78sc died while driving no spark

outhway · 02-06-2005, 01:41 PM

Long time reader first time writer. While driving home last night my 78sc just died. Like someone turned the key off. No warning at all. I had it towed home and have performed the following test.

Swapped CDI prematume with original Bosch...nothing
Checked power to the CDI....ok...Bosch whines
Checked coil to specs....ok
Checked green wire and lead....ok
Checked cap and rotor.......ok
Reseated 14 pin connector....nothing
Checked tranny ground.....ok
Checked/ rotated all fuses....ok
Rebuilt distributor lees that a year old.
Not to sure what the relay at the left of the engine is but it was cracked and broken so I swapped it with my power window relay with no results
Fuel is not and issue as with the key on, an injector pulled and pressing up on the fuel dist bar I get a nice spray.
I am runnig out of things that I know of to check.
All of your guys offer great advice and I hope someone has an idea.

AFJuvat · 08:43 PM

Check your coil - if you have a brownish goo around the top (where the wires connect) your coil is shot.

One other thing to check is your dizzy rotor again - make sure that the rev limiter isn't shorting to ground anywhere.

AFJuvat

Early_S_Man · 02-06-2005, 07:05 PM

Did you test the magnetic pickup coil in the distributor??? It should be 600 Ohms +/- 10% at terminals #7 and #31d in the connector for the CDI unit.

Here is a schematic that may help ...

ruf-porsche · ruf-porsche's Garage

Does your car have the factory alarm system or an after market alarm? Check to make sure it is not grounding the ignition.

outhway · 02-09-2005, 04:07 PM

Thanks guys! I will try all of them. The car does have an aftermarket alarm that is disconnected but I will check farther. I did not check the mag pickup as I did not know how......but now I do. I will recheck the rev limiter wire again as that was done in haste as I was VERY frustrated.....but hey 3 days in Tahoe have fixed that and I will beat the machine. Again THANK YOU.

mede8er · mede8er's Garage

Check your point gap........

Lorenfb · 07:15 PM

The diagnosis is very simple:

1. CDI buzzs, so it gets power, test #1 passes
2. pull coil wire from dist. & place 10 - 15mm from engine case
& crank, still no spark indicates four possibilities;
a. bad dist. pickup or bad green wire from dist.
b. bad coil, hopefully not the dreaded Bosch silver coil
c. bad wiring to coil or coil missing a ground
d. bad CDI
3. to check for "a" use a voltmeter set to A.C. volts & check for 1.0VAC while cranking
pins 7 & 31 on the CDI connector
4. to check for "b" use a test coil like a VW bug coil (most any coil will work)
5. if "a" & "b" & "c" check O.K., then the CDI is bad. Get a test CDI to verify
its' being bad.

Steve Marshall · 02-09-2005, 07:55 PM

I had the same thing happen. Twice. First time was with my 68 911. I could refire after about half an hour and then within a lap or two it would misfire and then die. I would test the coil and it appeared fine. However this was after it was towed to the paddock so some time would elapse. After consulting people smarter than I, I replaced the coil and that cured it. In this instance it would run crappy before it died. IE failing coil, but not totally failed.

The second time was with my 74 Carrera. It just died. DEAD. I was towed in and parked it for the day. The next morning she fired right up. I ran all morning, until the exact time of day, and on the exact spot on the track where it died the day before. It died again. DEAD. Just quit running. After consulting folks much smarter than I, I replaced the CD box. It's been running great ever since. Knock on wood.

outhway · 02-11-2005, 03:30 PM

It was the dreaded Bosch silver coil. The nasty brown goo was present but was not noticed as it was dark. A replacement was obtained and she now purrs like a kitten. The question I have is why the old coil and the new coil read ohms the same when checking them? If if was not for you guys I would never have looked at the coil again. Thanks again.

Lorenfb · 02-11-2005, 07:53 PM

"The question I have is why the old coil and the new coil read ohms the same when checking them?"

As I posted before, most coils don't short/open and usually check O.K. when
measuring resistance. Most coils breakdown when high voltage from the CDI
is applied, which CAN'T usually be determined from a resistance check.