accuracy of engine computers???

[drm7]

All,

I know all '96+ cars are OBD II and that there are dozens of readings that can be seen in programs like AutoTap. My question is this: how accurate do you think the readings are? For instance, if you were to place an oil pressure guage on a car would it read the same as the computer? (assuming it is a quality guage/install). You can get rpm, oil temp, oil pressure, coolant temp, intake air temp....etc. I would think that due to emissions, knock, etc that the ECU would have to be pretty accurate.

What does everyone think?

 

[brianl703]

It's not the accuracy of the computer that I'd question, it's the accuracy of the sensors themselves.

If any of the sensors aren't providing correct readings, you'll likely get a code. So the first thing I would check is that there are no stored codes before I would trust the readings the OBD-II system is giving.

I have heard that in some cases the OBD-II system will even substitute calculated values for a reading that it's getting from a sensor that it has determined to be bad.

For example if the engine coolant temperature sensor has gone to lunch, the OBD-II system will use subsituted values and these are what will show up when you use a scantool to display that reading.

Of course, in such a case, the OBD-II system will have a code to inform you that this has happened.

 

[drm7]

Thanks for the reply.

One more assumption I would like to add is that everything is new and in proper working condition with no error codes. I'm just looking for if the ECU is considered accurate in general, or at least as accurate as aftermarket guages if both are in good working order.

 

[cryptokid]

i think the ecu is generally very accurite.

however alot of sensors like oil pressure, water tempature, etc can be basically on or off sensors. for instance the oil pressure sending unit on most cars isnt a sending unit at all but a "on-off" pressure actiated switch.

additionally, alot of cars feature 2 or more water temp sending units, one of which may be another type of on-off switch, and another would be a real sending unit.

then there is the oxygen sensor, most cars use a narrow band o2 sensor which is so narrow in its accurite range, you could basically call it an on off sensor. very rare to factory cars, a wideband o2 sensor gives real info about a cars rich or lean condition, instead of a narrowbands readings which are basically rich or lean, but dont tell you HOW MUCH you are rich or lean.

so in short, the computer is probably very accurite, the sensors on the other hand, usually leave more to be desired.

my car does not even have an oil temp sending unit. most cars dont monitor this at all.

 

[drm7]

Crypto: what do you mean by on-off switch? Wouldn't the computer need "actual" readings? I know Ford uses idiot guages but I would think the ECU would need real readings.

If you look at AutoTap and see the types of readings available it's amazing. I'm looking to see if I had a palmpilot hooked up while driving if the readings would be as good as aftermarket guages. AutoTap for Ford will show trans temp, oil level, oil temp, and tons of other data.

 

[drm7]

Crypto: what do you mean by on-off switch? Wouldn't the computer need "actual" readings? I know Ford uses idiot guages but I would think the ECU would need real readings.

If you look at AutoTap and see the types of readings available it's amazing. I'm looking to see if I had a palmpilot hooked up while driving if the readings would be as good as aftermarket guages. AutoTap for Ford will show trans temp, oil level, oil temp, and tons of other data.

 

[cryptokid]

what i mean by on-off is that in the case of an oil pressure sending unit for example, on means there is enough oil pressure (usually 10psi) to keep the electrical connection on the dash idiot light disconnected. i think this is what is refered to as a hobbs switch.

my car unfortunately, uses this type of oil pressure monitoring. if the oil pressure in the car drops below 10psi, the oil light comes on. this is probably used on the majority of all cars today. i did elect to install an aftermarket oil pressure gauge and sending unit however.

the obd2 protocall does not need or require oil tempature or oil pressure. most obd2 computers dont even monitor these things.

it really just depends on the car. most all cars have atleast an idiot light, which is usually just an on-off type sending unit.
some cars go father and actually monitor oil pressure, some monitor oil tempature too, and even transmission temp and pressure.
it just depends on what the manf put into the car.


i got to borrow my friends obd2 datalogger for a month. its a great tool to have but dont let youreself think that its the end all of car tools. a simple code scanner is almost as good.

with an obd2 data logger and a laptop you can see in real time things like youre air to fuel ratio, but if you think about it, this is almost useless because of the narrowband o2 sensor.
you can also see things like the ignition advance in realtime. but again , this is useless. you cant reprogramme the ignition advance curve on most obd2 cars.

so having a obd2 hookup to a computer is nice sure, but most of the info it shows is useless because you cant do anything with it. thats the downfall of obd2. it isnt really adjustable.

this is why people go to adftermarket fuel injection, like haltech. not only can you read realtime info like youre ignition curve, you can reprogram the curve on the fly.

[ August 23, 2004, 02:24 PM: Message edited by: cryptokid ]

 

[friendly_jacek]

quote:

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Originally posted by drm7:
AutoTap for Ford will show trans temp, oil level, oil temp, and tons of other data.

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Are you sure that Ford has all these sensors installed?
Yes OBD-2 scanners will show you all the data ECU gets, but in most cars, oil pressure or temp is not included. BTW, you get real engine temp, it is not an off-on switch.

 

[cryptokid]

quote:

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you get real engine temp, it is not an off-on switch

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my car uses 2 tempsending units. one is real like you say and measures water. and another is an on-off switch that is used to tell the computer when the car is warmed up. the on-off switch actually monitors the physical metal temp of the engine though, not the water temp. the real sending unit is also used by the computer.

like i said, it depends on the manf on whats installed or not installed as far as sensors go.

 

[drm7]

Thanks for all the info.

friendly_jacek: no, I'm not positive but if you go to the AutoTap site and look at the Ford specific readings the chart shows these other readings on top of the base OBD II readings. I think it varies by manufacturer what additional things they monitor.

I'm thinking if I were to tow a trailer that I could use this program to monitor things like engine temp, oil temp, trans temp, etc. Just like hooking up autometer guages, you can't change anything, just monitor the various systems for potential problems.

 

[mechtech]

Not only can the sensors can be a little off, but the gauges as well.
The computer would be the least likely to be off.

 

[cangreylegend]

Whenever I cut something and it's not correct I buy a new tape measure. Funny thing is that it keeps happening again and again..... I wonder what's wrong with the ruler?

 

[QuadDriver]

quote:

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Originally posted by cryptokid:

quote:

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you get real engine temp, it is not an off-on switch

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my car uses 2 tempsending units. one is real like you say and measures water. and another is an on-off switch that is used to tell the computer when the car is warmed up. the on-off switch actually monitors the physical metal temp of the engine though, not the water temp. the real sending unit is also used by the computer.

like i said, it depends on the manf on whats installed or not installed as far as sensors go.

---

It is likely one is the CTS which is actually very accurate, the other is simple a grounded thermistor that is calibrated to the gauge to move a needle and is not very accurate. (in the case of an idiot light and no needle, a simple logic circuit is now used to gate current below certain temps.

btw to other posters: I dont know of any ECM that is aware of oil pressure. I know the GM ECM masks have had space for it for years, but GM tends to use 2 sensors: 1 to keep the fuel pump relay on, the other to run the gauge. Usually the ECM is aware of neither.

 

[Gary Allan]

quote:

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I dont know of any ECM that is aware of oil pressure.

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Err ...hmmm...well all the data in a jeep dash comes from the PCM. Now whether it's just "passing it through" to the dash computer via the bus ...or "digesting" the data for some adjustment

I'll list the inputs:

A/C request
A/C select
Auto Shutdown (ASD) sense
Battery Temp (this is to adjust alt output)
Battery Voltage
Brake Switch
CCD bus + circuits
CCD bus - circuits
Camshaft position sensor
Crankshaft position sensor
Fuel level
Generator (battery voltage) level
Ingition circuit sense (ignition switch in the run position)
Intake manifold air temp sensor
Leak detection pump
MAP sensor
OIl pressure sensor
Oxygen sensor
Park/neutral switch (auto trans only)
Power Ground
Power steering pump pressure (2.5 only)
SCI receive (DRB scan tool 16 way connection)
Sensor return
Signal ground
Speed control multiplexed single wire input
Throttle Position Sensor.

Source: 1999 Jeep Wrangler factory service manual

 

[brianl703]

I had an idea a few days ago.

I have a ScanGauge. It works quite nicely in my 1996 Ford Contour, but it's no good for my 1988 Ford Mustang for obvious reasons.

It occurred to me that Ford has used the same sensors with the same operating parameters through the years, and that a PCM will pass data to an attached scantool even when the engine isn't running.

I can get a Ford OBD-II PCM out of pretty much any 96-up wrecked Ford vehicle at the U-pull-it yard for $25 including the harness connector (and the OBD-II diag connector).

Then I can take this OBD-II PCM and connect it's sensor inputs to the various sensors on the 1988 Mustang that I wish to monitor, right in parallel with the existing sensors.

Then I can hook it up to power and connect the OBD-II diag connector, plug the ScanGauge into it, and I should be able to get readings for those sensors I connected to the OBD-II PCM.

Essentially I am using the PCM to simply process the analog inputs and spit them out into OBD-II format that the ScanGauge can then display.

Gary, your post about the oil pressure sensor and how the PCM might just be sending that data to the bus without doing anything with that data reminded me of this idea I had.

 

[XS650]

quote:

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Originally posted by brianl703:
I had an idea a few days ago.


I can get a Ford OBD-II PCM out of pretty much any 96-up wrecked Ford vehicle at the U-pull-it yard for $25 including the harness connector (and the OBD-II diag connector).

Then I can take this OBD-II PCM and connect it's sensor inputs to the various sensors on the 1988 Mustang that I wish to monitor, right in parallel with the existing sensors.

Then I can hook it up to power and connect the OBD-II diag connector, plug the ScanGauge into it, and I should be able to get readings for those sensors I connected to the OBD-II PCM.

---

Neat idea. Once you have done that, then you can add used stock sensors to the car in new places to measure other parameters, maybe even with switches if you end up with more sensors than the OBD II can handle.

 

[drm7]

Thanks for all the info!

I was hoping someone has used a tool like AutoTap and could give me the low-down on it. I may have to experiment myself. I did find other makers of these types of tools for less money.

Would anyone be interested in a panel that displayed OBD II readings while you drive?

 

[QuadDriver]

quote:

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Originally posted by drm7:
Thanks for all the info!

I was hoping someone has used a tool like AutoTap and could give me the low-down on it. I may have to experiment myself. I did find other makers of these types of tools for less money.

Would anyone be interested in a panel that displayed OBD II readings while you drive?

---

good scan tools tell you the parameters at any time, driving or not, but it would be worthless to have a simple panel in the car telling us what the O2S is reading at that time - the data is meaningless unless there is a problem.

I am currently writing a comprehensive OBD1 scan tool (runs on laptop or PC with at least DOS) to return all the data in the ECM. All OBD2 does (besides provide some new mandated info) is answer questions on the ALDL the same way as any other car, whereas OBD1 was proprietary.

 

[drm7]

QuadDriver: I'm not really interested in all OBD II readings, just specific ones like oil/coolant/trans temps, oil pressure, etc. The kinds of things you would buy AutoMeter guages for. Trying to avoid all the plumbing and possible headaches if the plumbing were to fail. If I can get the computer to tell me this data then why not? I'm not interested in O2 readings and other types of readings that don't help me in normal driving. I'm just tired of idiot guages like Ford uses.

BTW, what does ALDL mean?