Inside out feature

 

Cas unit

 

The cas unit is a much talked about part of the zed engine but quite what it does and how it does it is more of a mystery for many than its needs to be.

 

It`s closest comparison is the more commonly recognised distributor on engines in the past, but the cas unit does a whole lot more than the lowly distributor.

 

The ecu takes signal pulses from the cas unit pick up, these signals are used to trigger the coil pack at the correct time (ignition timing ) and to feed earth triggers to the injectors again at the correct time ( fuel timing)

 

To know how it does this take a look at the photo's, the reluctor disc has a series of slits and gaps which rotates at cam speed (half engine speed) the disc passes through the pick up sensor, within this sensor is light emitting diode one side of the disc and the other side a light receiving device. As the disc spins the light passes through the series of slits and gaps and fall on to the receiver which produces a signal output at the pick up which is then sent to the ecu to interpret the information and trigger the injectors and ignition coils accordingly.

 

If you take a more careful look at the reluctor disc it can be seen that the larger gaps are not evenly spaced which initially would look incorrect however quite the opposite, by doing this the ecu can not only calculate when to trigger the coil packs and injectors but gives it the information to know exactly what the position the engine is to a very fine degree.

 

Using this finer timing accuracy gives improved performance and works in conjunction with the injectors double-action fuelling, all fueling when cold starting and sequential fueling when warm starting and running.

 

The cas unit is a very hardy unit and will give many years of service but as with any electronic part can fail, the connection is one area that can be cause of cut outs and non starting.

 

But a lesser known fault is drifting of the output signal, this can show itself as a constant small increase in timing angle and then lowering, also it can suddenly spike 10 degrees and then return back to the correct idle setting of 15 deg. this however should not be mixed up with a faulty connection to the tps hard idle switch that can produce the same effect.

 

For testing purposes you need to monitor the ignition timing with a conventional timing gun and with Conzult at the same time to check it what is actually going on, strangely later series III cars that have a different looking cas unit seem particularly prone to signal drifting and although the unit can be the cause we have found later wiring looms use less shielding and this is more likely the problem and as a consquence paying particular attention to earths and cable looms on series III is very important.

 

Finally in the photo you can see the rear bearing, this can be the cause of many timing issues from signal drifting to noise by audible and electronic interference noise.

 

 

So all in all a very important bit of kit which is often overlooked as a potential problem as the engine will often apear to run ok even when the cas unit has issues, inspection, timing and Conzult is the key.

 

Jeff

 

 

 

 

 

I

Wondered what the CAS did..lol Haven't had a problem (so far) with mine, but another great read Jeff!

An excellent and very informative thread. :cool:

:bow: great info as always!

You the man jeff!:bow:

how can i test if my CAS is receiving a signal from the ECU/loom?

 

or how can i test if my CAS is transmitting an output signal?

 

 

my injectors are not receiving a power signal, but ive got spark so im asuming the CAS is giving out a signal to that?

The ecu send an engine position signal that the ecu interperates, it send the signals to the ptu ( ignition ) and the injectors. Hence it would appear that the cas is ok as your getting spark.

 

Jeff TT

thanks jeff.

 

can you adivse why my injectors are not getting electrial signal?

 

really stuck, dont want to hijack this thread, ive got my own one if you could post on that id appreciate a great deal

 

http://www.300zx.co.uk/forums/showthread.php?t=127192&page=2

Is this the part that goes on the cambelt cover??

 

You have removed the top cover. Is it just the 4 screws that hold the cover on?? If so would i be able to wipp it off and polish it up??

 

Dave :)