CKP/crank position sensor output (TECHS please!) - PeachParts Mercedes-Benz Forum

pentoman · #1 02-27-2006, 06:12 PM

My 16v is not sparking after having the gearbox reconditioned.

I have narrowed it down to the ignition computer - you know the one mounted on top of the wheel well, or the crank position sensor. I suspect it's the position sensor since that is very near the gearbox and causes the spark, but want to test it before going through the cost and effort of replacement.

A resistance test confirmed it has correct resistance.

I hooked it up to an oscilloscope to confirm that it's not working - the test steps say there should be signal with ~1.5V peak voltage. To my surprise it produced a signal, although there was more like a 3V+ peak, which suggests it is working.

If the sensor is indeed working, it means I need to replace the ignition computer, which equals ££££££££ so I would really like the problem to be the CKP sensor, especially as it's so likely to be disturbed by the gearbox replacement! I hooked the scope up to my 8 valve daily driver, and its CKP sensor produced a similar signal but it was however much smoother. The peaks tailed off nicely. Attached are 2 crudely drawn demonstrations of the traces.

My question is, what should the CKP sensor's signal look like, and is the odd spiky one I'm getting on the 16v enough to cause a no start? The 16v did start once, right after the gearbox was put back in, but then after switching off has not been able to fire since.

Help!

Rus

pentoman · #2 02-28-2006, 07:00 AM

Bump!

mbdoc · #3 02-28-2006, 08:15 AM

Try switching the sensors between the 2 engines....they are the same...only the length of the wire is different!

Moneypit SEL · #4 02-28-2006, 08:37 AM

I'm gonna say that the sensor is probably OK. It's either the leading or trailing edge of the waveform that triggers the ignition, and not the slope or duration. At least, that's the way most ignition systems work.

pentoman · #5 02-28-2006, 08:44 AM

Thanks DOC - yeah thought of that, bit of an effort though. Is there an easy way to get to the sensor to remove (I have small hands) or do you have to remove the starter motor?

Russell

pentoman · #6 03-02-2006, 07:25 AM

Bump!

I'd really like to know the best way to remove the CKP/CPS sensor. Since it looks like I might have to take apart both my 190s at once and swap the sensors it will be a right pain, so would appreciate any info regarding this.

Remember that my flywheel/clutch was removed and replaced so the problem could be something to do with them. Don't want to buy new parts if they're not the problem!

Just found out the cost of the items I'm looking at - a CKP sensor is £100 (can live with that), the ignition computer control box £906

That's ~33% the value of the car.

.

I notice one of the pins from the ignition computer outputs to the diagnostic plug (round one). How do i diagnose using this?

please help

Quote:

Originally Posted by Moneypit SEL

I'm gonna say that the sensor is probably OK. It's either the leading or trailing edge of the waveform that triggers the ignition, and not the slope or duration. At least, that's the way most ignition systems work.

True unless it's a clocked digital system, in which case a brief peak can go unnoticed. Nice login name by the way!

thanks

Russell

MrCjames · #7 03-02-2006, 09:56 AM

I am going to go out on a limb and ask if the flywheel was installed correctly? If there is no spark and the sensor voltage is that high then what I mention may be irrelevant; however, if spark is present then it is possible that it is being delivered at the wrong time. If the flywheel was not indexed to clyinder 1 TDC then your CKP signal could be off by 60 degree increments for 6 bolt mount or by 45 degree increments for 8 bolt mount. You did mention that it ran before you performed the gear box repairs, my thought circles back to flywheel and it's position so, is it possible to have inadvertantly misaligned it upon installation?

The CKP sensor is not that difficult to exchange however I would think that the voltage output is indicating that it is working well enough to operate. Have you tried a coil?
Just another .02

pentoman · #8 03-02-2006, 10:10 AM

Quote:

Originally Posted by MrCjames

I am going to go out on a limb and ask if the flywheel was installed correctly?

Thanks - there is no spark at all, and I assume even if the flywheel was in at the wrong angle there would at least be a spark?

Is it possible the new clutch in there could affect things??

MrCjames · #9 03-02-2006, 10:57 AM

I do not see how a new clutch would impact ignition operations, a new flywheel yes. Have you tried an igniton coil? Have you confirmed that the spark is absent from the igniton coil? If spark is absent at the distributor then it is possible that the coil wire is faulty. If the spark is absent at the plugs then the cap and or rotor might be the problem. If the spark is absent at the coil then it might be the coil. Your CKP signal appears to be strong enough to operate the ignition.

Is the coil getting voltage? Is it getting enough amperage to operate? Perform a voltage drop test between battery positive cable and the ignition coils terminal 15 to determine if the circuit is the cause.

pentoman · #10 03-06-2006, 09:53 AM

Quote:

Originally Posted by M.B.DOC

Try switching the sensors between the 2 engines....they are the same...only the length of the wire is different!

We tried this this morning - they aren't the same!

I guess a 2.5-16 must be different to the 2.3-16, or US/Euro are different or something.