Today saw the start of fabrication for this project. The first thing to do is modify the distributor to act as a low resolution trigger and still operate as basic distributor to fire the correct plugs. Here's how.....
First of all mark where No 1 plug lead on the cap is in relation to the distributor body.
This is important as both advance mechanisms have been removed and locked down and advance will be handled by the ECU. This means that when running at maximum or minimum advance the rotor must still be close enough to the plug connection to transfer a spark.
To cut a long story short, a few calculations show that my average advance will be about 25 deg BTDC and the idea is to accurately align the rotor with the plug lead at 25 deg BTDC so that when advance is at minimum or maximum the tip of the rotor will still be close enough to the lead to fire reliably and not fire on to the next lead instead.
Next thing to do is fit the Accuspark module which I'll use as the trigger for the ECU. The distributor has to be completely dismantled to do this.
As the distributor runs at half crank speed this trigger will only provide 2 signals per revolution just like points. When cranking the engine to start, the ECU cannot calculate an accurate figure for revs using only 2 signals because the revolution speed changes considerably as the pistons come up to and go off compression. This means the ECU cannot calculate an accurate sparking point. The trick is to tell the ECU to fire immediately it sees a trigger signal at cranking speed, i.e. below about 500 to 600 RPM.
This leads on to the next task, aligning the Accuspark to trigger at the correct time in relation to the rotor at a static ignition timing of 10 deg TBDC. To help with this I made a simple timing disc from a clear plastic CD disk using a protractor.
To use the disk TDC on No1 cylinder has to be set correctly on the engine. This can be determined with various mechanical methods but on the Slant 4 Vauxhall very kindly machined a TDC timing mark onto the block so that when the centre of the crank pulley keyway is aligned with the mark and the camshaft alignment marks are correct then you've found No1 TDC. I've highlighted and extended the mark with paint on the old engine I'm using to experiment on so that it can be seen easily,
I marked TDC, 10 and 25 Deg BTDC on the disk, enlarged the hole in the centre and fitted it close to the block behind the lower crank timing belt pulley.
Next up rotate the crank clock wise almost 2 full rotations to the 25 deg mark BTDC on No1 cylinder and install the distributor so that the rotor aligns roughly with the No1 plug lead mark on the body, then swing the distributor to accurately align. Bolt the distributor down tight.
You'll notice that it will be at one extreme end of adjustment or the other depending on which gear tooth you engage on the aux drive shaft. I suggest that you engage the gear wheels so that it at this extreme end of adjustment to avoid later problems such as the distributor cap clip being under the thermostat housing and not being able to get it on or off! It also aligns the base plate better for the locking procedure described later.
Next up align the 10 degree mark, this is where we align in the Accuspark.
Attach the block or distributor to the negative side of a 12v battery to provide a ground/earth and the RED wire on the Accuspark to the positive side. Then connect up a voltmeter as shown, red lead to Accuspark red (12v+) and the black lead to Accuspark black.
You should see a very small residual leakage voltage if all is OK.
The black wire is the wire which sends the trigger signal to the ECU and we must align that trigger to 10 deg BTDC to get a fixed cranking advance of 10 deg. To do this I gently and slowly swung the base plate with the Accuspark in place CLOCKWISE until I saw a large momentary voltage spike up to 12v. The meter may not be fast enough to read the full 12v so you could see anything from 6 to 11v register. This is the leading edge trigger point. If you swing anti clockwise you will see the trailing edge trigger which is about 15 deg out. You may need to practice this to get it accurate. Once found I marked the base plate and distributor body and took the distributor out.
I then drilled a hole through one of the redundant holes on the base plate and the bottom of the distributor body and locked the base plate in place with a self tapper.
Job done!