vb921 x 2, heating problem...wrong trigger config?

[J. Pulkkinen]

I have a MS v2.2. I soldered wires to r25 and r28 resistor pins which are closest to prosessor, connected them with 330ohm resistors to vb921's.

Using vr-sensor with lm1815 circuit and 60-2 wheel.

I switched spark inverted and dwell control on.

While I am starting the engine, the vb's get very damn hot.
I am getting about 200rpm while starting and the two leds flash on the ms board.

What's wrong. I am using a Bosch 0 221 503 407 coil which has three pins.

[JeroenBosma]

Check the dwell. For a DIS coil it should be something like 2.5 ms but it can vary. If the 921's get hot your dwell is to high. This becouse they limmit the current at 7 amps. The rest of the current will be dispensated as heath and the VB921's cannot stand that for a long time.

Do not use the 50%/75% dwell option for DIS coils.

[J. Pulkkinen]

In spark setting, I have put cranking timing to trigger return. What is the difference between that and Time Based? Mayby the problem is with the triggers. Putting too much spark through the vb's?

My current configuration is this.
Trigger A = 5
Trigger return = 13
Trigger B = 35
Trigger return 43

Trigger Angle = 60
Cranking Timing = Trigger return
Cranking Advance Angle = 12

Can't remember which tooth is under the sensor at TDC...
Mayby it is number 8. That propably means that my triggers are wrong.

EDIT: With my configuration, there should be 15th tooth under the sensor at TDC. Right?

How about the vr-sensor wires. It has three wires. One is ground and the other two go to lm-circuit? Now I have only one to ground and one to lm-circuit.

The wheel looks like this.
http://smg.photobucket.com/albums/v608/ ... sku012.jpg
So it really doesn't have tooths but holes...

[jsmcortina]

For the moment, try running with timebased cranking instead and set the trigger returns to zero.

James

[J. Pulkkinen]

For the moment, try running with timebased cranking instead and set the trigger returns to zero.

James

Your absolutely right. When I put trigger return on, It leaves the vb:s on for a second when I stop starting, and burns them up in the process. But when I use time based it works perfectly without any heating.

Can you explain what is the difference between the two options? Don't quite get it.

[jsmcortina]

With timebased cranking the code gets a trigger at (say) 60BTDC. It then works out how long until it should fire the coil at (say) 10BTDC. It then works backwards to find out when it needs to turn the coil on and sets a delay to the start of dwell.
Once the trigger has been received the calculations are done and no matter whether the engine speeds up or slows down in the next milliseconds the coil will still turn on and then fire at the same delay time.

With trigger return cranking it is similar to the above except that the actual firing of the coil is controlled by the trigger return tooth you specified.
So, if the engine slows down or speeds up the actual dwell time can be altered, but the spark will accurately happen on the designated tooth.
If the engine stops turning after the dwell has been scheduled it will still turn the coil and until the stall timer kicks in after 2.5 seconds.

With decent heatsinking this 2.5 seconds should be ok.

James