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Crank Position or RPM Sensor.

B2XX Series Engines. Symptoms of Failure: No Hot Restart. Suppose the car already has the upgraded silver-terminal fuel pump relay but won't restart promptly after a hot soak. Take an educated LH2.4 RPM Sensor, Courtesy FCPGrotonguess on a weak crank position (RPM) sensor. If you have Bosch LH2.4, then replacing this permanent-magnet sensor located in the top of the bellhousing fixes many hot restart complaints on 700/900s. [Tip from HansW] Check the lead to the sensor: it can rub against engine components and expose the wiring. Newer sensors come with plastic cable holders to prevent this.

Testing Bellhousing RPM Sensor. [Inquiry:] I decided to find and remove my flywheel sensor. Externally it's in perfect condition. The cable and ends are pristine-looking. Is there a way to test it with a multimeter?

[Response: Ivan K] Often these rpm sensors fail intermittently, usually when hot. This makes them hard to test. Since the sensor is cheap enough, just replace it. With high mileage cars, it's only a matter of time before they fail.

[Response] Measure the signal while the sensor is still installed by unplugging the sensor at the cable connections at the firewall to the drivers side of the engine. Measure voltage here as the engine is being cranked. Use a low voltage setting on a digital volt meter, preferably in an alternating current setting if available in a low voltage setting. If no signal is detected, the unit is suspect. The other simple measurement is to unplug the unit (without removing it from its location behind the engine on top of the transmission housing). Check all 3 terminals for resistance or an open circuit. The new sensor measured about 160 ohms between the center lug and a side lug. My old defective unit had no continuity and no measurable resistance.

Removal of Bellhousing RPM Sensor. [Tips from Jay Simkin and others] The rpm sensor is awkward to access as it is below the cylinder head on top of bell housing. It can be removed by using 21" of 1/4" or 3/8" extensions, then a universal joint, a 3" extension, and a 10mm standard depth socket to turn the retaining 10mm hex head screw. First clean the area around the sensor using brake cleaner. Without inserting the ratchet in the extension end, pass the tools between the firewall and the aluminum-colored air-conditioning pipe. Once the tools enter the space beside the engine, bend the universal at 60 degree angle. Pass the tool string behind the engine block. There is a black cable that can be pushed out of the way. The 3" extension and 10mm socket will align pretty easily with the top of the hex head machine screw. Set the socket on the machine screw head. Gently, insert the ratchet into the extension string. Turn gently to remove the machine screw. Remove the tools. Use your left hand to grip the sensor. The sensor usually will come free as soon as the screw is removed. If it does not, grip it close to where it enters the housing - not at the top, into which the wire enters) - and pull straight up. Do not pull on the wire. If the sensor will not come out, it may be needful to break any seal between the sensor base and the bell housing. Turn the sensor gently about 1/16" turn, in either direction. You may rock the sensor gently, but no more than 1/16", side-to-side. Do not use any tool or brute force and DO NOT PRY since the housing is plastic (see the Cautionary Note below). The sensor goes only about 3/4" into the bell housing. The sensor has smooth sides and no locking lugs, so it is removed by pulling straight up. If the sensor still will not come out, use a small amount of lubricant (WD-40 or PBlaster). Allow it time to flow along the sensor's sides. Rotate sensor slightly and try to remove. Do not use brute force: if the sensor snaps and plastic fragments fall into the bell housing, the transmission will have to be removed to allow removal of the fragments.

Caution: Some Cars Have a Flimsy Sensor Mounting Bracket. [Another Tip] Trouble started when I couldn't get the old sensor out of the hole into which it fits over the flywheel. Where it's located makes it hard to get much of a grip on it. I tried rotating it in the hole to free it and then prying it up with a screwdriver. To make a long-story short, it's not mounted in the beefy Volvo bell housing like I thought, but in a flimsy, Fiat-like, aluminum casting that bolts into the back of the block. And very easy to break, I've discovered. And designed so you need to separate the bell housing from the engine to R&R it!!! Moral of the story is go easy on a stuck sensor

[Chris Herbst] NEVER PRY the RPM sensor out of the bracket. The poor quality bracket that they use to hold the sensor is SO weak that can break if you force the sensor around. Prying side to side on the sensor can break either the bracket or the sensor, leaving you helpless to do anything but pull the trans back to replace the bracket. When you really pry that thing is usually when it breaks. But because those brackets are so flimsy, it can still get you when you don't expect it. Usually (99 out of 100) is a piece of cake. But when you're the one who has #100... it sucks. Just be careful and you'll be fine.

Turn it lightly back and forth to remove it, but don't force it at all. Wait until it moves. On a standard trans cars use a tiny bit of silicone spray to get the thing out, and work it gently back and forth until it gradually loosens. If it is an automatic, get an Easy Out screw extractor into it, then douse the thing with PBlaster penetrating oil and wait for it to work its magic. The penetrant won't hurt anything like the clutch on the standard trans car. On the other hand, you want to avoid harming the timing ring on the automatic, because it is not very strong. You still run the risk of demolishing the bracket if you pry at it too hard, so make sure you are patient and let any penetrant or silicone get in there before you pull it. Once they move side to side, they can gently be removed. This sometimes takes a while, but the rewards are that you don't have to pull the transmission. On installation, put a light coating of grease on the perimeter of the sensor where it contacts the bracket. That will make your life a lot easier when you go to get it out of there next time. And if you have the trans out for any reason, take the RPM sensor out--whether or not you reuse it--and coat it LIGHTLY with a film of grease. That will avoid the problem, should the sensor need replacement in the future.

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