Speed/Reference sensor test

[mikseymono]

Hi team!

First post...from the other side of the pond.....

Trying to work out why car seems to hit wall at 4000rpm...almost like a limiter...have read many several thread on this subject and have checked quite a few parts as per Clarks. Think it could be the reference sensors.

Just on the "jump" tach bit when you start the car.....I understand that if this jumps when you start the car say 1/8th inch the sensors are likely to be OK. Things is when I start the engine and it starts up mine in neutral the tach jumps to about 1200rpm...so about 2 inches.........is this what they mean by starting...as in turn the engine on...or just turn the key to on and this is when the needles come to life on the dash.

Sorry for the noob question....

Cheers,

M

[DannoXYZ]

Just key to ON, don't start engine.

Measure impedance of sensors. Better to plot their output waveforms with oscilloscope.

When you hit 4000rpm, is it smooth up to that point? Then abrupt stop and holds at 4000rpm?

[Pauld_94S2]

Testing:

https://www.youtube.com/watch?v=zeFOOcLSk60&t=5s

Gapping:

https://www.youtube.com/watch?v=-MbghejYiqk

[mikseymono]

Originally Posted by DannoXYZ

Just key to ON, don't start engine.

Measure impedance of sensors. Better to plot their output waveforms with oscilloscope.

When you hit 4000rpm, is it smooth up to that point? Then abrupt stop and holds at 4000rpm?

Thanks for this...well....no jump at all with key on..so I suppose that is positive in a negative way!

My car is a 924S 1988.

It is smooth up to 4000 rpm but really quite slow....no abrupt stop but really sounds like it might explode. Feels like it has hit the rev limit.

Here is my UK thread https://porsche924.co.uk/forum/viewtopic.php?f=46&t=31276

Quite detailed and have tested quite a few areas with your excellent Clarks website.

Will check the sensors from the DME this morning as per Pauld924s advice.

Thanks,

M

[mikseymono]

Originally Posted by DannoXYZ

Just key to ON, don't start engine.

Measure impedance of sensors. Better to plot their output waveforms with oscilloscope.

When you hit 4000rpm, is it smooth up to that point? Then abrupt stop and holds at 4000rpm?

So, without sounding too thick...just turn the engine to ON, but not start it, and the tacho needle should jump. Yes? If this is right....mine doesn't........

M

[mikseymono]

Originally Posted by Pauld_94S2

Testing:

https://www.youtube.com/watch?v=zeFOOcLSk60&t=5s

Gapping:

https://www.youtube.com/watch?v=-MbghejYiqk

Thanks for this...really good informative videos.

Just trying to get to the bottom of why my 924S 1988 seems to hit rev limiter and sounds really strained at 4000. It is smooth to get there but not fast at all.
This is my UK thread..https://porsche924.co.uk/forum/viewtopic.php?f=46&t=31276
Thought it was fuel related and now more inclined towards senors....

Cheers,

M

[mikseymono]

Originally Posted by Pauld_94S2

Testing:

https://www.youtube.com/watch?v=zeFOOcLSk60&t=5s

Gapping:

https://www.youtube.com/watch?v=-MbghejYiqk

So tested as per the vid...and all good!

However, as posted by DannoXYZ the Tacho will not jump when you turn it to on. So what does this mean please?

Thanks,

M

[Pauld_94S2]

You need to crank the engine.

If the flywheel isn't turning, then there won't be any sensor pulses.

[mikseymono]

Well...a bit confused now..as Danno said orherwise! Engine running or just to on......

[DannoXYZ]

sorry, yes, need to crank.
If no tiny little bounces, sensors aren't sending proper signal to DME.

Follow Clark's Garage guide to:

1. measure impedance of sensors
2. space sensor from flywheel properly.

[mikseymono]

no worries danno...when I crank engine it goes to about 1200rpm...not tiny little bounces as you say. Already measured impedance, at the plugs as well as DME, now just need to check spacing. Running out of ideas though...

Anyone have any clues re my revs not going more than 4000rpm..as per my thread?

Thanks,

M

[mikseymono]

Originally Posted by Pauld_94S2

Testing:

https://www.youtube.com/watch?v=zeFOOcLSk60&t=5s

Gapping:

https://www.youtube.com/watch?v=-MbghejYiqk

OK..well, just cant get the sensors off! Have removed bolts but feel iImay break the sensors. Have tried wiggling/pulling etc. Is there a special tool /technique please?

Thanks,

M

[944 Ecology]

The special technique is to have a couple of sensors nearby, as the likelihood of breaking sensors that have been in place for at least 30 years is quite high.

My success rate (for sensors in engines/drivetrains that had been removed as a whole from chassis) is around 50 percent.

If you don't break the bracket for the sensors, it will help to open up the hole into which they fit... there is no advantage to having the sensor as a friction fit.

[mikseymono]

Thanks for reply.....guess I really need to find out if the sensors are defective first before playing parts Lotto.............just don't have an oscilloscope...

[944 Ecology]

https://www.picotech.com/products/oscilloscope


In the giant scheme of things, cheaper than two sensors and a bracket.

[mikseymono]

Just a question please:

If I have bad speed and reference sensors I thought the car would not start. Is it possible for the car to start with faulty sensors or sensor?

It is because of this thread Help me diagnose - poor idle/rough running/low power from engine

Other than the 'pop' he mentions it is the same........just don't have an Oscilloscope to test...

Thanks,

M

[DannoXYZ]

Without oscilloscope, you can at least measure impedance of sensors. Will at least 100% confirm they are bad if out-of-range.

[mikseymono]

OK...so..managed to get sensors out...not too bad.....the one nearest the bulk head in hole 'D' was badly eroded on the top as in it was rubbing against metal. I gaped new sensor as per Clarks. I stuck an 8mm thick cut up credit card to gap them. I cranked the engine. Straight away the tacho jumped up to around 4500 on start with no foot on accelaration...first time I have ever seen that...exciting, engine felt much more alive.....however there was this high pitched metal grinding whine. Sure enough I checked new sensor and the new one was eroded on the top as well. It turns out the aftermarket one is slightly...maybe...1mm longer than the OEM.

There does not seem to be the amount of play on the up down motion of the bracket on mine against this instruction vid https://www.youtube.com/watch?v=-MbghejYiqk

I have now put the old one back on as I have no more!

Convinced it is the speed sensor that is the problem. So question is, how hard is it to gap and if the sensor was scratched eroded would this make the difference? Was a bit adverse paying top whack for OEM Bosch sensor so bought these instead https://www.ebay.co.uk/itm/CRANKSHAFT-S ... 2749.l2649

Thoughts please.

Thanks,

M

[mikseymono]

Hello!

So, it seems I don't have the 0.3mm washer that goes between the sensor and the bracket as per the PET diagram. No 29. Is this really going to make any difference?


Thanks.

M

[944 Ecology]

If your bracket has the sleeve (#28), then it should have the spacer (#29) in order to maintain the relative relationship between the two sensors.