FLUFF

Wouldnt mind the DME checklist. Its stopped raining so I'm going to give those sensors another squirt of WD40 and elbow grease.

FLUFF

I also remember seeing a late model 924 in my local breakers yard. It had the porsche engine not the audi engine. Would that have a compatible ECU?

Terry Jamison

A couple months ago there was an article on these sensors and their replacement in Porsche 911 magazine out of the UK. Might be worth looking into. There was some talk about the article on this board. Talk was centered around construction of a shop tool to re-install pick-ups with the correct spacing.

Good luck.

Terry '84 Guards Red

Bri Bro

If you don't remove the bracket, the sensors shouldn't move out of position. If you want to check the gap, glue a washer to the bottom of an old sensor that is the same size as the gap. Place it in the bracket and check if the washer hits the flywheel with the sensor completly in the bracket.

VectorGuy

Whatever you do, DON'T forget the 2mm spacer that goes on the rearmost sensor when you reasseble from a clutch job. The flywheel makes a darn MESS out of a sensor set with a gap of -1.2, and it's impossible to remove the whacked stainless sensor housing. You'll end up removing the bracket and cleaning all the shrapnel out, then re-gapping all with a VERY difficult access angle and hole. OUCH..been there, done that..and not proud to admit it.

I'm guessing if someone were to have a 1.2mm spacer for the rear sensor, it could be re-assembled with the sensor on the flywheel, (spun to rest on top of a tooth) and then remove the 1.2mm, ad the 2mm and then presto, a .8 gap.

FLUFF

I seem to be destroying the allen heads rather than extracting them. Before I totally destroy them can anyone check the cranking voltages from the disconnected sensors with a DVM on AC range.
I know that you have given me the peak voltages but there should be a corresponding RMS AC voltage.

(I dont have a scope and the car is nowhere near a mains supply anyway)

FLUFF

After a night of WD40 - finally got allen screws out - tightened them up until they "cracked" and then they came out by finger.

Position sensor withdrew OK, but Speed sensor is well and truly stuck - cant lever or prise it out.
Eventually got a 17mm combination spanner across it and levered the other(ring) end with a socket bar. Rocked the spanner in both directions with copious quantities of WD40 but no joy - I can tell its stuck towards the bottom end and dont want to break off the plastic! I shall leave it overnight to soak.

I'm pretty cerain that there is NO spacer under the drive sensor (worked fine for years though).

The sensor Bosch part number for your records is 0 261 210 003 (cheaper to buy it from a Bosch dealer than from Porsche)

bs

ok fluff, hope this doesn't give you a bad flashback or something but if you do the integrals you will see that:

for sine wave:
V_rms=V_peak/sqrt(2)
=V_peak/1.41

for square wave:
V_rms=V_peak/2

now i'm not sure what kind of ac signal you are dealing with here as i don't know much about these sensors. if they are hall effect (which it kind of sounds like) then they should be outputting a sine wave; if they have their own internal signal processing (unlikely for something that old) they probably output a square wave. make sure that your dvm measures true RMS because some of the cheap ones just measure the peak voltage and then divide by the square root of two, assuming that you will never measure anything but perfect sine waves.

Manning

I am really blown away by the fact that people have trouble pulling those things out. Mine slipped out after having soaked in a (very) little bit of Liquid Wrench for about a minute. And I don't think that was even necessary.

I don't understand what gets stuck. On the sensors in my car there isn't really any metal-to-metal contact to seize together. Mine are held to the bracket on the bell housing by a plastic housing with an O-ring.

One thing I have noticed that seems like a really stupid design is the fact that one of the drains for the well in front of the windshield dumps right onto the sensors.

EDIT: I live in the very salty NE Ohio area, where I would expect that things should corrode and get stuck together.

Bri Bro

if you do the integrals you will see that

The problem is it isn't even close to a sine wave, it is a pulse. Your point about how a DVM reads is well taken. Also, the voltage read by the DVM will be determined by the cranking speed since it is the integral of the square root of the sum of the squares over time.


On the sensors in my car there isn't really any metal-to-metal contact to seize together

The metals don't seize. The space between the sensor and the Al bracket gets packed with grit, oils and "stuff". It is better then most glues. My car came out of Texas were there was NO salt, but plenty of sand and dust.

FLUFF

I'm thinking the speed sensor might be rusted in - so I'm taking a little care. If WD40 doesnt work I'll use diesel. If diesel doesnt work I'll use brake fluid. The car had been sitting for a few months before I used it and it stopped only a few days later - perhaps an extra rusty flywheel is enough to grind the end off the sensor - especially if it had no washer!

The refence sensor looks like new.

A friend has told me that he thinks these sensors are the same as on the front of the Ford Fiesta - so I will be down the breakers yard for a look - these arent cheap.

FLUFF

I'm after empirical measurements by someone with a healthy car rather than "theory" so that I can do a "real world" check on my sensors with the tools that I have available. (How many people out there have an oscilloscope in their pocket?) It might not equal the manual specs, but it will be a "real world" measurement that any of us can refer to in the future.

The cranking speed is fast enough that a modern DVM should give a reading on the AC range that is the RMS voltage equivalent of the pulse train from the sensor. Modern DVMs owned by different 944 owners should all give the same reading (unlike analogue meters which can vary depending upon whether they are moving-coil or moving-iron meters, etc) and hence comparisons between owners is possible (making minor allowances for differences in cranking speed, sensor gap and sensor tolerance)

Unfortunately it look like evidence that the coil resistance of the sensor being OK, ie around 1k ohm, is not sufficient to determine that the sensor is OK - there may be damage, loss of magnetism, iron filings, etc which reduce the efficiency of the voltage output.

Bri Bro

Why don't you call Porsche and ask them why they didn't do the test with the tools that you have. I am out of here...CLICK!

a1984944

FLUFF!!
You got it nailed.
I just got my car going after the same symptoms.
No fire, got compression and fuel. Replaced the coil and relay and got nothing.
Replaced the sensor and it cranked right up!!

TheJonesBoy

Its easy to diagnose a bad hall effect pickup. Just crank the engine and look at the tach. If it wiggles, you are getting a signal. If it lies flat, your troubleshooting is over.

Hall effect sensors usually get flaky before they die completely. That gets hard to diagnose.

Oh, and the signal on a scope is a sinusoidal wave, with increasing voltage as the tooth approaches the sensor, and a negative swing as it leaves. One complete + to - cycle per tooth. There is then a break before the next tooth. It is not a sine wave, but it is sine-like. AC setting on a DVM would be better than DC. You might be better off with an older analog gauge, as it shows low level impule based signals better.