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Full Version: Crank / Rpm Sensor Replacement
Alfa Romeo 145 - 146 Forum > Technical > FAQ
A big thanks to the blokes on this forum, The advice, the pictures and the knowledge. I did manage to change my RPM/Crankshaft sensor. So I thought I would post some pictures and some simple things to look for, If any one else ever has the problem.

The sensor was not expensive, And took about an hour from start to finish. the thing that can catch you out is, When I tested the resistance of the faulty sensor, It gave me the correct value of 860 ohms ( If you have the motronic 1.5.5 injection/ignition ), the problem being is when it heated up, only then does it break down.

Its held in by a 5mm allen key, And it appeared they put on some loctite of some sort, could be the loctite 243 (medium strength) I cant confirm that, i beleive that as there was a small blue dried up piece on the actual bolt, it was not very tight, And JAM was right, there is just enough space to squeeze your hand through. Most difficult part is trying to get the sensor cable out from the clips they in.

Not sure if its important, but where the sensor connects to the rest of the system, there is actually a number on either side of the brown connector( just a simple 1 on the one side and the other had a 2 ), I put it in the exact same way, with the number facing the same way as the previous one.

I lowered the car from the ramp, connected the battery, and its been running fantastic since.

Thanks a million

The pictures are of the actual sensor, The space your hand has, And what it looks like from above where the sensor goes in


Pictures of the crank sensor location on the back of the engine block can be found in the following post link...
great job there mason.

am I right in saying that the prefacelift crank sensor is positioned differently from the facelift.if I remember correctly I would need to remove the belt cover to get the sensor or do I have it all wrong.the reason for me asking that I have been experiencing the same problems every now and then, if stuck in traffic for a long time, the car would become a bit slugglish and then jirk and then switch off, I would a need a few minutes before I could the start and pull off...

how much did you pay for the sensor mason?

Good day,

I think firstly you should check your bbob, ( just go to the search bar and look for bbob ) as everyone has mentioned, Do you have any warning lights that come on when that happens?
My opinion is that you should go to the Alfa dealer closest to you ( or phone the garages in your area ) and ask them to do a diagnostic test, if the engine had a fault, the system should record the problem. So when they do the test it will come up, They can clear the fault, but obviously depending on the problem, it does not mean the problem is solved permanently. ( they never charged me to do the test, But some advice, when they do it, ask them if you can stand there and see for yourself on the computer screen if there are any faults ), If its the RPM/Crankshaft sensor, Then it will come up as three different errors.

About the position of the sensor, I am not sure if there is a difference in position between the new and older models, I would assume not, but I speak under correction. Maybe you referring to the cam angle sensor. But all this I am not sure

The RPM/Crankshaft sensor from the dealer was R350 ( 25 pounds ) And you can change it yourself fairly easy. I never removed any covers to get to it
The Crank sensor won't alsways throw up an error on the diagnostic system, even if it fails completely. I am not sure why this is though maybe the ECU just thinks that the engine is not running.
To try answer your question, The system is suppose to have a 'limp' mode, if something goes wrong it changes its parameters on its sensors to allow you to get to safety of some sort, Maybe the system cant distinguish between the two, The normal and the limp mode. So then, when it goes to limp mode and it changes the parameters, the fault might then be 'solved' by mistake. These systems are installed in the cars, but I dont know how well they tested.

When I had my problems, there was no sign of any 'limp' mode, I just sat in traffic looking like a bloke showing off my car...... Wasn't funny

Just a theory......
The car can't go into limp mode if the crank sensor fails, as this is the only thing that tells the ECU when to fire the injectors and spark plugs. This is why a car with failing crank sensor won't run or will fail to start.

The ECU will go into limp if the coolant temp sensor, MAF, Lambda sensor, or cam angle sensor fails as the values fed to the ECU are replaced by default values if they fail. Obviously you cant have a Default engine speed programmed into the ECU, as engine speed is not constant whilst driving, and any variation would have the engine firing all over the place.

If you see what I mean... rolleyes.gif
Thats most likely one of the most pleasant ways that some one has told me I am the idiot. blink.gif

You make very good points, very interesting. Thank you for that.

My comments were purely a hypothesis, But seeing it was shot down constructively, I cant help with any more possible reasons why the crank sensor would not come up as a fault all the time on the diag test.

If you find out, or if anyone else has any ideas, please inform us wink.gif
Hey. I have a weird problem Connected with RPM sensor.
Sometimes my car wan't run because injector light wouldn't turn off.
I connect my 145 to PC and always I see an error with RPM sensor. I deleted that than make an injectors test in fiat ECU scan.
After taking this action injector light disappears and my car works again.

My question is how to fix that? That is really annoying. For example you wake up fastly go to car and you see that s**t. Go for the laptop cables and you're late...
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