79_911_SC_Targa

My '79 SC sat for a few years in the garage before I decided to revive it. It was running, just not running well. I like to work on my 356, but the SC is more complex, of course, so I trailered it to a shop about an hour drive from home. After a few months and $4500, got a ride to the shop and drove it home. It seemed to be running well, though the idle was little high --12-1400. There was an accident on the interstate, so it was stop & go traffic, mostly stopped. The gas gauge showed more than a 1/4 tank. The car cut off suddenly. Would crank and run for a second or 2, then cut off again, as if it was out of gas. I rolled it off to the side, went to get some gas and added 4 gallons. It took a lot of cranking, but each time it would run a little better but with lots of spitting and backfiring, and finally it ran well enough to get to a gas station with one final very loud backfire comming off the exit ramp. It ran well then next 50 miles, getting up to 90mph sometimes. Then when I got just about home, it started cutting up pretty badly and I barely made it to my parking garage. I parked, but then couldn't start it again. Next day, same thing, it would turn over quickly, but not fire on any cylinder. Next day after that, I cranked it and it started immediately, within a fraction of a second of turning the key, ran smoothly for a minute, then cut off again. Since then, it hasn't started yet, won't even fire on one cylinder.

More info... the mechanic suggested that since it seemed to run out of gas, there must have been dirt/gunk in the tank that got stirred up and clogged up the system. I tested this by removing the lines from the injectors (I couldn't pull out the injectors at all, so that was easier), turned the key on, and lifted the mechanism in the breather box. The fuel flowed easily and evenly into the 6 jars I set up, and there was no visible gunk in the fuel. I finally pried 1 injector out and tested it the same way, just for a couple of seconds, and it sprayed strongly.


Your thoughts and suggestions would be greatly appreciated!!!

Mitch in Asheville, NC

theiceman

sounds liek your throttle wire is your cruise control, ( i cant see pics on this device ) so that is unrelated.

I think really after that long and that much money , they should be sending a tow truck over to get it and fix it right... sounds like you might be losing spark so CDI or even the dreaded coil if it one of these sliver ones..

should be pretty easy to tell if your not getting spark. after that its CIS. do you have a pop off valve ? maybe check to see if that is seated properly and not dislodged...

a few things to start with

79_911_SC_Targa

Yes, that WAS my cruise control wire, duh, after some research I figured that out, so I edited my original post to remove that picture and text. That broken cable seems to be a common problem.

I don't care about cruise control at this point. What's the best way to tell if you have spark? Hold the distributor wire out a little and watch for spark? The pop off valve is seated ok.

Regarding the shop, I guess I can't hold them responsible unless I know that is was something they replaced that has gone bad already. Otherwise it could be some other item that was working but just coincidentally failed that would have failed anyway. Won't know that until I figure out the problem.

Thanks!

theiceman

pull the wire from the coil to the distributor at the distributor end, put it close to a ground and turn it over. you will know pretty instantly .. but of course that has to be when the car wont start... you will hear it cracking, better yet, have an assistant turn the car over... but we careful that's a lot of juice coning out of that coil

to be safer get an old plug and stick it on one of your leads and ground it. same routine, have someone crank it over.

hard to believe they had it that long and now you think its something unrelated though..
is your coil sliver ?

if they replaced it with a silver one that's likely your issue , they are terrible and last anywhere from 1 mile to 100 miles typically.

79_911_SC_Targa

The coil is black, but looks might old, maybe original. Cables look brand new. I think I have an old plug I can use, or if not I think I have insulated wire holder. Same as the old cars, didn't knwo if it'd be different with these new fangled CIS systems...
Thanks!!!

79_911_SC_Targa

There doesn't seem to be juice coming from the coil. How do you test a coil other than just buy a new one? This one looks mighty old, not opposed to replacing it anyway, but would like to know if that's the problem. Otherwise, it's probably the CDI box? Any way to test that? I see that you mostly have to send in your box to get rebuilt? Any favorite sources anybody recommends for coils and CDI boxes? Avoid the silver ones, I understand.

systemsc

1. With key in the run position, do you hear any buzzing sound from the CDI unit? If so, this would indicative that the unit has power (12V from the ignition switch).
In some cases it's not possible to hear the buzzing and the unit is OK.
2. If no buzzing and no spark, check the middle pin (bottom side of unit - no fins) for 12V.
3. Check the two pins where the green wire attaches to the CDI connector;
a. for an ohms reading of 600 ohms
b. for an A/C voltage reading of about .60 volts when cranking
4. The original black coils rarely fail.

The CDI can sent in for testing. https://www.systemsc.com/products.htm

79_911_SC_Targa

Yes, with the key in the 'on' position, i do hear the high pitched continuous tone. I realize that doesn't mean that the CDI is necessarily good. If the black coil rarely fails, then it's most likely the CDI unit. We'll see.

systemsc

Additional tests when evaluating the functionality of the CDI unit:

1. With the key in the off position;
a. remove both wires to the coil
b. using a VOM on the ohms scale measure each coil wire to ground (coil bracket)
c. one wire should indicate zero ohms, while the other should indicate an open
d. redo this test by reversing the test leads
e. if either test doesn't indicate a high resistance on one wire, the CDI is bad.

2. Use the VOM in VAC mode on the 100V scale;
a. with the negative meter lead grounded, connect the other lead to non-ground lead from test 1 above
b. turn the key on
c. the voltage should momentarily read about 40 VAC.
d. if there's no voltage (change) appears, the CDI is likely bad.

3. For better accuracy in test 2, repeat test 2 with the meter setup for recording min/max values

79_911_SC_Targa

I'm still trying to figure out how to use my new VOM (AstroAI DM6000AR), but in the first test, one wire was definitely 0, and the other one showed numbers all over the place, not just a single #. I'll have to study up on this VOM, I'm new to those in general.

Meanwhile, as I removed the wires to the coil, I noticed a solid brown wire that wasn't connected to anything. I see a brown wire in the CDI connecting 6 pin plug... what does this wire do and where should it connect at the coil end? See pic of wires.

So today, I went to try the tests above suggested by systemsc, and thought I'd drop in a new coil -- figured worst case, even if this problem is not the coil, i'd have a spare in the trunk. First, before doing anything, I tried cranking it just for fun. It turned over quickly and right before I gave up, it actually fired on a cylinder. I tried again, and it fired on a couple more but wouldn't really start. So I went ahead and replaced the coil and tried to crank it again... After a good bit of turning over, it fired a few times, then actually sputtered a little, tried to crank, and evenutally actually started running, albeit very badly. It kept going, however, and finally warmed up and ran strong and smooth! Aha... it was the coil (I thought). So I cut the engine off, tighted down the coil bracket, closed the lid and was going for a drive. Went to crank... nothing... no firing cylinders at all. Back to square 1. So now, seems it must be the CDI unit, or the wiring in between. Maybe it's not unusual for the CDI to run sporadically, sometimes runs, then doesn't?






Ignition coil wires from '79 911 SC. The solid brown was not connected to anything... where does it go?

systemsc

Not really.

79_911_SC_Targa

Thanks again Sys! You're pointing to the nut holding the coil bracket down, right? What does that wire do... just a ground? if it's not connected, will the car run? If not, then I guess by chance it is sometimes touching a ground and sometimes not... can't be that lucky for this to be my problem, though?

systemsc

Correct

Correct

79_911_SC_Targa

Problem solved, car is running great. I went ahead and just ordered a new CDI+ from Pelican, installed right out of the box with no timing adjustments. Car runs again at least as well as it did before. I'll have my old CDI rebuilt to keep in the trunk.

So it seems that the CDI can indeed work/fail intermittently. My biggest concern now is wondering if there is something else that caused the original CDI to fail, and if so, can it damage the new one.

thanks for all the dialog everyone.

systemsc

Great, and good luck!