How to tell if crank position sensor is bad??

Discussion in 'SN95 4.6L Mustang Tech' started by Mustang5L5, Jun 9, 2007.


  1. Mustang5L5

    Mustang5L5 Car used in adult film "Highway Gangbang-InDaButt" SN Certified Technician Founding Member

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    I assume the crank position sensor is what gives the tach it's signal right?


    I have some issues with my cars tachometer. Usually it would happen in the winter time when it was VERY cold out until the underhood area got some heat. I have determined it's not the needle dragging on my gauge faces.

    When spring/summer comes, the problem only happens on rainy days.


    Tonight, i washed under my hood real quick. I didn't really spray the motor down, just washed the front radiator area and inner fenders and gave the motor a light mist.

    I got in the car to drive, and the tach was sticking again. It likes to stick at 3000RPM and will only move if i drop or rev up or down ~1000 rpm (drop to 2K or accelerate to 4K). After a few mins, when there is some heat under the hood, the problem goes away.

    It only really happens when it's very cold (under 40 degrees) or raining/wet out.....or if i wash my motor.

    Anyway it's bugging me and i'd like to fix it. Are the crank position sensors responsible for the tach readings?? How can i test them? I do know where they are on the motor, but don't understand how they work. They look like 2 small black boxes with a single wire coming out.
     
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  2. '01 Steed

    '01 Steed Founding Member

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    If your crankshaft position sensor was bad, your car wouldn't run. The problem is probably between the PCM and tach. It is curious that humidity seems to affect it.
    I wouldn't know where to begin. Electrical gremlins suck!

    BTW, nice avatar!
     
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  3. Mustang5L5

    Mustang5L5 Car used in adult film "Highway Gangbang-InDaButt" SN Certified Technician Founding Member

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    That's kinda what i figured that the car wouldn't run.


    I have no idea what this is then but the symptions are just as you said...humidity and cold weather.


    Like i said, get a little heat in the engine bay and it goes away. The colder the day, the longer it takes. A 40 degree day may take 5 minutes, a 5 degree day could take 20-25 minutes.

    Usually when it rains in the summer, it only lasts 2 or 3 sweeps past 3K rpm and then it's fine. That seems to be the magical "sticking" number. :shrug:



    Also, one more thing i notice. When it's cold out (freezing or below) the tach never returns to idle. It stops at 1200RPM. Once the car warms up, it will drop to actual idle RPM.

    It is not the tach needle sticking on the gauge face. I checked 3 times already
     
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  4. sicK96GT

    sicK96GT New Member

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    my cps when out on my car when i first bought it.

    The car would run fine, but die when I would put it out of gear or dropped in RMP fast.

    Yours sounds more like Idle Air Controller or Throttle Position Sensor.
     
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  5. cjones

    cjones Founding Member

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    when you say "sticking" is it the tach that is stuck or the engine actually reving at that RPM??

    i don't see a prob with the idle at 1200 RPM with cold engine and below freezing.
     
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  6. Mustang5L5

    Mustang5L5 Car used in adult film "Highway Gangbang-InDaButt" SN Certified Technician Founding Member

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    No, the engine RPM is normal. ONLY the tach is sticking. It's not an issue with the engine at all.

    The engine will idle at 800 or so RPM (normal rpm) but the tach will show 1200ish.

    The tach will stick at 3K rpm, but the engine will be revving freely throughout the range.

    Nothing is wrong with the engine, just the tach needle is acting funny
     
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  7. ToplessStang

    ToplessStang Member

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    As was mentioned, if it were your crank sensor you'd most likely have driveabilty issues. Most likely wiring or the tach gauge itself.

    What I would try is to use the cluster diagnostic in tach mode and see if it corresponds with the needle. If the tach needle is stuck at 3 000 rpm but the cluster is ok I would say the tach itself is at fault.

    Just a suggestion.

    Here is the cluster diagnostic info http://www.mustangworld.com/ourpics/fcar/dtcodes.htm
     
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  8. '01 Steed

    '01 Steed Founding Member

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    DAMN! That is a excellent suggestion!!
     
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  9. Mustang5L5

    Mustang5L5 Car used in adult film "Highway Gangbang-InDaButt" SN Certified Technician Founding Member

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    Ok well i tried the digital tach out. It was a perfect day since it was rainy and cold here. I know it would stick again

    It seems the fault is in the analog tach then because the digital tach kept accurate RPM while the analog stuck and seemed to fall behind at times. It would read 2800RPM while the digital tach read 3300RPM.

    After the car got warm, it went away and the readings were within 100RPM of each other.


    Which is more accurate anyway? The digital tach or the analog?
     
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  10. Superhereaux

    Superhereaux chicks make me feel inadequate Founding Member

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    digital


    you absolutely sure it isn't the gauge faces (which i assume are aftermarket)?
     
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  11. Mustang5L5

    Mustang5L5 Car used in adult film "Highway Gangbang-InDaButt" SN Certified Technician Founding Member

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    I've checked them 3 times. There is plenty of space between the needle and face. I have gauge overlays, but after checking 3 times it's time to start looking elsewhere i guess.
     
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  12. Mike97gt

    Mike97gt it doe snot Mod Dude Founding Member

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    your going to need a new cluster.

    As soon as you saw the digital readout on the cluster diagnostics was right then that is a dead giveaway that the analog tach is junk. Since there is no way to fix this yourself you need to replace the cluster.

    Only other thing you can try is to get rid of the overlays.
     
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