Saturday 21 July 2018

Porsche 911 (996) CPS Crankshaft position Sensor replacement DIY guide - fix no start problem

So my 911 has been hibernating the garage for about 6 weeks now, as that's the last time I could start the car.

Initial problems: Wouldn't start when hot

Would have to wait 30-60 minutes for the car to cooldown, then would start as normal. Would turn the key, and absoultely nothing would happen, not even a starter motor crank.

So I'm not a mechanic, but have enough time to research this problem as it's very common.

The most likely cause of this problem was a bad starter-alternator cable. When old and corroded, when it gets hot the cable resistance would max out, preventing any current reaching the starter and thus not even a crank would happen. However, when cold the car would crank and start quickly, so not likely a starter motor problem.

Another problem: Battery not recharging

Car dash display would read 14V for about 30 minutes after being started, then would slowly drift down to 13V or even less. Another reason why it's likely to be a starter-alternator cable problem, as again when reaching a hot temperature, this cable would reduce current flow.

So I've had a few parts to fix this in stock for a while - new starter-alternator cable, new drive belt, new alternator regulator.

Then a new problem appeared - car wouldn't even start when cold.

Some research then told me it's a likely CPS (crankshaft position sensor) problem. From what I can figure out - when you turn the key, the starter motor turns over the engine (and crankshaft), which turns the flywheel which has a bunch of cutouts in it. The CPS reads this signal, and when appropriate to kick in some fuel and spark the computer would then fire up the engine. However, with a bad CPS the computer can't tell when to kick things on.

Main telltale sign is the tacho doesn't budge when trying to crank the engine. Engine cranks (so starter is OK) but not moving tacho = (probably) bad CPS.

This is a bigger problem than the first two... I can (kinda) live with the car not starting when hot for a little while, as I have other cars I can get around in. Battery not recharging - hook up to a CTEK battery maintainer every now and then. But a bad CPS means I can't even start the car - no bueno!

So I tackled this job today - replacing CPS. For the full start to finish video see bottom of this post.


I'm not a mechanic. If you follow this guide, be prepared for everything to go wrong. I take no responsibility for what you're about to do. What I write here is fully hypothetical, invented, imaginary and not to be taken internally. In fact, stop reading this now, go outside and catch public transport instead of trying to work on your car.

Tools required

  • Jack and stands
  • Wheel nut remover tools - I use my impact air gun for this (lazy!)
  • 5mm hex socket and a loooong extension tool or two
  • 10mm socket (I used a small ratchet 10mm socket as there's not much space to get in there

This is the part I ordered from eBay - took a few weeks to arrive from UK to Australia. About $110AUD delivered.

 Jack up car and make it secure. I've placed another jack stand at the back of the car just in case

Remove wheel and set aside somewhere safe.

Now you need to see where the CPS is. There's a lot of things in the way, but if you look towards the central housing behind the suspension components you can spot it.

To get the CPS out, you need a 5mm socket. I'd advise against using a small allen/hex key thing as you might slip and strip the bolt. I used a 5mm socket with a couple of extensions so I could get a wrench onto it to break the seal. Then was pretty easy to back it out.

After the bolt it out, remove the CPS. Follow the cable and unhook it from the plastic clip, then up the top (where it's pretty hard to get your hands in) you should see a ground wire bolted to a 10mm bolt and then a harness. Get your 10mm socket up there somehow and unbolt the 10mm bolt, then with the bracket loose it's easy to unclip the harness.

Then... "installation is the reverse of removal" as all the instruction manuals say.

Did it work?

Now [spoiler] at the end of the video I've got my 911 back and running! I drove it around for about 2 hours, stopping about 3 times during this drive, turning the car off and then restarting it no problem. Then I got home and parked it for about 30 minutes, then hopped in for another drive.

This time, nothing. No click, no starter motor engaging, no cranking at all. So this is a recurrence of the starter-alternator cable problem that I originally had. However, since the faulty CPS at least lets me start the car, then next project will be to replace the starter-alternator cable, alternator regulator and drive belt. Stay tuned.

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