Megasquirt and the standard MK2 ignition module - dwell control issues

Hi all,

This is a question I'm asking on the MSx/Extra EFI forum, but thought I'd try my luck here as there are many MS users using the standard Golf MK2 ignition amplifier and coil.

The situation is this. I'm using MS1 (V2.2 board) to drive the stock 7-pin ignition amplifier, exactly as described here.

My understanding is that these modules have inbuilt dwell control. Therefore, one can simply turn off dwell control in MegaTune, and just set the spark drive to 50% duty.

What's bugging me is this. I currently have the spark output inverted, and this means that the spark output to the module is asserted (high) when ignition is first turned on. I have always noticed that for the first second after turning the key, there seemed to be an oddly high current draw from the battery (as indicated by dimming lights and a change in tone from the fuel pump as it primes). I discovered that this was due to the ignition module grounding the coil for a silly amount of time, about one second, when I first turn the key.

As a test I turned on the ignition and I used MegaTune to toggle the spark output between inverted and non-inverted. As I did this, whenever the output went high, I would see my DVM indicate that the coil was being grounded by the module for well over a second, accompanied by dimming of the instrument cluster.

If these modules really do control dwell then surely this should not happen. My understanding is that the module should pull the coil negative to ground only for a matter of milliseconds whenever a rising edge is presented at the spark input.

The testing I have done suggests that maybe the module is supposed to be driven with only a very narrow pulse, i.e. one that is shorter than the range of dwell times. Therefore by using the 50% duty from MS isn't going to produce proper dwell control from this module.

Any experiences please? Has anyone used this setup and measured the dwell angle and confirmed it's sensible?

I have tried three modules; ALL produce the same results. These are all modules taken from Golf MK2s.

Siemens - VW part 191 905 351C
Telefunken - VW part 191 905 351B
Bosch - 0 227 100 142 / VW part 191 905 351A

Trev

 

Trev,
Did you put a current clamp or shunt on the battery terminal to determine the current delta
with the various calibratables?
Also is your vehicle's wiring mirror a VW digifant MK2.
I cannot say that these issues have been observed with any of the projects undertaken. What I have observed is with a MK3 coil/amplifier combo you need to run 50% fixed dwell. Running greater than this seems to quickly distroy the amplifier.
TCI-h units seem to work fine at 75% with no reported "deaths" to date.
I would have thought that TCI-h unit requires a non inverted PCM signal though vs the opposite for PCM controlled BIP373 or VB921.