DTAForum.com - a forum for the DTA ECU

A mate has fitted an external 60-2 SBD trigger wheel on his VX XE and mounted a Ford Fiesta crank sensor on a bracket. He's positioned the trigger wheel such that the missing teeth gap is in the same relative position as the normal internal trigger wheel in respect of the 117 degrees sensor position. The gap between the end of the sensor and the teeth is about 1mm.

The engine wont start. We're getting crank pulses on the oscilloscope but there are 2 blue spikes where the should be one (separated by a few yellow crank pulses) and there is no speed showing on the tacho.

The fuel pump relay is also not powering up when cranking (although it does when powering up the ecu) so were sure it's a crank signal problem. We can see the crank sensor resistance correctly on pins 19 & 28 of the S40 connector. We've tried holding the sensor close to the trigger teeth too but no change.

He's tried 3 more crank sensors but no difference.

One thing we thought about was that the trigger teeth are quite small as the wheel is only about 100mm in diameter, is it possible that the Ford Fiesta Zetec crank sensor cant see the teeth properly, ie the end of the sensor is very big. The one that SBD normally use has a very small sensor tip, like 2mm diameter, like a Rover K series one or the one used on a 1600 VX Ecotec engine. Just a thought but I would have expected this to be a well known and documented problem if it was the case.

I've given him a K series sensor to try but it's not so easy as it's not a direct swap on his bracket!

Anybody have any similar experiences?

cheers,

Martin

What does the scope trace look like ? Or better still, a proper scope with voltages etc.

You doi seem to suggest there is a scope trace, albeit not a good one ? So I'd have thought even a bad trace would still give an rpm reading, as for rpm it wouldnt care about missing teeth etc, just that it is seeing teeth

Obvious one, have you tried reversing the polarity ?

I havn't had a chance to get a proper scope on the cranks signal as yet, yes we've tried reversing the polarity and I've double checked the crank sensor screened cable (that was made by me!)

The gathered pulses look normal although the trigger wheel seems to be slightly eccentric and there's a sinewave ripple showing on the amplitude of the pulses. As you say, the missing tooth is of no consequence to the tacho.

Martin

i have had problems before when the gaps are smaller than the steel tip of the sensor, at one point i put the sensor in the lathe and turned the nose down!

lumley32 wrote:i have had problems before when the gaps are smaller than the steel tip of the sensor, at one point i put the sensor in the lathe and turned the nose down!

That does make sense....not sure I'd have tried that though ! lol

4" really isnt a huge diameter for a 60 tooth wheel though. I guess the teeth must be pretty small ?

More like an ABS trigger really and they do have a fine point for reading the wheels.

it did work stevie! later on i made up a bigger wheel and put it on the other end of the crank!

on the bike stuff the standard is about 22 teeth, you struggle to get any more as the roters are only ~60mm! why so many teeth? 36 is plenty!

lumley32 wrote:it did work stevie! later on i made up a bigger wheel and put it on the other end of the crank!

on the bike stuff the standard is about 22 teeth, you struggle to get any more as the roters are only ~60mm! why so many teeth? 36 is plenty!

More teeth, more resolution.

If you consider an old Cossie with only 4 crank teeth, for 25% of the engines rotation, the ecu has no clue what the crankshaft is doing. In human terms no big deal. In ecu terms that could be like a fortnight lol So it does matter especially in terms of ignition timing accuracy.
Fuel perhaps less so unless you are running fully sequential with tuned/timed injection.
But it can effect transients

And some modern ecu's are able to detect torque increases, detonation etc via measuring crankshaft acceleration. With a low number tooth trigger wheel that would be impossible. With lots of teeth it becomes possible.

i see that the more teeth the better in terms of working out where the crank is, what i was saying is there is a point of overkill! the sensor needs to match the trigger wheel as the two are matched to a certain extent! ie size of poles and teeth

also at high rpm's you are going to have more problems with more teeth, the ecu can only go to a certain number of teeth per second, no idea what that number is in this case! im guessing as this is a car engine of some sort so its not going to rev anywhere near this limmet, but its still there!

OK, so we sorted the problem! A crank sensor from a breakers yard Fiat Stilo fitted the bracket with slight modification, this time we got the correct pulses from the crank oscilloscope and 194rpm on the tacho. This crank sensor was quite a large diameter but with a 2 mm visible metal end pin embedded in the plastic but clearly visible.

Clearly the Ford sensor is not compatible with a 100mm diameter 60-2 trigger wheel in this application, strange that I have not heard this before. My mate cut open the end of the Ford Fiesta Zetec sensor and found that the metal end was say 8mm in diameter, obviously enough to see two teeth at once.

Something new learned every day!

Martin

The engine fired up and ran well today with the Fiat crank sensor.

Martin