Crank Position Sensor

Recent events underlined the importance of our Deltas' crank position sensor.

This little item controls the ignition timing, and if the CPS malfunctions,

the car is impossible to get running.

Magnetti Marelli SEN 8D 15mm diameter revolution sensor.

Air gap 0.5 to 1mm, output 400mV,

resistance 612 - 748 ohms

The CPS on our cars is a Magnetti Marelli product, in common use on many other vehicles. It is an induction sensor, which means the electric charge is induced, and it needs no electrical input from the car. The sensor makes its own electrical charge.

As one of the crank pulley's four teeth pass close to the sensor, the sensor's magnetic core has its polarity altered, which induces a small current in the copper winding around the core. This pulse is sent to the ECU which amplifies the signal and completes the spark plug firing sequence.

The power generated by the CPS is only 400mV, so everything must be perfect for this tiny signal to reach the ECU. The resistance of the winding and the magnetism of the core are carefully balanced, so if either lose their parameters the unit will fail. The resistance of the coil is about 700ohms, that and the 400mV signal can be measured with a multi meter.

The two wires from the sensor are shielded with a woven wire outer to protect the tiny signal from interference.

We recently couldn't start a Delta Evo1, it ran well and drove into the garage, then wouldn't drive out again! I checked it all over, then after a while we tested the crank sensor parameters at the top plug, and found no signal at all.

I removed the sensor and cable and found the cable had a clean cut about half way up. I've noticed wear at this point before on other CPS cables, I can only assume the cables have rubbed on the alternator pulley somehow.

We didn't have another sensor that worked well enough, so we spliced in a pair of wires to see if the car would fire up.

It was definitely a 'drive 'em in...push 'em out' moment!

testing crank sensor
Integrale cps wire

Testing the CPS with multi meter

at the cable connector

CPS cable cut behind alternator.

Well, the car tried to fire up, there was nothing at all previously, 

but it sounded like the timing was out, so our bodged up sensor cable had made a difference, but the joins, the length, or the lack of shielding must have corrupted the tiny signal somehow.

I have a new sensor on order, but this operation showed me what a sensitive and important item the crank sensor is.

The sensor's cable is difficult to feed up the back of the engine correctly, but I'll make sure it is routed safely, and I'll make sure the air gap between sensor and teeth is between the .5 and 1mm medians.

Video of attempted start up, with bodged sensor wiring

multi meter and cps wire

As a Delta owner, you'll know living with the car is almost

a spiritual experience!

Two days previously I'd done a Youtube vid about CP sensors,

the things I'd learned making the vid were very useful here.

It was like our Delta was putting me through a quick test

to see if I'd remembered everything!

Vid is here, you'll notice the cut on the cable shown in the picture on the left, which is a clip from the vid, is in the same position as the cut on our car's cable.

testing crank sensor