[Efi332] newbie (also Autronic EFI and Bosch ignition modules)

[Andrei Chichak]

 > The ignition system uses Bosch ignition drivers and Mitsubishi 
coils. I have been
 > having problems with the car eating coils.  I wonder if they are 
getting too hot. The
 > coils are mounted right to the head (they probably get pretty hot 
just from this) but
 > also I wonder if the ignition modules are working correctly.

The Bosch ignition modules will charge the coil then shut down after 
a short period of time. The dwell time is satisfied so it stops the 
current flow. After a little while longer it completely shuts down 
(as seen by my experiments). BUT the ignition module is basically 
just a great huge transistor switch that switches the coil to ground.

I'll bet that you are blowing coils because the coils need a ballast 
resistor and you don't have one. There are a couple of coil styles, 
low resistance and high resistance. Low resistance are about 0.5 Ohm 
on the primary and the high resistance ones are about 3 Ohms.

The low resistance ones are expected to either be run with a system 
that can handle them or with (typically) an external resistor. This 
coil is fed raw power during start then switched over to a 3 Ohm 
current limiting resistor during run.

Running without a resistor will have the coil drawing about 24 amps 
all of the time and it will make a big spark - for a while. Your 
di/dt will be huge compared to the normal 4 amps for the same period 
with the resistor, but you have to dissipate 36 times the heat if I 
have my math right.

With a high resistance coil you just draw 4 amps during start and running.

Andrei

 > Regards,
 > Erik Berg