MoTeC Global Forum

Hi guys,

I am looking after a Group A BMW fitted with the BMW S14B23 engine and an M48 ECU.
We are limited to using the original number/type of sensors as the car is 5th category.

When the M48 ECU was installed the installer chose to remove the SYNC and REF sensors from their original positions on the bell-housing and fit a chopper wheel to the harmonic balancer.
It now appears that this has damaged the balancer as we had two crank angle sensors wiped out on the weekend by the trigger wheel (it has been fine for the past two years but it's not now).
No SYNC sensor was fitted when they did the install.

Is it possible to return to using the REF and SYNC sensors on the flywheel and if so what settings do we need to enter into the ECU to count properly?
I believe that in factory “spec” the REF sensor counted the teeth on the flywheel and the SYNC sensor “looked” for a button off to the side of the teeth (as it is staggered from the other sensor).

We were told by the installer that the M48 is not capable of calculating injector and ignition properly as there are too many teeth for it to count accurately (I think about 166 teeth...).

All help/suggestions appreciated.

Thanks

Cristian

Hi Christian,

probably trying to do the same as you? ive got an m4 unit and was wondering if it was possible to use the ref signal (one per engine rev) along with an m5 cam sensor (for thr sync) as am sure read somewhere that the Motec only needs one tooth per rev? although plan B is going to be 12 tooth mounted on reverse of front pulley (shrunk onto the solid bit) along with mag cam sensor.

Andy.....

The CRt value can go up to 135 teeth. If your ring gear has this amount of teeth and you also have 1 tooth for the sync it will work in group fire mode.

Hi Christian,

You won't be able to get it to work with 166 teeth on an M48. If you have damaged crank sensors you need to look at how the trigger wheel is mounted and how the sensor is mounted. If the sensor is coming in contact with the trigger it means that either the trigger or the sensor mounting are not solid enough or that there is excessive runout on the trigger teeth.

The Sensor is mounted to the side of the block using a welded bracket then bolted to the block. Al the mount bolts were tight and there is next to no flex in the bracket (less than 1mm at the extremity with 100kg guy hanging off it).

My thoughts are that when the trigger wheel was "put" onto the harmonic balancer it started degradation in the rubber of the harmonic balancer and the heat last week accelerated the breakdown allowing the trigger wheel and outer balancer to "move" under high torque then striking the sensors.
I say this as we checked the second sensor fitment and clearance at idle and up to about 2500rpm in the pits. It was on the track during the warm up when the driver revved over 6k that the second impact happened.

As a side note the trigger wheel was attached to the harmonic balancer body using 3 dobs of weld (I have checked these and they have not failed).

I guess we are going to have to find a “better” way of mounting the trigger wheel.

Thanks for your help guys.

Cristian

I would not recommend mounting the trigger on to the harmonic balancer, it needs to be mounted firmly to the crank. This is likely to be the cause of your problem and can also cause ignition timing and RPM inacuracies. I would make up a new front pulley/balancer assembly with the trigger fixed to the part that is attached to the crank. I would also have the assembly balanced.

It's my car that Cristian posed the question about, so what's the best way to set this thing up then? we're having a solid harmonic balancer made and plan to fit the trigger wheel to this, but what's the optimum no. of teeth on the trigger wheel?
Cheers
David

Hi David,
I have spoken to you about this car in the past. There are a number of solutions that would be ideal for this car but there are various things that will make a difference to your choice. For instance the cranking speed of the engine, the type of sensor being used etc. I would normally go for a 12 tooth trigger on an engine like this, assuming that you have a sync sensor fitted. What is the current setup on the engine? If it has been working ok there may not be any need to change the tooth arrangement, just revise the way it has been mounted.

OK, now we've had new rules inflicted on us. Can an M48 run a 4 cylinder, normally aspirated engine without a cam angle sensor? I'm just using a TPS, crank angle sensor, an air temp sensor and a lamda sensor. Please correct me if I'm wrong but the use of a cam sensor allows for sequential injection which is now not allowed on my car even if it is set to group fire in the ECU.