ign coil?

usaporche · 11-03-2009, 06:03 PM

my 79 911 wont start, changed fuel pump,all fuses, relays good, turns over but it wont start , checked coil posts, i get two positves on my probe, should i not get a negative side and a positve side? help i need to make my porsche alive again!

tmctguer · tmctguer's Garage

are you getting spark to your spark plugs? pull a plug wire and use a spark tester, or pull the whole plug out of the head and SAFELY check for spark when the engine turns over.

that might help point you in the right direction.

docrodg · 11-04-2009, 03:15 AM

You will get 2 positives on the coil with a probe. You need a multimeter to check it, or just use a spark plug as suggested.

ossiblue · 11-04-2009, 06:35 AM

If you changed your fuel pump, did you bleed your fuel system? Are you sure it's a spark issue and not a fuel issue?

T77911S · 11-04-2009, 08:59 AM

i am not sure what you mean by 2 positives.

but, you should not get anything, either with a test probe or a multimeter, on the neg side. that side is grounded. the only way you will see voltage, or a test light will light, is if the ground is not making contact. in fact, i would not think there is enough resistance in the coil to show voltage on the pos side, at least not 12v. i am not that familiar with the 78 ignition.

jjrowe · 11-04-2009, 12:11 PM

coils are rarely bad. You need to figure out what the real issue is before you spend any more $. Are you sure you have fuel? To test the coil do a resistance test across the terminals with a multimeter, if there is no resistance (direct short) then your coil is bad. Again this is very rare. Was the car running fine and then just stopped or was there an action that caused this issue?

steely · 11-04-2009, 03:01 PM

dumb question - did it run before you changed the fuel pump?

T77911S · 11-05-2009, 10:37 AM

Quote:

Originally Posted by jjrowe

coils are rarely bad. You need to figure out what the real issue is before you spend any more $. Are you sure you have fuel? To test the coil do a resistance test across the terminals with a multimeter, if there is no resistance (direct short) then your coil is bad. Again this is very rare. Was the car running fine and then just stopped or was there an action that caused this issue?

normally i would agree with that statement. and definately any other car i would. but it seems like the porsche coils do go bad quite often. mine included. perhaps it is the high voltage that is applied to the coil, maybe too many people run too big a plug gap and strains it more.

even at my job as an electronics tech i would doubt coils going bad. they are usually the last thing, except in high voltage circuits.

i had a coil go bad in the crossoever on the horn side of one of my PA speakers. i would have put money that it would never go bad, but it did.

steely · 11-05-2009, 05:24 PM

I suspect his problem isn't with his coil, and not for technical reasons either.

glewis80SC · glewis80SC's Garage

I would check the fuel too, however I had two brand new coils go bad on me, Bosch but made in Brazil. Could it be the famous green ignition wire from the distributor. With me when the coil went bad I tried the "spark" test and got nothing. Does the 78 have a CD Box, is it humming?

T77911S · 11-06-2009, 06:36 AM

Quote:

Originally Posted by steely

I suspect his problem isn't with his coil, and not for technical reasons either.

me neither. more info is needed

ossiblue · 11-06-2009, 06:38 AM

If you haven't done a simple test for spark--connecting a plug to one of your wires from the distributor, grounding it and cranking the engine to see a spark--please do so and let us know your results. I am not convinced from your brief post that you have an ignition problem, given the work you did before the problem developed. If you, in fact, have no spark, please post that information.