OBDII Data: Ambient Air Temperature

 There was a time that if your Ambient Air Temperature sensor wasn't working the only noticeable thing was your outside temperature display was incorrect or non functional. These days however the Ambient Air Temperature is factored in to the engine control logic and an integral part of the system. The system monitors the engine coolant temperature, the intake air temperature and the ambient air temperature. Cold air is denser than warm air and that becomes part of the air density calculation of the engine computer. It affects fuel mileage, emissions and performance. The pid for AAT isn't as straightforward as you might think however. You would expect that a vehicle sitting overnight without running would have engine coolant, intake air and ambient air temps all pretty close to each other when looking at the data with key on but that isn't always the case. There are rules of engagement that vary a bit from manufacturer to manufacturer that prevent the AAT pid from showing the current state of the sensor reading. Now the sensor can be working just fine and the sensor resistance has changed to match the current temperature but the number you are seeing with key on usually won't update to current reading until the vehicle is driving under the particular manufacturers rules. Some may update on the starter engagement but others not until the car is driving at some distance, some length of time or some speed. However the engine computer used the current resistance in real time to calculate the conditions at the engine start up. It just isn't showing the current value yet on the scan data. It is a bit like the computer logic that prevents your fuel gauge reacting to the fuel sloshing around in your tank while you are driving. You don't have to see the gauge reacting to that because the sensor data is being filtered and slowed before it reaches the gauge. 

Bearing in mind that the AAT data you are looking at isn't necessarily what the engine control module is currently reacting to, know that the ecm is very capable of noticing a discrepancy that isn't rational if one temperature sensor reading is contrary to what would be logical compared to the others. If you want to see the current readings for your own comparison, you would meet the conditions required for the scan pid to update as the quickest and simplest way. The computer will set a code if it sees illogical temperature data but there are times when a sensor can be incorrect yet acceptable to the computer so it never hurts to check. 

A few things of note, the intake air temperature is measuring under hood temps of air in the air intake tube so not the same as the outside air temperature being monitored by the ambient sensor which is often located near the front grill or in an outside rearview mirror. Also, the rules of engagement for the climate control ambient air pid are not the same as the rules for the engine control even though it is often using the same sensor, so it is not unusual to see different temp readings in HVAC ambient air and Engine ambient air. Temps are also factored in to the computer deciding how long your engine should take to warm up, a P0128 being related. 

Thanks for reading!

Kenny@GGAuto.Repair

OBDII Data: Air Flow Rate

 Let's take a look at the data item  Air Flow rate from the Mass Airflow Sensor. The most straightforward way to look at this pid on a scan is grams per second however in my screen shots I have the tool set to imperial vs metric so we will need to convert. The reason I say it is more straightforward in metric readings is that your expected flow rate in grams per second at idle should be roughly the same as the engine size. Be aware of the caveats to that rule of thumb. The engine should be idling normally and steady, no accessories on and a fully warmed engine. Accessories on, cold engine, erratic idle, all those will affect your base air flow. I had a 2010 F150 in with a 4.6 and when the engine was idling cold the computer had raised the rpms for warmup and I was getting air flow of 0.86 lb per minute, which translates to 6.5 grams per second which, though steady, was significantly higher than the 4.6 expected. 

Once the engine was warm and the idle was slowed due to a warm engine the air flow was 4.0 grams per second and normal. See how closely that is to the 4.6 engine size? Now there was also a Toyota in the shop with a 5.7 engine so let's test that rule of thumb once again. The warm engine, good steady idle and no accessories brought me 0.66 lb per minute, or 5 grams per second. 

On a roll now so I took a look at the air flow on the 2011 GMC 5.3 and found a problem. The truck was here for an oil pressure switch replacement, which was done, but I have just found another problem simply by looking at the air flow pid. The idle speed was correct and stable but my air flow was erratic and varying from 0.86 to 1.06 lb per minute, or 6.5 to 8.0 grams per second. This Mass Airflow Sensor had not set a code but this signal indicated a problem. Being the GMC it would not be unlikely I was looking at a MAF that would soon fail but it was even more likely I was looking at a dirty MAF sensing wire. The sensor is easy to remove and very definitely needed cleaning. 

Once the sensor was cleaned and re-installed I had a steady airflow reading of  0.72 lb per minute, 5.45 grams per second. The sensor had been misreporting air flow by about 20%. So before the cleaning, and because the computer thought the air flow was more than it actually was it would have been adding extra fuel. We have just improved gas mileage and acceleration by providing the computer with an accurate air flow reading. 

Thanks for reading!

Kenny@GGAuto.Repair

OBDII Data: Accelerator Position

 Because there is no throttle cable on a drive by wire system the computer needs to see what position you are placing the accelerator and it determines that by looking at the signal from the Accelerator Pedal Position sensor. Similar to the Throttle Position having redundant sensors, the Accelerator Pedal Position has two outputs that can be monitored in scan data as Accelerator Pedal Position D and Accelerator Pedal Position E. The system is set up that way so that if there is a sensor problem it will be noticed because the two outputs will no longer correlate. A mismatch here would set a P2138 and put the system into a limp in mode, limiting how much throttle can be given. Depending on application the two signals may vary in voltage output but they will always move together in sync as you press the pedal. 

Thanks for reading!

Kenny@GGAuto.Repair. 

OBDII Data: Throttle Position

 Electronic Throttle Bodies or throttle by wire systems have no accelerator cable and are responding to commands from the powertrain control module. Contained in the throttle body are two position sensors that relay the throttle position information back to the powertrain module so that it can "see" the response of the throttle to its commanded actions. Absolute Throttle Position is the data item for the primary sensor monitoring the butterfly valve angle. Absolute Throttle Position B is from a secondary sensor reporting the same information but on a different circuit to the powertrain module. This redundancy is for safety purposes and allows the powertrain module to cross-check and verify the information. Notice in the wiring diagram that the two sensors do share the same reference and return circuits. 

If the computer notices a discrepancy in the two signals it will set a fault code and go into a failsafe or limp-in mode that doesn't allow but a minimum of throttle. P2135 would be a result of that discrepancy. You can monitor and compare the two sensor outputs by looking at Absolute Throttle Position and Absolute Throttle Position B. They should correlate closely as the throttle moves. 

 On this particular application the signals are a mirror image of each other. 

Thanks for reading!

Kenny@GGAuto.Repair

 

OBDII Data: Absolute Load Value

 If you are using OBDII scan data you will notice a pid called Absolute Load Value. This is an indicator of the engine's current pumping response based on the demand and presented as a percentage. Think of this as a "Lung Capacity" test for your car. It shows how much air your engine is actually pulling in compared to the maximum amount it was designed to handle. For example, if this value is at 80% it means your engine is filling its cylinders 80% of the way with fresh air. It's a great way to see if your engine is breathing well or if there is a problem causing it to struggle for breath. Calculated Engine Load is what the computer is trying to do under current conditions, like giving 100% to sprint up that steep hill (wide open throttle for instance).  A comparison of the two data items can help determine if you have a breathing problem and what may be the cause. If you are looking for the cause of your car struggling up a hill or losing power on hard acceleration, and you see 100% Calculated Engine Load but a Absolute Load Value much lower then you have a problem with lung capacity. At a Calculated Load of 100% you want to see 85% or better Absolute Load. 70% to 80% is a red flag and anything less than 70% you definitely  have a restriction in intake or exhaust. Now there are other causes for power loss at wide open throttle, fuel delivery for instance, but a weak fuel delivery won't show up in your Load Values. You look for hints of that in fuel trims or by looking at air/fuel ratio sensor data during the problem. In Load Values if you see both Absolute and Calculated low at wide open throttle during the power loss you likely have a Mass Airflow sensor problem. We'll cover those topics later. 

"If your car has a turbocharger, don't be alarmed if your Absolute Load goes over 100%! A turbo literally force-feeds your engine, so it can actually breathe at 150% or even 200% capacity."

Thanks for reading!

Kenny@GGAuto.Repair

2012 Ford Escape Scan Summary

Ok, I had this 2012 Ford Escape in last month for a stalling and low idle speed complaint which was repaired by cleaning the throttle body and a relearn of idle speed. Before doing the repair I had scanned the vehicle for any relevant codes to the stalling problem. I didn't find anything related to the stalling but I'd just like to give you a walkthrough of what was there. This is an XLT with 2.5 engine and 2wd, 218,000 miles. 

The ABS/Traction Control module has C1963. The C indicates a chassis code. Those are related to brake, steering or suspension systems and you should find a code in one of those systems as well. The C1963 is indicating a chassis system problem and a related or companion code should be set in the section of the chassis controls that has the problem. 

In this case, the code triggering the C1963 is stored in the Power Steering Control module, B2278. B codes are body system codes. The PSCM is inside the passenger compartment. 

 The other codes, the U codes are related to problems with the communication between modules. The U3000-04 indicates an internal module failure in the Accessory Protocol Interface module and the U3000-09 indicates an internal module failure in the Global Positioning System module. So, even though the system wasn't part of why it was here and I didn't use it, I expect the radio or navigation systems weren't functional. Not everyone would care. 

The B12B0  in the HVAC indicates an an electrical fault with the front blower relay. That would usually cause the blower to not work or to work all the time, depending on the actual condition. No mention was made of a blower problem. I'm going to assume the problem was intermittent and blower was working because you would want your blower to work in Arkansas where you can need both A/C or heat in the same day.

The B1600 is a problem with the Instrument Cluster/PATS module didn't receive a valid key signal which would cause a no-start and again no complaint concerning a no-start and it was running fine. I'll call that one intermittent. 

The P1000 in the Powertrain Control Module just tells me the PCM memory has been cleared and the monitors have not had a chance to complete since that was done. What I don't know is how or why the codes were cleared. I don't know if only the PCM was cleared or if all the systems were cleared.   

So while I can see some reasons for being aware of these problems, it is the B2278 that should be repaired next since it is related to your steering. The best thing about a B2278 is it doesn't involve a lot of diagnostic time to pinpoint the cause. It will need a Steering Shaft Torque Sensor. There was a time when you would have to replace the entire steering module for that but Ford has made the sensor available now as a stand alone part. Installing a new Ford sensor and the steering column mounting bolts, and a steering trim adjustment and calibration will be the fix. 

Btw, most of the sensors are replaced around 200,000 miles and this has 218,000. 

Thank for reading.

Kenny@ggauto.repair

Carnac the Magnificent Mechanic

 Anybody remember when Johnny Carson would be Carnac the Magnificent? He would use his power of mind to answer a question before seeing the question. Always funny.

In a similar vein, I'm going to take a look at some information I have on a car that I have not seen. For instance a random car for sale online with a vin 2C4RDGCG7DR798845. 2013 Dodge Grand Caravan with 128,277 miles. 

Alldata tells me the van was serviced and battery replaced 7/10/25. Alldata also shows a technical service bulletin related to a flash update to the fold and stow module to address a problem with the module sometimes staying in wake mode and discharging the battery.

ShopKey Pro charts the mileage of battery replacements on these vans, easy to notice the peak times to expect a battery failure. 

Based on mileage we can also expect it needs brake pads and spark plugs unless they have recently been replaced. It is also worth noting that 128,000 is the peak mileage that a timing chain is often needed. Good things to bear in mind when you are shopping and considering this purchase.