'83 911sc was running again...DEBUG UPDATE

[springerman55]

The car still will not crank (duh), so here are the results from tonight's further testing. To the new viewers, '83 SC with new fuel pump. She ran briefly and would not start the next day. The ignition system is dead. All stock ignition with the exception of a Permatune (doom) CDI unit. Below are the measurements:

*Battery voltage is 12.33V.
*Voltage between pins 15 and 31/1 of the CDI harness (PWR/GND) measured 12.23V with ignition key ON.
*Voltage between pins 7 and 31d of the CDI harness (pulse input/pulse shield) measured 1.36VAC with the ignition key in the cranking position.
*Resistance between pins 7 and 31d of the CDI harness (pulse input/pulse shield) measured 622 ohms (Permatune states the reading should be 600 ohms +/-100 ohms.)
*Resistance between pin 31/1 and the Permatune mounting hole (GND/GND) measured 0.3 ohms (Permatune states the reading should be 0 ohms +0.2 ohms/-0 ohms.)
*Resistance between pin 31d and the Permatune mounting hole (GND/GND) measured 0.3 ohms (Permatune states the reading should be 0 ohms +0.2 ohms/-0 ohms.)
*Resistance between pins 31/1 and A (GND/PWR) measured 3,256 ohms (Permatune states the reading should be 3,300 ohms +/-100 ohms.)
*Resistance between pins TD and 15 (TACH/PWR) measured 0.5 ohms (Permatune states the reading should be 0 ohms +1 ohm/-0 ohms.)
*Capacitance between pins 31/1 and 15 (GND/PWR) measured 200.4uf (Permatune states the reading should be between 220uf and 250uf, nothing else).

So the only real thing "out of spec" is the capacitance and I'm not sure if the 1.36VAC is sufficient enough voltage out of the distributor pulse generator. The only thing I have not checked is the ignition coil. Not sure exactly what to check there. Any opinions because I do not see any clearcut answers/solutions?

[techman1]

While someone checks battery voltage, crank the car. What does the voltage drop to?

[springerman55]

Techman1,

The voltage drops down to 10.75V during cranking. I put everything back together last night and pulled the coil wire from the distributor cap and isolated the tip of the coil wire very close the the coil mounting bracket clamp nut. There was bright blue spark during cranking. I re-installed the fuel pump relay and cranked again. This time the engine fired right up. It ran great for about one minute and then the engine died. It would not crank again. I pulled the coil wire from the distributor cap once again and tested for spark. No ignition spark. I have come to the conclusion that I have an intermittent 12V supply to the Permatune unit (maybe a marginal/old fuse) or one of the components in the Permadoom unit is being stressed while the engine runs briefly then the component fails. I am leaning towards the Permadoom needing to be replaced.

[turborat]

Before you change out expensive parts, completely check all of the termination and interconnect points of the wiring and the condition of the wiring itself. My guess based on my experiences in this area of these cars and the fact that the ignition in your car will intermittently operate correctly is that there is an intermittent fault with wiring to/from distributor or ignition coil. Check the end-to-end resistance of the wires while flexing the appropriate harness. The wire ends where soldered to terminals can get very brittle.

[springerman55]

I am somewhat puzzled but the SC is running again. I went out this evening and verified the 12V voltage to fuse #22 and also the CDI harness power supply pin to the box. All checks were good. I pulled the distributor cap and rotor. The rotor tip was pretty dull and corroded so I took an emory board to the tip and lightly cleaned it up. I also did the same thing to the individual cylinder contacts but that was harder to do since they are harder to get to. When I got done, I blew out the cap with my air hose and put it all back together. I made sure that all of the plug wires and distributor coil wire were tightly inserted and intact. I checked for spark at the coil wire to GND with the wire pulled out of the cap and there was spark. I put the coil wire back into the cap and she cranked right up! I let the engine idle for about 15 minutes with an occasional rev. All was fine. I turned the engine off and re-started the engine. It fired right back up and idled smoothly. So now I will check to see if she will start again tomorrow after sitting overnight and see what happens. If she cranks tomorrow, then I will drive her around the neighborhood. I will have to time that just right so a family member can come get me in my Super Duty truck to tow the SC back to the house if anything goes wrong. It is running a bit rich but after sitting a while and all of the cranking that's been going on, she needs a good "blowing out". I had the fuel pump relay out and the cold start valve unplugged during most of the tests but I'm sure more fuel got into engine than needed to be. Does anyone believe that a partially corroded rotor could have caused this intermittent problem(s)? Are older 911 rotors susceptible to this issue? Maybe she's just pissed because she sat so long during my surgery ordeal...

[JJ 911SC]

Originally Posted by springerman55

... Maybe she's just pissed because she sat so long during my surgery ordeal...

Bingo... and probably the cleaning of the rotor and cap may have help

Don't worry about the tow truck. Go out and take her to operating temperature for a while.

[mholbrook38]

And put a new cap and rotor on the "to do now" list.

Put a few ounces of Marvel Mystery Oil in the fuel. Maybe 12 for a full tank. Do that for a couple tanks of fuel. Will really help clean out CIS fuel injection. Also avoid gasohol if at all possible.