Why it is not possible to auto makers to use only camshaft position sensor?

[napalmv8]

Hi folks!
I have get troubled sleep because one question wont come out of my head- whe car manufacturers use both crank and camshaft position sensors instead of using the only camshaft.
I know that usually camshaft pos. sensor only determines TDC of 1 cylinder, but why not to connect the same toothed disk as for systems with only crank sensor to the camshaft and use only one sensor for injection/ignition etc.?

 

[bazzateer]

Why have them at all? My classic cars do fine without them. Better economy than many moderns too.

 

[rockchucker]

Timing Belt. If the Belt Skips a Tooth it will throw a code and possibly save an Interference Engine from disaster.

Realistically it is just more expensive parts to go bad and have to replace with a Dealer item.

My question is "Why are there more than say 10 different styles of Wheel Bearings?"

 

[Beaubeau]

Back up, if your cam or crank sensor stuffs out you got your back up for running right timing.

 

[Iroc-Z]

Some vehicles can run on just a crank sensor. But a lot of vehicles have variable cam timing. Thus needing to monitor where the came are when running.

 

[reflexdb]

Some vehicles can run on just a crank sensor. But a lot of vehicles have variable cam timing. Thus needing to monitor where the came are when running.

Bingo.

 

[srmofo]

Many pre 2000s ran only a cam sensor. Other cats run on just a crank sensor.

It just depends on the design of the engine

 

[Falcon67]

Crank is really the only one that knows TDC on any cylinder. They are now using variable cam timing, so I imagine the computer just needs to know where the cam is in relation to the pistons along with engine speed and load.

 

[DanarchyCustoms]

I was told in Diesel Tech school that its a redundant system just in case one fails. One could have failed and you never would know it. It would **** to be chasing a no-start situation and it turned out to be a cank sensor.

 

[pipsters]

When my crank sensor went out on my VW the car did not start. Fortunately it thru a code. Cam sensor will also create a no-start.

 

[Milton Shaw]

I had a cam sensor code and it turned out I wish that was what it was. On a GM 3800 engine the code was set when the magnet in the cam gear fell out. It goes in from the back side of the cam so a compete front engine tear down to replace a $5.00 part. Which it could have gone in from the front of the engine, then it could have been installed through the cam sensor hole.

 

[agoins]

The crank sensor helps to detect misfires also.

 

[ddawg16]

You don't have enough angle resolution with just a cam sensor....

On older systems that still had a dist, they used the dist to tell them TDC for #1....and then the crank sensor for more accurate timing. Once the engine was going it could just keep track of pulse counts to know the postion of the crank.

Newer engines don't have a dist...so you need the cam....but you still need the crank for accurate timing....

 

[theoldwizard1]

Many pre 2000s ran only a cam sensor.

Any engine running on a sensor inside of a distributor (all Fords with TFI) are really running on a "cam sensor", because the distributor is run directly off of the cam.

 

[theoldwizard1]

From someone who has "been there, done that" (I used to write PCM software for one of the Detroit 3), engines can run on just the cam sensor, but there are a combination of reasons listed above, that they use both.

First, with only one sensor determining engine speed/crank angle, you are DOA if it stops working. Manufacturers don't like vehicles coming in "on the hook" for a sensor failure. EPA/CARB now mandate that if a failed senso/actuatorr will effect the engine's emissions, it must be detected, recorded and reported (turn on the check engine light).

Second, with variable cam timing, you could mount the sensor to pick up the position of the cam pulley (and therefore the crank) but you would not know exactly how far advanced/retarded the cam was. This is the primary reason for both a crank and cam sensor.

Third, they "validate" each other. If a signal is not there when t is supposed to be or if more signals appear than is physically possible (electronic noise) if will be recorded. Ford goes as far as using a "36 minus 1" tooth crank wheel, to make certain the cam sensor input shows up in that "missing tooth" window.


Let me add that during the period of time from when the starter just starts to move the flywheel until the engine is fully running on its own, a lot of complex things are happening. Much of it is trade secret (i.e. unique to each manufacturer). Let's just say that "it is complicated" !

 

[bob_mp]

You don't have enough angle resolution with just a cam sensor....

On older systems that still had a dist, they used the dist to tell them TDC for #1....and then the crank sensor for more accurate timing. Once the engine was going it could just keep track of pulse counts to know the postion of the crank.

Newer engines don't have a dist...so you need the cam....but you still need the crank for accurate timing....

Agree.

The crank sensor usually has a lot more accuracy. The camshaft position sensor (CPS) commonly only resolves the intake/compression vs. power/exhaust cycle ambiguity.

Some ignition systems don't use a CPS and fire the spark plugs both at the end of the compression stroke and the end of the exhaust stroke. They are often referred to as "wasted spark" systems, the motivation being that you can reduce the number of coils.

 

[Trey T]

Generally we use sensor in this world just because everything is not perfect. If the components are designed by theories, there's no sensor to be needed. The more you want to work toward the theories, more sensors is needed. That's the general idea of sensor in cars! The theory is that to get the most efficient engine you can get. Today's 4-stroke has about 40-50% efficiency, the rest is lost to heat by one way or another.

Like several people said, variable valve timing (VVT) play a major role of having camshaft position sensor.

 

[jimindm]

With OBD2 it is for misfire detection. Not only for a misfire but what cylinder is misfireing. In the early stages of OBD2, I think it was honda, that had both sensors in the distributor for a while. That was interesting to find a crank sensor in a ditributor during diagnosis.

 

[wesalexleft]

For the same reason they can't make the headlights turn on with the windshield wipers (state law for most states). Seems simple, but I guess I'm missing something.

 

[493 scamp]

I think the main reason is quick starting. Having both alows fewer degrees of rotation before the pcm knows which coil to fire.Second the resolution is twice as accurate.Can you imagine what effective a loose chain would have. Look at some of GM cars with 18X sensors and multiple sensors,quick starting,quick sequential fuel,more precise fuel timing.

 

[Big-Foot]

I worked on the code for an ECU once that used the cam sensor for timing only the ignition and the crank sensor to control the EFI. It was an industrial engine but still relevant.
I can see one backing up the other. That would be smart. However it would take a bit more code to have them back each other up. More code = slower code, more expense, etc..

 

[DodgeMech]

On a B Series Cummins it has both the cps and ckps and both are used for timing of the injectors and for the tach...funny thing is that they are the same sensor...and sometimes if it throws a code for one, the other is bad...

 

[Beaubeau]

Some vehicles can run on just a crank sensor. But a lot of vehicles have variable cam timing. Thus needing to monitor where the came are when running.

I was going to note that aswell but in some brands I've been taught that they run off rpm to activate variable timing, Toyota is one I've been told, land rover BMW v8s, mercedes, all activated from rpm and a certain oil pressure range, which does not include just the cam sensor. Correct me if I'm wrong but the cam sensor is for injection and spark timing, crank for tdc and rpm for most cases, but it can vary from manufacturer, that being I stated the most obvious reason is just have a functional engine if a sensor stuffs out

 

[BlindViper]

Engineers don't do things because they should, they do them because they can.

 

[ddawg16]

Some vehicles can run on just a crank sensor. But a lot of vehicles have variable cam timing. Thus needing to monitor where the came are when running.

The jeep Renix system was like that...sort of. It had the sensor in the dist....this told the engine where TDC was....after that it depended on the crank sensor....

If the system lost the signal from the dist...it would keep the engine running....but if you shut if off...it would not restart without the dist sensor.