Am I supposed to have 12 volts....

[944freak]

on both sides of the coil or on just one? I read somewhere in an old thread that both sides are supposed to have 12 volts. I'm pretty sure mine only has 12 volts on the + side.

[ae1969]

with the ignition on.... voltage on both.

[944freak]

what if i only have power on one of them. I have power on the larger + conection.

[carsontc]

unplug the DME unit and check it again

if voltage on both sides...DME final output transistor is probably NG

if no voltage on the green wire still...could be a bad coil...not too often but possible

or look for a short to ground on green wire...do you have an aftermarket alarm?

btw...we are talking about the thin black wire and the green wire...not the fat black high voltage wire

[944freak]

yep, we're talkin about the thin wires. Where do the green and black wires come from, and where all do they conect?

[carsontc]

black comes from ignition switch term 15 IIRC goes to coil term with larger nut

green from pin 1 of DME (supplies a pulsing ground to the other side of coil to produce spark when engine is turning) goes to coil term with slightly smaller nut

[Moneyguy1]

Just curious....

The primary of an ignition coil is an uninterrupted coil of wire. With a ohmmeter (ignition off) the coil should show continuity.

IF the system is "between sparks" and the coil is floating, then there would be 12 volts on both. If, as in a conventional system, one end of the coil is grounded, then yes....there would be 12 volts on one pole only when the ignition is "on".

[944freak]

im confused.... if the green is a floating ground how are you supposed to get 12 volts from it?

[DannoXYZ]

"if the green is a floating ground how are you supposed to get 12 volts from it?"

Well, it's not so much floating as saturated. The current is an inductor. Current will flow into the primary coil until the induced EMF field is maxed out, then both terminals will be equal and no more current flows. When the DME grounds the green wire, this discharges the current and the EMF field collapses back into the coil. This collapse through the secondary generates the high-voltage that goes through the spark-plugs (the ground for the secondary coil is the block).

[carsontc]

Originally posted by 944freak
im confused.... if the green is a floating ground how are you supposed to get 12 volts from it?

in other words...

the coil is just a transformer with a primary winding made of copper wire, and a secondary winding also of copper wire. it's job is 'stepping up' the low input voltage (primary) side to many thousands of volts out the other side (secondary/high voltage).

current flow through the coil is interrupted...the DME making a repeating series of 'ground' voltage (0 volts) 'pulses'

as a 'pulse' is sent through the primary side, magnetic waves are produced (ElectroMagnetic Force - EMF).

then as these 'waves' move from the primary winding they cut into the secondary ‘inducing’ a current to flow in that winding as well

the small black wire supplies +12volts to one end of the copper wire on the primary winding. other other side of the primary winding is connected to the green wire.

when there is NO ground present on the green wire, the circuit is 'open'.

so as NO 'current' is flowing, the voltage on the OTHER end on this copper wire is the same...that is, the green wire should have +12 volts too as no electrical voltage is being used/lost (no 'voltage drop').

when the DME unit supplies a momentary ground to the green wire, then what Danno said...except that (per the 944 wiring schematic) the ‘ground’ for the secondary is actually shared with the primary and will get to the chassis and block through the final ‘Darlington Amp’ switching transistor in the DME unit.

aren’t you glad you asked?

[944freak]

Um yea...... ok so basically if my battery is hooked up and my key is on the on position either side of the coil should see 12 volts. If only the black side has 12 volts then i should unplug my dme and test again. then if only the black side has 12 volts replace the coil. After that if only the black side has 12 volts my dme is the problem......or the green wire is shorted out.

[Zero10]

If you want to save the cash for a new coil to rule it out, you can unhook kit, and check the resistance across the low voltage terminals. No continuity means your coil is no good. I don't know for sure what you should see, but I would imagine well under 50 ohms.

You could also see if the green wire is grounded with the DME unhooked. If it is, start checking for where it's shorted.
Might save you a few bucks if it's not the coil.

[carsontc]

zero is 100% correct ;-))

also with DME unit unplugged and green wire disconnected...check green in reference to ground by setting meter on ohms scale

connect one lead to green the other to ground...should read infinity (and beyond)

check that before replacing coil...else, what you said is also 100% correct

[DannoXYZ]

"If you want to save the cash for a new coil to rule it out, you can unhook kit, and check the resistance across the low voltage terminals. No continuity means your coil is no good. I don't know for sure what you should see, but I would imagine well under 50 ohms."

Primary coil impedance should be around 0.7 ohms. if you get aftermarket coil with less than that, you should use a ballast resistor to get it in the ballpark.

[944freak]

OK.....I was wrong and it was all my fault....... As soon as my car didn't start I put in my spare DME relay. Later on I found a loose speed or reference sensor. After tightning that and trying many other things I was stumped and posted this thread. Well today I double checked everything again.....I did have 12 volts on bot sides of the coil and everything seemed fine. I pulled out my relay and took it apart...it looked burned on the inside so i put my other relay back in and it started right up................I love my car........Thanks for all of the help guys, soon I will be an expert.

[carsontc]

Originally posted by 944freak
Thanks for all of the help guys, soon I will be an expert.

and you will help us answer questions...RIGHT ?

glad the problem is solved ! cheers Steve !