring and adjusting the engine's performance.

A problem or failure of the ECT sensor will usually cause one or more of the following symptoms:

• Check Engine Light (CEL) illuminated by one of the following trouble codes:
  • Code 14: ECT (Engine Coolant Temperature) High Temperature Indicated.
  • Code 15: ECT (Engine Coolant Temperature) Sensor Signal Voltage High.
• Poor engine performance: Inaccurate temperature readings can cause the engine to run too rich or too lean, resulting in poor engine performance, reduced fuel economy, and increased emissions.
• Engine stalling or rough idle: A malfunctioning ECT sensor can cause the engine to stall or experience a rough idle, as the ECM may receive incorrect information about the engine's temperature, leading to incorrect fuel delivery.
• Hard starting (extended cranking time): Inaccurate temperature readings can cause the fuel injection computer to assume that the engine is either too hot or too cold and deliver an incorrect amount of fuel to the engine. This incorrect fuel amount may cause the engine to take longer to start.
• Engine does not start: Inaccurate temperature readings can cause the fuel injection computer to assume that the engine is either too hot or too cold and deliver an incorrect amount of fuel to the engine. This incorrect fuel amount may not be enough for the engine to start.

What Does The Engine Coolant Temperature (ECT) Sensor Do?

The engine coolant temperature (ECT) sensor's primary purpose is to measure the temperature of the engine coolant and provide that information to the engine control module (ECM).

The ECT sensor can measure the engine coolant temperature because it's a thermistor. As you may already know, the electrical resistance of a thermistor changes with temperature.

Upon receiving 5 Volts from the fuel injection computer, the ECT sensor creates a voltage drop within its circuit as a result of its internal resistance.

This voltage drop is then measured by the fuel injection computer, allowing it to determine the engine coolant temperature based on pre-programmed voltage values.

The accuracy of the ECT sensor is critical for the proper functioning of the engine, as it helps the ECM adjust the fuel delivery, ignition timing, and other engine operating parameters.

Common Engine Coolant Temperature (ECT) Sensor Problems

There are several common problems that can cause an ECT sensor to fail.

• The ECT sensor simply wears out and its performance degrades over time.
• The ECT sensor suffers an internal open-circuit problem.
• The ECT sensor suffers an internal short-circuit problem.
• The insulation on the wires of the ECT sensor connector has peeled off and the wires are shorted together.
• The locking tab on the ECT sensor connector is broken, resulting in an intermittent connection between the sensor and the ECM.

Any of these conditions will cause the sensor to send an incorrect signal to the engine control module (ECM), resulting in engine performance problems, as well as setting diagnostic trouble codes (DTCs).

What Tools Do I Need To Test The ECT Sensor?

To test the ECT sensor, you'll need a few tools:

• A multimeter.
• A code reader (optional).

A Live Data capable OBD-I system scan tool can be quite expensive, often costing thousands of dollars. Fortunately, you don't need one to test the ECT sensor.

Although a code reader isn't required to test the ECT sensor, it can save you time by quickly reading ECT sensor diagnostic trouble codes rather than the slower method of manually decoding the check engine light ON/OFF flashes.

What is a must-have tool for the ECT sensor test is a multimeter. I have a few recommendations if you're looking for a fully-featured yet budget-friendly option: Buying A Digital Multimeter For Automotive Diagnostic Testing.

Initial Checks Before Starting The ECT Sensor Tests

Before performing the ECT sensor test, there are a few initial checks that should be made.

• Inspect the ECT sensor connector wires for any damage, such as cuts or fraying.
• Inspect the ECT sensor connector for any damage, such as cracks or a broken locking tab.
• Inspect the ECT sensor for any damage, such as cracks or corrosion.

A frayed wire is a wire that has become damaged and its metallic innards have become exposed and is a common problem with the engine coolant temperature (ECT) sensor connector.

You can see an example of wire fraying here:

• IAT Sensor Wire Insulation Problem And Wires Shorted Together.

Frayed wires can cause issues such as a short-circuit issue, causing the ECT sensor to report an inaccurate temperature reading and illuminate the check engine light.

To determine the severity of the fraying, inspect the wire carefully. If the fraying is severe or near the connector, it's best to replace the entire connector.

If the fraying is not severe and far enough from the connector, wrapping the frayed section with layers of electrical tape can provide a temporary fix.

However, it is important to note that electrical tape is only a temporary solution and that replacing the connector is the only way to ensure a permanent fix.

TEST 1: Checking For ECT Sensor Trouble Codes

NOTE: If you have already confirmed a DTC is present for the ECT sensor, you can skip this section and go to TEST 2.

When diagnosing a potential issue with the engine coolant temperature (ECT) sensor, it's best to begin by checking for diagnostic trouble codes (DTCs).

Retrieving the trouble codes can be done with a code reader or manually. If you need to retrieve the codes manually, I've written a tutorial to help you interpret the ON/OFF flashes of the check engine light:

• How To Retrieve OBD I Trouble Codes (1986-1993 2.8L Chevy S10 Pickup, GMC S15 Pickup, GMC Sonoma).

Alright, these are the test steps:

1. 1

   Locate the onboard diagnostic (OBD) port in your vehicle
   
   This is usually located under the dashboard near the steering wheel.

2. 2

   Connect the code reader to the OBD I port.

3. 3

   Turn the ignition key to the ON position but don't start the engine.

4. 4

   Follow the code reader's instructions to retrieve the diagnostic trouble codes.

5. 5

   Write down the codes.
   
   Is code 23 ECT (Engine Coolant Temperature) Signal Voltage High present?
   
   Is code 25 ECT (Engine Coolant Temperature) Signal Voltage Low present

Let's see what your test results mean:

CASE 1: ECT sensor codes are present. Now that you've confirmed that the ECM is aware of an ECT sensor problem, your next step is go to: TEST 2: Checking For An Internal Open-Circuit/Short-Circuit Issue.

CASE 2: ECT sensor codes ARE NOT present. It looks like we've ruled out the engine coolant temperature (ECT) sensor as the cause of your engine performance issue.

Since no codes related to the ECT sensor were found, you can conclude that at this moment the ECT sensor is functioning properly.