Recently my car wouldn't start, replaced the crank sensor, and that fixed the problem.

After reading a lot of posts regarding the sensor, it sounds like, when the car is running and the transmission is rotating, if the sensor stops working, it doesn't see the tranny spinning, and the car stalls, sounds ok to me.

My question is (and let me start by saying I don't know all that much about how all this works), like in my situation, the car isn't running in the first but just won't start, (sothe tranny isn't spinning) how does the crank sensor prevent the car from starting?

Thanks

Lots of good responses. It sounds to me like you're looking at what happens at the very beginning of the cycle, to prevent the car from starting.

When you turn the key to start the car, the starter motor turns the flexplate or flywheel. If the RPM sensor is functioning, it can "see" this motion and give a signal to the computers to provide spark, etc. If the RPM sensor is not functioning, it can't see the flexplate being turned by the starter, and no signal is given to start the spark.

At least I think that's how it works...

- Brian

You ask how does the crank sensor prevent starting?

Simply put, no sensor = no spark. The ECU is set to not operate the fuel pumps with no spark. Prevents flooding and is a safety thing, too.

Think about how the ignition system knows when to create a high voltage (40,000 volts) pulse at just the right time to go from the coil to the distributor and then from the distributor to the correct cylinder? And the "right time" can vary according to engine load and whether it is accelerating.

All internal combustion car engines have this need, Model T to the newest. Even a Diesel engine, although they use different system since Rudolph Diesel's invention uses no spark plugs.

That crank sensor tells the Ignition Control Module (ICM) the position of the crankshaft so that the ICN can make the high voltage pulse fire the right spark plug at just the right time.

Cars without a crank sensor have another way of doing this. Cars before on board comupters did it a different way. I started with a '47 Chevvy and it had a way to do it, too.

Today's lesson. Wasn't that easy?? Hope it didn't seem condescending.

Regards,

Bob

:>)

I understand now, but, one of the replies mentioned that there would be no fuel flow if the sensor wasn't working. While troubleshooting I checked for fuel flow by disconnecting the fuel line coming into the injectors, then tried to start the car, and I had plenty of fuel spurting out of the line!

The fuel pump pressurizes the injectors as soon as you turn the key. The injectors however are only triggered by the ECM if the crank sensor senses rotation. So you will have fuel to the input side of the injector, but none should be comming out of the injector if there is no crank signal.

DEWFPO
--
1998 S90 066,800 and 1995 964 150,200

"The fuel pump pressurizes the injectors as soon as you turn the key."

I may be spliting hairs here, but that isn't exactly so. The fuel pump sometimes runs for a second when the key is turned on, but that only occurs when the ignition system is powered and the sensor detects a signal at power-on (happens when the sensor and vanes are positioned just so). This produces a single spark pulse, which triggers the fuel pump relay.

That is a random event and doesn't always occur. And it's most likely NOT an intended design feature.

The fuel pump and pressure accumulator have a check valve intended to maintain fuel pressure when the engine is shut off -- and that's so the system has pressure for the next start up.

If you have no crank signal, you have no spark. No spark means the fuel pump relay does not power the pump. No pump means no pressure beyond what's held by the check valve.
--
Don Foster (near Cape Cod, MA)

It sure can prevent starting. See my message below:
http://www.brickboard.com/RWD/index.htm?id=720290

I found my sensor had deteriorated with age and heat. What still puzzles me is why the sensor didn't cause stalling as well.

The crank sensor actually senses rotation (and timing, or position) of the flywheel -- or flexplate, if an A/T. Because the flywheel or flexplate is bolted to the crank, the sensor detects crank rotation and timing. Hence it's called a "crank sensor."

The sensor doesn't respond to rotation of the tranny, per se, because the tranny is linked to the engine through the torque converter, which is a fluid (slipping) coupling.

A defective or failed crank sensor will fail to detect the flywheel rotation, so the electronic control unit will not produce a spark pulse and also will not trigger the FI system to inject fuel. That's a pretty good reason for your engine to not start.

--
Don Foster (near Cape Cod, MA)

where is this sensor? Can they be purchased used for a spare as a test unit?

Dear crow1,

Good p.m. The crank (RPM) sensor can be bought from a Volvo dealer. I have never changed one of these. I gather that they are delicate. If twisted or shocked during removal, the external parts can break off. The transmission then has to be removed, so the internal part can be removed. Apparently, the replacements are more robust.

The crank sensor sits on top of the bell housing, which connects the transmission case to the rear of the engine.

To see the crack sensor, open the hood (bonnet) and stand on the passenger side of the engine, close to the firewall. Taking care to avoid striking one's head on the hood, bend over and look towards the ground, at the corner of the engine. You will see the crank sensor perched atop the bell housing. The sensor's wire runs straight up the firewall, and turns towards the driver's side of the firewall, where it joins a harness.

Hope this helps.

All the Best for a Good and Healthy 2004: for you, yours, and your car(s)!

Yours faithfully,

Spook