AfterMarketNews 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

Diagnostic Dilemma: The Case of the Missing Code

When doing mobile diagnostic work, no-code stalling complaints are a major part of your agenda. In most cases, the client shop is simply too busy to duplicate the failure or, in some cases, a long test drive will yield nothing in the way of useful...

Read more...

Secondary Ignition: The Art of Spark

What is a coil? From the beginning of the internal combustion engine, several different ignition systems have been used to create a high-energy spark. The most popular system, and the one that’s in use today, is a step-up coil. A coil is nothing...

Read more...

Diagnosing Catalytic Converter Failure Symptoms

Although construction can vary according to engine application, the common three-way catalytic converter contains a reduction and oxidation stage. To create maximum surface area, each stage is generally a ­ceramic or stainless steel honeycomb substrate...

Read more...

Automotive Pet Peeves 2: Reader Feedback Is Overwhelming

How many auto repair pet peeves are out there? Well, enough of them that one article wouldn’t hold them all. I’ve received so many emails, texts and phone calls about my article in the February issue that I thought: why not put everyone’s pet peeve...

Read more...

Air Filter Show & Tell: Seeing Is Believing

Air filters are normal wear items that ­require regular checks and ­replacement. Their role is to trap dirt particles that can cause damage to engine cylinders, walls, pistons and piston rings. In fuel-injected vehicles, the air filter also plays...

Read more...

Searching For 'Black Holes': Job Totals Reveal Missed Selling Opportunities

The concept for Maintenance Chronicle is simple: We ask one shop to record their maintenance sales for a two-week period, and then we see what we learn from the results. This edition of Maintenance Chronicle also proved to be valuable for the shop we...

Read more...

Electronic Proportioning Valve: Doing More With Less Hardware

Anti-lock brake systems (ABS) and the HCU are replacing proportioning, combination and other valves to change the braking forces in the front and rear. This is called Electronic Brake Distribution (EBD) and it can dynamically change the proportioning...

Read more...

NHTSA’s GM Brake Line Corrosion Investigation: Reading Between the Brake Lines

There will be no recalls on some GM vehicles for brake line corrosion. Instead, we received an advisory from the National Highway and Traffic Safety Administration (NHTSA) about brake line inspection and car washes. What was not discussed was the corrosion...

Read more...

2007-2011 Ford Expedition and Navigator Air Suspension

The Ford Expedition air suspension does more than just lift and raise the vehicle. The system levels the vehicle under loads and when a trailer is attached. The system uses only two air bags on the rear axle unlike previous models with air bags on all...

Read more...

Maintaining Your Spray Guns

If there’s one piece of equipment that epitomizes the painter and the paint shop, it’s the spray gun. Over the years we’ve seen many spray guns. Although there are operating principles and functions that remain the same, some have been improved...

Read more...

Wheel Bearing Adjustment Tools & Equipment

A recent survey showed that more than half of the bearings on the road today are adjusted incorrectly. A wheel bearing that’s out of adjustment can reduce bearing life and can affect more than just the bearing. An out-of-adjustment bearing affects...

Read more...

ETI's ToolTech 2015 Focuses on the Connected Vehicle

ETI’s Annual ToolTech conference remains the premiere event in the tool and equipment industry with more than 115 professionals from over 50 companies in attendance at this year’s ToolTech 2015 in Austin, TX. The Equipment and Tool Institute's...

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:

Ultimate Underhood: From Mechanical Fuel Injection to Putters

Mechanical constant stream fuel injection is the pinnacle of pure mechanical engineering. The mechanic setting up the system must optimize the amount of fuel for a given throttle position, rpm and engine...More

Induction Cleaning Service For Direct-Injection Vehicles

You may have seen it before: misfire codes, stumbling and suspicious fuel trim numbers. On a scan tool, the engine may show a loss in volumetric efficiency. The driver may complain about a loss of power,...More

Snap-on Partners With CRKT And Ken Onion To Design The Rave, Exclusive Compact Pocket Knife

Perfect for everyday carry, yet tough enough for life in the shop, the new Snap-on Rave SEK60 series knives are an exclusive Ken Onion design. This compact, folding pocket knife features a 2.3-inch blade...More

Auto Care Association And ASE Recognize World Class Technicians

Of the more than 840,000 automotive technicians working in the United States, 17 outstanding individuals have qualified for the prestigious 2015 World Class Technician Award. The Auto Care Association...More

Electronic Proportioning Valve: Doing More With Less Hardware

Anti-lock brake systems (ABS) and the HCU are replacing proportioning, combination and other valves to change the braking forces in the front and rear. This is called Electronic Brake Distribution (EBD)...More

NHTSA’s GM Brake Line Corrosion Investigation: Reading Between the Brake Lines

There will be no recalls on some GM vehicles for brake line corrosion. Instead, we received an advisory from the National Highway and Traffic Safety Administration (NHTSA) about brake line inspection and...More

Deluxe Maintenance Carts from Homak

Homak Manufacturing’s Big Dawg series includes its 44" 10-drawer Deluxe Maintenance Carts. Built to handle the demands of professional shops, these carts feature thick-gauge steel frames, heavy-duty...More

K-Seal by Solv-Tec Offers One-Step Permanent Coolant Leak Repair

The company calls it 'The Miracle in the Little Blue Bottle' – K-Seal by Solv-Tec is a one-step permanent coolant leak repair that permanently seals most leaks in the engine block, cylinder head, head...More