72 911 T all the sudden no spark

[traderdan]

Went to run check measure and adjust but car wouldn't start no spark. the car is out of a wrecking yard that we've been restoring so all electrical is suspect - started up ok the night before. Permatune never really made a noise before so it's hard to tell if it's working or not. No spark out of the coil wire.

Which wires should I check for power, I assume one wire provides 12V to the CDS and there should also be a hot wire going to the coil will it hut if I at least try these with a 12v test light? It wouldn't hurt the CDS if I did so that way I at least knew 'some' power was making it through. If it ends up being the permatune who rebuilds them???

Thanks in advance to any help you can give.

[porchedan911]

Maybe this wil be of some help.. My car-78 911 sc with similar problem. First, pull cdi box out and test it in another car if possible. That's what I did to determine if it was bad. Mine was. I also had a similar problem unrelated to the cdi. There is a serviceable plug connected to the back of the ignition switch. The plug was faulty and after replacement car started fine. This problem, however, took a very long time to figure out. Good Luck!

[Jim Sims]

I assume the engine turned over. Check the ignition points (dwell) and timing before tinkering with the CDI. If you don't know what you're doing you can "smoke" the CDI. Jim

[Early_S_Man]

Dan,

Since this isn't an emergency roadside repair ... I suggest you remove both CDI unit and coil for cleaning and bench testing.

There are three marked terminals at the CDI 3-pin connector ... wipe dirt/dust off the top of black phenolic connector ... A, B, & C, plus unmarked ground lug.

With box upright, looking into connector, left to right ... A on left, C on right ...

A -- output to terminal '15' or 'A' of coil

B -- +12 Volt power

C -- trigger signal from points

D -- unmarked twin ground lug

First, remove ground lug from case, and clean case with stainless steel toothbrush. Clean lug [twin .25" Faston connector] with ink eraser, stainless brush, or both.

You will need a spark plug cable to plug into coil and a new spark plug to use as a load -- under no circumstances operate the CDI unit and coil without a proper sparkgap load on the coil ... the spark plug should be securely grounded at its' case for all testing!!!

Test jig wiring should be new, all connectors crimped securely or soldered. A momentary push button switch should be used to simulate the points closing and opening to ground. Either 14 ga or 16 ga wire can be used ... the CDI unit isn't supposed to draw more than 1.9 Amps at 12 Volts, per Bosch and Porsche specs.

When power is supplied to the CDI unit, it should audibly whistle ... an indication that the DC-to-DC converter circuit is working! No whisle usually means that a wire is broken or that the 2N3055 power transistor needs to be replaced -- about $1 to $4 from various sources.

When wiring is complete and spark plug load is connected to coil ... one momentary push and release of the switch should produce a healthy blue-white spark at the test spark plug!

The following diagram can be copied to your computer and printed for reference ...

[traderdan]

ok I'll give those suggestions a try, I just recently replaced the electrical portion of the ignition switch and the pin connectors in the dash because I had problems with power not switching on at that point. Right after I did that everything worked and the car ran (ran as good as you might expect from a porsche that sat in a salvage yard for 20 years lol) Is it possible I still might have problems related to the ignition switch? Can I check for 12V power getting to the cdi would that be where to start? It looks like it's the red wire going to fuse 8 on the block according to the diagram Warren supplied.

[Early_S_Man]

Dan,

You can check to see if 12 Volt power is getting to CDI connector by pulling it off and check at center terminal -- the red 'B' wire.

[traderdan]

Awesome, thanks Warren you are the man. I was browsing through Wayne's 101 projects book in the section that talks about the MSD ignition replacement for the Bosche CDI or permatune.

I don't know if this pertains to any of my problems (and I'm only now getting familiar with Porsche and CDI's etc) but I ran onto the following passage:

"Other Porsches such as the 356, 912, and 914 use a standard inductive ignition system that cannot match the performance of the capacitive discharge design. On these older inductive systems, the coil must step up the voltage and store the electrical charge at maximum strength between each firing. When the rpm of the engine increases, the coil doesn't have enough time in between firing to ramp up to it's maximum voltage. This results in a weaker spark and a loss of power at higher rpm."

I know this much be reaching but I'm curious if it relates to another problem I posted about. When the Porsche is running (doesn't idle by the way at least not for long) it's timing seems fairly smooth until it starts getting in the higher rpm bands. After reading this in the book I was curious as to weither the permatune might have been malfunctioning somehow and perhaps just finally failed. Now keep in mind though I haven't had a chance to check the adjustments on the MFI or the timing on the distributor. When we had the engine out and unsiezed it of goo and grime we replaced the distributor with a known good used one, the old one was shot and static timed it along with setting the points gap. I also adjusted the valves to the spec's given in the Haynes manual and installed a new MFI belt and set the timing marks as indicated in one of Waynes books I believe (might have gotten the info on the forums too).

From what I understand permatunes have two functions, working and not working. The latter function is very frustrating ^_^. But just in case do you think it is possible that this could be causing my higher rpm fluctuations?

[Lorenfb]

If the Bosch CDI doesn't buzz, the following could be the internal problem
of the CDI (assuming the unit has a good +12 volts & ground);

1. the inverter transformer is bad
2. the inverter transistor is bad (not a 2N3055 as it lacks adequate SOA)
3. hign voltage diodes bad
4. input 22 volt zener was overvoltaged the result of bad alt. regulator
5. bad solder connections for inverter resistors
6. SCR shorted
7. capacitor shorted

Check out the Diagnostics page at this web site, www.systemsc.com.

[Early_S_Man]

Dan,

Permatune is another problem scenario ... and they can be intermittent, as reported by more than one Pelicanhead! Was it hot (above 80°F ) when the cutting out occured ... the intermittent Permatunes that have been reported all seemed to be hot-weather related???

Is it a blue one, or a newer aluminum finish version?

I have an intermittent silver Permatune [sent to me by a Pelicanhead] that only fails in hot weather, or under heat lamps for several hours ... haven't tried to dissolve the potting compound an determine what the problem is. Nice paperweight!

[techweenie]

Any number of things can cause your problem.

The first place I'd check is your points. If you've been leaving the ignition on for testing, they could've fused. And even if you haven't they could go out of adjustment pretty fast after sitting so long.

[traderdan]

The permatube came with the car, it has an aluminum case I'm guessing it's pretty ancient. The car has been run at several temperatures ranging from 45 degrees to 85 degrees ambient temp, sometimes it was run for a few minutes sometimes half and hour while we toyed with it trying to get it to run better (using expert advice found here of course). Also the car does seem to take a bit to get started (when it did spark) I was guessing it had to build up fuel pressure - once it did it would barely go 1500 rpm for the first moments but then you could run it up to 4000 but it would start cutting out at 2500 progressively missing more as it accelerated. When it is warmed up it actually sounds pretty decent at around 1000 to 2000 rpm -- also if it's warm and shut it off then immediately restart it starts up just fine usually. I was curious if perhaps it wasn't sparking right maybe the permatube couldn't catch up as rpm increased -- could be worn out. The car hasn't been left in the on position for very long while we've worked on it only for breif moments but it certainly wouldn't hurt to check the points again. I've been pretty busy with work so I'm guessing I won't be able to test for the 12V to the cds till monday unless I get time tomorrow evening.

[Lorenfb]

Blue Permatune failures:

1. overvoltage by bad alt. regulator
2. SCR shorted because ground/+12 volts applied to coil
3. battery jumped backwards

Silver Permatune failures:

1. all the above
2. highly intermittent at high temp
3. total failure because of consistent #2

Check points signal to CDI by using a test light connected to
+12 volts (checks for ground) while cranking (light will flash).
If points signal, +12 volts input, & ground are O.K., then CDI
is bad. Coil failures are rare, except the silver Bosch ones.

[traderdan]

Just checked for power going to the cds by turning the key on (hear fuel pump running) and using a 12v test light at the center red wire of the cds and nothing no power. Where should I check now? oh by the way the connectors on the cds were extremely loose so I'm gonna pull it anyway and open it up to take a look inside. Wish I knew how to post pics.

[Early_S_Man]

Dan,

Silver Permatunes are potted. Good luck ... if you find a broken wire! Be sure to clean off the ground lug tabs above the 3-pin connector ... remove screw and wire brush thoroughly!

Otherwise ... the individual 0.25" Faston connectors (all four, including braided ground cable) can be tightened up by squeezing slightly on each side ... so they grip the tab tighter. Clean with 9% pickling vinegar and old toothbrush, then rinse with distilled water. Blow dry with compressed air, and finally 90% isopropyl alcohol, blow off with air again.

[no substitute]

My internal coil Permatune would suddenly shut down the entire vehicle. Boom, as John Madden says. According to the Permatune guy there is an internal resistor or capacitor, I forget which, I believe the former, which will shut down when it gets too hot. It will then reset itself and will run again--for awhile. Rinse and repeat, meaning when it resets, it resets at whatever value it "chooses" and will shut down again when it overloads. Eventually mine died permanently (hence the term?). I plugged in the twenty some year old OEM Bosch unit and it still has never failed after twenty-five years of use. Thank you Bobby B. for electrical engineering very well done. BTW, my car passed ALL the ground tests recommended by Permatune. Permatune says their unit produces no heat, but mine was always hot to the touch when it failed. All failures were in hot weather, which means eighties plus for those of us in the upper left corner. I understand some folks, I believe Warren for one, have run Permatunes without problems for lengthy periods. Me, I'm buying Bosch. Of course I may not need one for quite a few more years, but I'm okay with that. YMMV.

[traderdan]

yeah I'm hoping that it's something that I can fix in the permatune otherwise I'll go with the MSD. I was just thinking that I tested for power to the red wire while the 3 pin connector was unplugged. Should I havetested for power while it was plugged in still does it need to complete a circuit in order to draw 12V??

[Early_S_Man]

No, Dan, the power should have been there if ignition switch is on!

Going backwards towards the ignition switch ... there are connections to check at:

1. a single-wire disconnect under the relay/regulator/CDI panel,

2. at the 3-fuse block ... bus connection at 1 - 2 'hot' side

3. forward 14-pin connector

4. fuse block #1 by battery ... bus connection 7 - 8 on the diagram posted above ...

5. ignition switch

It is sounding more like you have an intermittent connection somewhere, and that the Permatune box may not be the problem, yet!

[traderdan]

well I was having ignition switch problems before, I took the switch apart and repaired the connections and reinstalled and the car ran fine. The very next day I ran into the problem of the CDS not getting power wonder if the ignition switch problem isn't fixed yet.

[traderdan]

On a side not I found a used Petronix CDS on eBay for 70.00 I'm the high bidder though I suspect snipers abound.

[traderdan]

Well good news and bad news.... first thing Warren you save the day again. I checked the fuse panel in the battery compartment fuse was ok but there was damage to the fuse panel causing an intermittent connection which I have repaired. Now full power is restored to the cds but it still wont fire. Also there is no sound from the cds so I suspect it's that flimsy 3 pin connector I metioned before in the cds itself preventing it from getting power. If I cannot repair it myself then it would seem I'll be replacing that ignition box sooner than I thought.