GEMS - General Engine Management Systems Ltd - Forum

Hello all,

I am upgrading to a 12-1 crank trigger wheel. Intake cam has VVC and a 2+1 (early extra tooth) trigger pattern. 4-cylinder 4-stroke engine. Car ran great for months with a 2-tooth (!) crank wheel with VVC running, and now I want to upgrade the crank trigger wheel for better dynamic timing accuracy.

My tooth control table is
54 54 54 54 54 53
(Divide by 2; with 6 5's; total number of entries = 11, without counting the final '3'; 5's sit on even numbered teeth, 0,2,4,...)

Sync MX = on
Missing =1
Sync teeth = 2
MX Sync test = 10
MX time = 50%

It syncs and generates spark outputs, but... please see attached datalog of "A Tooth" and "Fuel Tooth".

If I understand the tooth control table properly, it's supposed to increment Fuel Tooth on every even-numbered A tooth - i.e. when A Tooth transitions 1->2, 3->4, and so on, including 10->0 (the first tooth after the gap)
wherein the first tooth after the missing tooth is tooth #0.

However, look at the datalog. Note that I do get Stat Sync and coil output pulses.

"A Tooth" is the blue "staircase". It goes from 0 to 10 then resets. (horizontal lines are at 0,3,6,9). As expected, the reset from the missing tooth comes after tooth #10 (after transition 9->10), thus the extra long tooth step at A Tooth #10. (top of the blue staircase).

"Fuel Tooth" is the red staircase. It goes from 0 to 11, as expected. However, note that it increments on 10->0 like it's supposed to, but then subsequently it increments on odd, instead of even, teeth (0->1, 2->3, 4->5, etc.) The end result is that the Fuel Tooth has a step 1.5x longer than the others - at Fuel Tooth 5 (halfway up red), and at Fuel Tooth 11 (top of red) - when A tooth goes through its missing tooth. After those extra length steps, Fuel Tooth 6 and 0 are half the length of the other Fuel Teeth.

I looged RPM (not shown) and it doesn't seem to waver due to the wrong length fuel teeth.

Any clues where my problem lies? Is there now way around it? Divide by 3? Is it safe? My missing tooth (and thus wrong length fuel teeth) are just after TDC where no spark occurs...

TIA

I figured it out. I was mis-interpreting the tooth control table. The entry in position X tells the software what to do when the *next* tooth comes.
If you consider the first tooth after the gap as tooth 0.

The tooth control table that worked was

01 01 01 01 45 533

The consecutive 5's make FuelTooth increment on the A teeth that straddle the gap.
I don't know what the extra 3 in the end is for, (doesn't work without it) but I got the idea from a 36-2 setup I found. It doesn't work without it.

My next question is, how do I choose the best value for?:
- Sync R/S
- Sync MX

I got the car to run, but it seems to sometimes log timing errors when starting - when it does this, it seems to "catch" but then struggles - like it doesn't see the missing tooth due to RPM jumping due to the cranking. It seems to be affected by Sync MX; it gets worse if it's >65 or <50.

Tooth Control table is encoded:
bit0 = use tooth for internal teeth, that are even angles, so for 12 -1 every other tooth is not used internally.
bit1 = used by Crank_Alt_Fire mode, however if Bit0 + Bit1 are ON "3", reset A_Tooth counter and increment Timing_Error.
Bit2 = Check for Synchronisation, if Test_not_sync'd = On, then only required near sync point at end before "3".

It is important that the correct Crank_rising/falling_edge is selected, and Miss_Time/MX_Tooth_Time is close to expected.
In this case with one missing, ratio about 1.95 to 2.05 in normal running. As Missing and MX_Time make a discriminator to detect the missing tooth. Note this edge problem is more common with variable reluctance crank sensors, but not unheard of with some Hall Effects, or optical sensors with saturated photo transistors as found in older Nissan Skylines.

The internal teeth count-up in Fuel_Tooth_No, I_Tooth_No, Wheel_Teeth no, etcetera.

Sync_R/S is the number failed sychronisation attempts before going Stat_Sync'd = Off, so typically 4.

Rich A

Hi, if I were to make a 12+1 trigger wheel, or otherwise have an extra tooth welded onto my 12T trigger wheel, what is the best position of the extra tooth wrt to its neighbors? (I don't mean between which 2 teeth, I know that it should be away from the cam teeth over its VVC range).

Halfway between its neighbors, or closer to the previous tooth e.g. 40% of the way between the previous to the next tooth?

TIA

Hello,

It depends on the resolution of your timing sensor but you should position the extra tooth close to the last tooh. A triggering edge of about 10 degress after would be about right for 12+1 setups.

Hope this helps
Ed

The hall sensor seems to put out 3° wide pulses. Does this imply it's plenty fast?

So 10° after the last tooth is 1/3rd of the way between neighboring teeth, closer to the previous one, correct?

Thanks again.

Hello,

Correct you would be good on that setup.

Hope this helps
Ed