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RPM signal pick up for Duratec HE
MK9R - 17/12/09 at 09:12 AM

I've asked something similiar before, but i'm a bit further down the line now and still not got a result so i thought i'd ask again

I am fitting a piggy back ECU for an LPG system and need to pick up an RPM signal on a 1.8 Duratec HE engine. I've been told i can and can't do it from the coil pack depending on who i ask. I have mistakenly broken into the ground wire into the coil already and it doesn't work, i could try the other 2 wires which feedback to the ecu, but i was hoping to get a bit more info first and if its even possible from the coil. I've trawled the net and tried to get some info of Burton Power, but nothing that helpful has come out of it yet.

turbodisplay - 17/12/09 at 09:22 AM

There are 2 disadvantages to the coil as a pickup:
1) electrical noise fron the coil can cause things to malfuction if not designed for it.
Also voltage spikes of greater than 30 - 100v are possible
2) when rpm cut occurs if the sparks are cut (normally fuel, but could be both)

Darren

MK9R - 17/12/09 at 09:26 AM

quote:
Originally posted by turbodisplay
There are 2 disadvantages to the coil as a pickup:
1) electrical noise fron the coil can cause things to malfuction if not designed for it.
Also voltage spikes of greater than 30 - 100v are possible
2) when rpm cut occurs if the sparks are cut (normally fuel, but could be both)

Darren


Yeah just been told this, luckily i have broken into a 12v wire at the moment, if i gone into one of the others i icould have had a massive volt spike fired into the ECU!!!

This is proving to be a real pain in the ass!!!! So close yet so far!!

turbodisplay - 17/12/09 at 11:09 AM

Most competent designs will be okay to prevent damage.
But the question is will the ecu keep on reseting due to the noise.
A filter is simple, a 1k resisor, with a pair of series 12v zender diodes back to back,
connected between ground and the ecu input will limit peak voltage to 12.6v.
Darren

MK9R - 17/12/09 at 11:14 AM

quote:
Originally posted by turbodisplay
Most competent designs will be okay to prevent damage.
But the question is will the ecu keep on reseting due to the noise.
A filter is simple, a 1k resisor, with a pair of series 12v zender diodes back to back,
connected between ground and the ecu input will limit peak voltage to 12.6v.
Darren

spoke to reseller and told to connect it to one of the other wires from coil and should be fine. If it screws it up, i can at least tell them they told me to do it!

MikeRJ - 17/12/09 at 12:47 PM

quote:
Originally posted by turbodisplay
Most competent designs will be okay to prevent damage.
But the question is will the ecu keep on reseting due to the noise.
A filter is simple, a 1k resisor, with a pair of series 12v zender diodes back to back,
connected between ground and the ecu input will limit peak voltage to 12.6v.
Darren


1k is a bit on the low side, primary voltage on the coil could be as high as 300v; a 1k resistor would be enough to actually reduce spark energy.

Dusty - 17/12/09 at 12:51 PM

The coils are switched on their earthing side by the ecu. This was used in the old days for the tacho signal when dizzy points were doing the switching which must have been crude but was effective. Can you really get that big a spike 100v on the earth side? If so how does the ecu which is doing the switching survive under these conditions.
Presumably the primary petrol ecu has a tacho out signal. Can you split that, insert some diodes to isolate the outputs and feed one to the gas ecu and one to the tacho?

MK9R - 17/12/09 at 12:57 PM

quote:
Originally posted by Dusty
The coils are switched on their earthing side by the ecu. This was used in the old days for the tacho signal when dizzy points were doing the switching which must have been crude but was effective. Can you really get that big a spike 100v on the earth side? If so how does the ecu which is doing the switching survive under these conditions.
Presumably the primary petrol ecu has a tacho out signal. Can you split that, insert some diodes to isolate the outputs and feed one to the gas ecu and one to the tacho?


Ideally i'd like to break into the tacho signal from the ecu, but i can't find out which pin it is, even though i have a full wiring diagram!!

Dusty - 17/12/09 at 12:59 PM

You could find it at the back of the tacho, usualy coloured green in the olden days. Then trace it back.

[Edited on 17/12/09 by Dusty]

MK9R - 17/12/09 at 01:05 PM

quote:
Originally posted by Dusty
You could find it at the back of the tacho, usualy coloured green in the olden days. Then trace it back.

[Edited on 17/12/09 by Dusty]


easier said than done on a modern car, just getting to view the back of the dash is a serious amount of stripping out!!!

ashg - 17/12/09 at 02:07 PM

ring a clifford alarm installer they will know which wire is the speed puse off the ecu.

Dusty - 18/12/09 at 01:41 AM

Find it in the wiring diagram perhaps and trace it back.

MikeRJ - 18/12/09 at 12:39 PM

quote:
Originally posted by Dusty
Can you really get that big a spike 100v on the earth side? If so how does the ecu which is doing the switching survive under these conditions.


Easily, typically it will be somewhat higher than this under normal operation. The coil is a essentially a transformer, turns ratio is of the order of 100:1 (varies between different coils). Thus a 30kV HT voltage requires a primary voltage of 30000/100 = 300v.

The ECU survives this by having special high voltage, high current drivers with voltage clamps to prevent the voltage rising too far. This is why it's a bad idea to pull off HT leads on a modern engine whilst it's running; without a suitable spark gap the HT voltage rises considerably, which also causes an increase in primary voltage. This stresses both the insulation on the HT side and the protection circuitry on the primary which has to absorb a lot of energy to clamp the voltage.