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S40 + 3 wire bosch coils

Posted: Tue Oct 16, 2018 2:17 pm
by Rob Stevens
I've not used active coils before and I think these 3 wire bosch are? Any reason why I can't run 2 coils from one output of an s40 on a 4 cylinder. If that is ok do I just check the external amplifier box, no need for pull up resistors or anything else?. What worries me is that the loom the coils came from had a fairly thick wire that is the trigger wire.
Worried also of damaging something, I'll definitely wire coils +12v via fuel pump relay.
I can't find any info on the coils.

Re: S40 + 3 wire bosch coils

Posted: Tue Oct 16, 2018 3:36 pm
by stevieturbo
If you test with a current clamp/scope, you'll soon know with a brief test as to what mode of charging is correct.

Problem with 3 wire...it could be either. And as you say a heavy trigger wire might seem a little out of place for a coil with an internal ignitor.

Do you get a resistance reading between that trigger and any other terminals ? I dont think you should get a reading if it has an ignitor inside.

Regular inductive you'd defo get a reading.

Re: S40 + 3 wire bosch coils

Posted: Wed Oct 17, 2018 2:13 am
by Rob Stevens
Thanks, yes silly me, 1st thing to do is put a meter on them! They were definitely fed with an earth and 12v and the 3rd wire. So if they are passive can I hang 2 on one output? I guess I need to find how much current they draw.

Re: S40 + 3 wire bosch coils

Posted: Wed Oct 17, 2018 3:24 am
by Alex DTA
Bosch 3 wire tend to be inductive.

There are a number of people running S40s with paired active coils. It does work, but puts a lot of strain on the coil drivers as there's no heat sink on them. The S40 has thermistors to monitor coil temps under normal operating conditions for this reason.

Most of them are fine just being paired up, no pull ups required.

Re: S40 + 3 wire bosch coils

Posted: Sat Oct 20, 2018 12:38 pm
by Rob Stevens
As Alex suggested they are passive coils but with an extra earth for something or other. The primary resistance is 0.8 ohm and 2 of those would be 0.4 ohm,
So I chickened out and got one of those ford wasted spark coils.
Started up first time :)