bigbronco

I have a 1985 porsche 944 that i bought two weeks ago. I have had no problems with the car until yesterday. Driving to work, it was running perfect. After work, when i went to start it, it would turn over but not start. Upon getting it home, I checked for fuel and spark and had no spark. With a voltometer, I checked the positive side of the coil to the negative side of the battery and had 12 volts. I then checked the negative side of the coil and the positive side of the battery and had nothing. I assumed the coil was bad so I removed it. Between the two sides of the coil I had 1.0 volts resistance. On the high side I had 5.8 volts resistance. Out of curiousity, I checked the two coil wires while they were disconnected from the coil and had 12 volts between them.

Long story short...when the negative wire touches the negative terminal of the coil, I lose my voltage on the voltometer. When it's not touching it, I have 12 volts...???

Does this mean a bad coil?

Thanks,
Michael

Sorry about the double post, I initially put this in the wrong forum.

Chris_924s

I would suspect the coil. I was running one day and it quit.. had spark, fuel and the coil seemd to pass all the tests. It's cheap, replace it and rule it out. Make sure the rotor and cap are OK. the rotor can loose it's set screw. You'll need a shortie screwdriver to get to the bottom screw on the cap (I modified a shortie blade screwdriver so It's 2" long- you'll need a large blade to turn the screws on the cap.

Welcome to Rennlist!!.

Rock

I think a sensor is fuxxored. Try jiggling the plugs and give it a go to see if it starts, or check to see if theyre still even plugged in!. If they look good, then Try testing for .999k resistance

http://www.clarks-garage.com/shop-manual/ign-02.htm

(the plugs are going to be right on that bracket, 2 of them, rectangular shaped)

Chris_924s

True- the speed/ref sensors wouldnt trigger a spark thru the DME.. make sure thay havent wiggled loose.

ROCK- 'sup with the black/yellow cam tower there hoodie?

Rock

Youre not going to have crizzap on the negatizzive (green wire) because when the car is off the DME grounds that out. It will read 0 on anybodys cizzar if im not mistaking. Ive heard some people say Coils are like tanks and very rarely go out. I had a whole episode a while ago and i dont trust the speed and reference sensors anymore...

The speed and reference sensors read the flywheel for the DME. If they DME senses the engine movement above 200rpms, then it will give the car the good old sparky. If one of those goes out then the DME is blind and doesnt see anything so it doesnt know when to sparkity spark. Couldnt hurt to check them.

Dont listen to me though, because man, im just a kid

Rock

Methinks it looks neato

bigbronco

Oh! One thing I forgot to mention was when I tried to hot wire the coil (neg bat to neg coil, pos bat to pos coil), it sparks like I'm shorting out the battery. Is this correct?

When I was testing for power, I always had the ignition key on.

Rock: I'll try that tomorrow and see if the car reacts to it.

Thinking back, I can think of only one problem with the way the car ran. I blamed it on altitude difference between its old home(California) and here(Oklahoma). It was actually getting a little worse as time passed. When shifting and getting back on throttle, it would hesitate like a carb might do(too small of squirters).

I am used to carbs so I'm a little overwhelmed with this already. I greatly appreciate the help on this.

Have a good day!
Michael

Rock

DOOD! You could have fried your DME doing that! I think...

Chris_924s

Michael, use the tried and true methods. http://www.clarks-garage.com/
http://www.clarks-garage.com/shop-manual/ts-01.htm

Zero10

If you connect the coil + to your battery +, and - to -, you will get a BIG spark, under DC power, it is essentially a dead short. Very bad for the coil if you keep doing that. The coil has a high impedance, which you can't measure with most multimeters. Long story short, it limits the rate of change of current passing through it, so when you pulse the power to it, it won't allow a very large current to pass through the coil, thereby roasting it.

Coils are like tanks, unless they get water in them, or are hit, or something else physical, they really should run forever. They are just a transformer.

Does the tach wiggle when you turn it over? No wiggle = speed/referencce sensors

bigbronco

Ok, here is what I've found out so far:

1) the coil is not the problem, I replaced it with a new one and nothing changed.

2) the green wire that goes to the coil is supposed to be positive when the key is on...it is negative. What would cause this? I think it's a safe bet to say that this is my problem. I just don't know what ot make of it.

3) I checked the connections at the back of the motor and they were fine.

Thanks for the help,
Michael Dunn

Rock

Naw man the green goes on the negative. the black is what goes on the positive.

Rock

i would check the resistances of those wires in the back. mine looked good but were sure toast

bigbronco

I understand that the green wire goes on the negative of the coil, but according to the Chris_924 sent me...a voltometer should read 12 volts between the green wire and ground.

Rock

the green wire and the ground are both grounds methinks

The green wire is for the DME to ground the coil out, and the ground is a ground man. I dont think current will flow there at all, but im just a kid.