AfterMarketNews Auto Care Pro AutoCareCareerHub Brake&Frontend BodyShopBusiness Counterman EngineBuilder Fleet Equipment ImportCar Motorcycle & Powersports News Servicio Automotriz Shop Owner Tire Review Tech Shop Tomorrow's Tech Underhood Service Speedville

‘Green’ Bullets: Tips for Servicing Hybrid Vehicles

Newsflash: Hybrids are here to stay. The oldest Toyota Prius models are now 14 years old and many other hybrids from other manufacturers are crossing the 100,000-mile mark every day. Here are a few of my Silver Bullets, — or in this case “Green...

Read more...

Legendary Spark Plugs: The Six Toughest Vintage Spark Plug Replacements

1. AMC Pacer: The Pacer was supposed to get a GM Wankel rotary engine. If this were the case, the four plugs would have been easy to access. But thanks to emissions problems, the Pacer would get the regular AMC straight six. This forced the engineers...

Read more...

Battery Service and Diagnosis

A good battery with an adequate charge is absolutely essential for reliable cold starting. A weak battery, or one that is rundown, may not deliver enough amps to crank the engine when temperatures plunge and the oil thickens. Cold weather can be hard...

Read more...

The Problem With Living In The 'Now'

I once had a shop manager who concentrated on the “now.” Every day was a mad dash to complete the jobs at hand. He wanted to know who was working on what, where the parts were and when everything would be done. He was constantly reacting to a customer’s...

Read more...

ASE G1: Drive Belt Inspection, Replacement

The ASE G1 Certification test contains 55 scored questions, plus 10 unscored ­research questions, that cover a range of skills and knowledge related to maintenance and light repairs in engine systems, automatic transmission/transaxle, manual drivetrain...

Read more...

Amateurs and Hacks Provide Job Security For Automotive Service Professionals

Two cars pull up in front of my shop. The drivers didn’t come in, but I heard the commotion from my office window. The boyfriend opens the hood of his girlfriend’s car. They both stare at the engine; she tells the boyfriend that she was supposed...

Read more...

Rotor Failure: Why Rotors Crack and Make Noise

The prices of rotors seem to be dropping the past few years. Call just about any parts supplier and they can quote you a vast range of prices for the same application. And when you compare the rotors side-by-side, they may look the same, but the difference...

Read more...

Wheel Bearings: Measurement and Torque

Wheel bearings are either of the ball or tapered roller variety. Front wheel bearing applications are an angular-type ball bearing, which will accept greater thrust loads than a Conrad-type bearing, and will accept a 100 percent load in the radial...

Read more...

Why Alignment Angles Change

An alignment angle doesn’t change randomly. There is a cause-and-effect relationship between external and ­internal forces that can alter the geometry of a vehicle’s suspension. Having the alignment reading for only one angle on one corner is just...

Read more...

Bendpak Breaks Ground On New Warehouse and Shipping Complex

    BendPak, Inc. announced the recent groundbreaking to celebrate beginning construction of a 67,000 square-foot multipurpose warehouse and shipping center located on 3.7 acres of land in Santa Paula, CA. The new 67,000 square-foot...

Read more...

Pulling Codes: 7 Common Causes of Misfire Codes

A flashing check engine light and a P0301 to P0312 diagnostic trouble code (DTC) is a surefire indication that one or more cylinders are misfiring. Occasional misfires may pass unnoticed, but a steady misfire is hard to miss. The engine usually feels...

Read more...

Analyzing the Cylinder Pressure Waveform from a Running Engine, Part 3

By Vasyl Postolovskyi and Olle Gladso Contributing Writers and Instructors at Riverland Technical and Community College in Albert Lea, MN   In Part 1 of this Maximizing Tools series, we discussed an alternative approach to diagnosing an engine...

Read more...

Home European Diagnostic Solutions: Engine Coolant Temperature Sensors

Print Print Email Email

Considering that roughly 1,500 or more ­different vehicle models are introduced into our domestic market each year, it’s becoming more difficult to predict how a Powertrain Control Module (PCM) will utilize data from a particular sensor or detect an out-of-range sensor in any single vehicle platform. The engine coolant temperature (ECT) ­sensor provides a good illustration of how many on-board diagnostic strategies have changed. Keeping in mind that an of out-of-range ECT ­sensor can, among other things, affect the PCM’s fuel and spark mapping, variable camshaft timing, transmission, radiator cooling fan and evaporative emissions functions, it’s important to develop an awareness of how the PCM self-diagnoses the ECT circuit and how the ECT data is integrated into a vehicle’s ­operating strategy.

Much of any on-board diagnostic strategy ­depends upon the computing capacity of the PCM. Most pre-1996 OBD I and many early post-OBD II Engine Control Modules (ECMs) and PCMs had only enough computing capacity to detect hard or intermittent circuit voltage faults.

In many cases, early ECMs didn’t have enough capacity to rationalize the performance of the ECT sensor with other data inputs. So, in some applications, it’s possible that an out-of-range sensor can affect the operation of many OBD II test monitors and the operation of many vehicle components without setting a trouble code. In passing, remember that the ECT input is part of the freeze-frame data that accompanies most ­diagnostic trouble codes.

OPERATING STRATEGIES

diagram 1: the coolant temperature should rise steadily as the engine warms up.Most modern automotive ECT and intake air temperature (IAT) sensors are generally two-wire, “negative temperature coefficient” (NTC) thermistors in which the electrical resistance of the ECT and IAT sensors decreases as temperatures increase. See ­Diagrams 1 and 2.

diagram 2: the coolant temperature should level out as the thermostat opens.At the extremes, an open-circuit ECT should indicate a scan tool data of approximately -40°F coolant temperature, since the PCM is receiving a zero ­return voltage. In contrast, short-circuiting the ECT connector from the PCM’s 5-volt reference terminal to the PCM’s voltage return terminal should indicate a scan tool data of approximately +300°F coolant temperature.

Both temperatures are programmed into the on-board diagnostic strategy as the most extreme ­temperatures under which the engine might be ­expected to operate. The first ­series of “Global” circuit-related codes include P0115 (ECT circuit fault), P0117 (ECT low input voltage), P0118 (ECT high input voltage) and P0119 (ECT sensor or ­circuit ­erratic).

ELECTRICAL/MECHANICAL FAILURES

Electrical failures include low ECT return voltages caused by corroded ECT connectors or, at another extreme, a low reference voltage caused by another sensor shorting the reference voltage circuit. In some cases, a P0116 DTC will be set if the PCM detects an error in the range or performance of the ECT sensor.

Mechanical failures include low ECT return voltages caused by low coolant levels and stuck-open thermostats, which are often represented by a second ­series of P0125-128 DTCs. The low coolant level will cause a much lower-than-expected ECT return voltage because the ECT sensor is no longer in contact with the coolant.

Presumably, the driver will see a “low coolant” warning light on his instrument cluster. Perhaps the “Check Engine” light will be illuminated and a DTC set, or perhaps not. In contrast, the stuck-open thermostat will cause a slow warm-up time and might store a P0128 DTC simply because the PCM sees a lower-than-normal coolant temperature for a predetermined length of time.

Because the ECT sensor is a primary input data, practically all ECMs and PCMs are programmed to detect open and shorted circuits in the ECT­ ­circuit. But, when detecting an out-of-range ECT sensor, the actual ECT test monitor can vary among applications. The PCM can, for example, measure the time, speed and load required to bring an engine up to a predetermined coolant temperature of, let’s say, 194°F.

If the indicated ECT data hasn’t reached the ­desired operating temperature during a specific time limit and at a specific engine speed and load factor, the PCM might set a P0125 (insufficient temperature for closed-loop operation) or a P0128 (coolant temperature below thermostat-regulated temperature), which in most (but not all) cases ­indicates a bad thermostat. If this diagnostic strategy sounds complicated, that’s because it is complicated, and also because it can vary widely among different applications.

ENABLING CRITERIA

Enabling criteria are simply the types of sensor ­inputs required by the PCM to run a test monitor and to set a specific DTC. Since enabling criteria are application-specific, an appropriate technical database must be consulted before making any assumptions. The engine coolant temperature is important because it forms part of the enabling criteria for many component test monitors and is part of the freeze-frame data for most DTCs.

diagram 3: this sharp drop in the ect signal caused the air/fuel mixture to momentarily lean out, which caused an intermittent, no-code stalling complaint on this 1997 toyota camry. this very same driveability complaint might not exist on a 2013 vehicle.As ­illustrated above, if the ECT sensor is indicating a momentary dip in coolant temperature on a 1996 OBD II vehicle, the result might be a no-code, cold-engine driveability complaint because the PCM has increased fuel delivery to meet the fuel map for the indicated (not the actual) engine operating temperature. See Diagram 3.

If the ECT is indicating a lower than actual ­operating temperature, it’s possible that the PCM might increase the pulse width to enrich the fuel mixture only until the oxygen sensor provides a data input to the PCM so it can assume fuel control. With early OBD II vehicles, an over-rich condition might also depend upon how much authority software engineers programmed into the PCM for the ECT input. On low-authority systems, the effects would be negligible, whereas on high-authority systems, the effects might be profound.

OLD VERSUS NEW

But let’s fast-forward to 2013 when a vehicle has a far greater capacity to detect a sensor fault than does the PCM in a 1996 model. Here’s where experience can lead us astray. For example, a 1996 engine might compare the data inputs from the IAT sensor and the ECT sensors to determine if the engine is starting from a cold-soak or a hot-soak condition. If both temperatures are within, let’s say, eight degrees of each other, the PCM ­assumes that the engine is starting from a cold-soak condition. This data allows the PCM to adjust the spark and fuel maps to start and run from a cold-soak condition.

But, let’s say that the ECT ­resistance is lower than specification and is therefore indicating a higher coolant temperature. In this case, the PCM might assume that the engine is starting from a hot-soak condition, when, in fact, it is not. This false data might cause a cold driveability complaint, and, among other things, possibly prevent the evaporative emissions monitor from running.

With 1996 vehicles, it’s also conceivable that an out-of range ECT sensor or stuck-open thermostat can prevent a DTC from being set for a defective oxygen sensor because the system never reaches closed-loop operation. Similarly, many 1996 automatic transmissions might not engage the torque converter lock-up clutch or transmission overdrive gear until the ECT sensor indicates that the engine has reached a specific operating temperature.

On the other hand, because modern heated ­zirconia oxygen or air/fuel ratio (AFR) sensors on a 2010 vehicle allow the PCM to assume fuel control practically as soon as the engine is started, the oxygen or AFR sensor is given more authority than the ECT sensor for entering closed-loop operation. Multiple A/F and oxygen sensors also provide a backup data stream and allow the PCM to compare the data inputs of each sensor.

So, an out-of-range ECT sensor on a 2010 vehicle would likely not affect driveability or performance as much as on a 1996 model. ­Instead, the 2010 PCM might project a value for the ­expected engine temperature by monitoring ­enabling criteria like intake air temperature ­engine speed and engine load. Furthermore, the additional computing capacity of the 2010 vehicle might allow its PCM to overlook a momentary glitch in the ECT data input (See Diagram 3 on page 28) and ­instead simply store an ECT-related trouble code in its diagnostic memory.

BASIC ECT DIAGNOSTICS

The simplest diagnostic strategy for diagnosing IAT and ECT sensors is to compare their data ­inputs after the vehicle has cold-soaked overnight. A ­second strategy can include using a scan tool to graph the ECT voltage. A third, but less reliable, method is to use an infrared pyrometer or “heat gun” to compare both intake air and engine cylinder head temperatures with the data stream displayed on a scan tool. But, remember that due to the “reflectivity” of various surfaces, the heat gun ­approach will not indicate the exact temperature indicated on the scan tool.

Lastly, make sure you’re testing the correct ­sensor. Keep in mind that the IAT sensor is usually integrated with the hot-wire mass air flow sensor assembly on most current vehicles. Many pre-1996 OBD I vehicles included a separate temperature sensor for activating the ­radiator cooling fans. Early OBD I and OBD II ­vehicles used a ­single-wire ECT sensor to supply data to the ­instrument cluster temperature gauge and a ­separate two-wire sensor to supply data to the PCM. Thanks to multiplexing, which makes it possible to share a single datastream among various control modules, modern ­vehicles generally use a single ECT sensor to supply engine coolant temperature data to ­various modules. 

The following two tabs change content below.

Gary Goms

Gary Goms is a former educator and shop owner who remains active in the aftermarket service industry. Gary is an ASE-certified Master Automobile Technician (CMAT) and has earned the L1 advanced engine performance certification. He also belongs to the Automotive Service Association (ASA) and the Society of Automotive Engineers (SAE).
Latest articles from our other sites:

‘Green’ Bullets: Tips for Servicing Hybrid Vehicles

Newsflash: Hybrids are here to stay. The oldest Toyota Prius models are now 14 years old and many other hybrids from other manufacturers are crossing the 100,000-mile mark every day. Here are a few...More

Legendary Spark Plugs: The Six Toughest Vintage Spark Plug Replacements

1. AMC Pacer: The Pacer was supposed to get a GM Wankel rotary engine. If this were the case, the four plugs would have been easy to access. But thanks to emissions problems, the Pacer would get the regular...More

FDP Introduces Front Brake Pads For 2015 Ford Expedition, 2015 Lincoln Navigator, 2014 Ford F-150

FDP Friction Science has announced that it now offers front brake pads for the 2015 Ford Expedition, 2015 Lincoln Navigator and 2014 Ford F-150. In addition to brake pads, FDP also has added rear brake...More

US Motor Works Named Water Pump Supplier For The New Elio Ultra High Mileage Vehicle

US Motor Works LLC, a manufacturer and distributor of water pumps, fan clutches, electric fuel pumps and timing kits for the automotive and heavy-duty industries, has been selected as the water pump supplier...More

Drift Correction with Cross Camber

One of the common handling-related complaints brought to an alignment shop is ‘drift’ — usually meaning the vehicle fails to continue straight when allowed to choose its own path. Many times, an...More

Independent Rear Suspension Theory

Independent rear suspensions are becoming the norm on cars and SUVs of all sizes. The two main advantages of ­independent rear suspension are ride and handling. When you’re looking up at one of...More

Rotary Lift To Showcase Updated Four-Post Car Lifts At SEMA Show 2014

Vehicle service professionals will have the first opportunity to see Rotary Lift’s updated 14,000-lb. capacity four-post general service and alignment lifts at the SEMA Show. The enhanced lifts will...More

Bendpak Breaks Ground On New Warehouse and Shipping Complex

    BendPak, Inc. announced the recent groundbreaking to celebrate beginning construction of a 67,000 square-foot multipurpose warehouse and shipping center located on 3.7 acres of...More