Help, No Spark!

[silver356]

Hi all, Im hopeful you guys can help me. Im working with a friend on his recently acquired 71T, original engine, that has been sitting for 2-3 years, but ran when parked (yeah, lol). We have new batteries, and after we had no spark to begin with, we switch out the old CDI and coil with ones from a running car, 1970T. The CDI is making a faint squealing sound with ignition on.

I looked over the long article from EarlySman on CDI,s and it looks like the center wire into the three prong CDI connection(B) is only giving us around 4 volts, when it looks to me like it should be getting 12v. Whats up the food chain that we can check? Is it something on that electronics panel? (sorry, dont know what thats called). Is there a voltage regulator that could be the problem? Should I switch out the whole panel from the other car? Could it be the ignition switch?

Help!
David

[dicklague]

If you only have 4 volts at the red wire in the center on the 3 pin connector, you have found your problem.

This is switched 12 volts from the ignition switch. Try jumping this connector with a known 12 vdc and see if it starts.

It could be the ignition switch contacts.

I don't know why Porsche wired it this way. the CDI draws a lot and should not be going to the switch without a relay.

There should be 12 volts all the time at the top fuse in the stack of 3 in the rear. There is a thick red wire in the bundle back there that goes straight to the starter connection. I does however go through the 14 pin connector at the back of that panel right around the corner from the 3 fuses on that chassis in the rear. this connector can get dirty as can the one at the other end of the panel. Sometimes just unplugging and re-plugging will do it.....but they should be cleaned. and use some dielectric grease.

[dicklague]

You can download the wiring diagrams from here:

Pelican Parts - Porsche 911 Parts Listings & Diagrams

This best diagram for you is part 2 for 1971........ You can see the red wire going up to fuse #8 and #7.

As I said I would try to get 12vdc to the center pin red wire and see if the car starts.

[dicklague]

Keep us posted on what works.

[E Sully]

4V is definitely a problem. Get a meter and find out where the voltage drop is occuring. You may have a pin on one of the connectors that needs cleaning. With a strong 12V the CDI will make a louder high pitched whine.
Power is supplied from the ignition switch, terminal 15. This then goes to the fuse panel by way of a R/Bk wire supplying positions 7 and 8 by way of a strap on the back of the fuse block. The Red wire on the unfused side of 8 then goes to the 14 pin connector on the front of the engine consloe, pin #13, then engine fuse block, pin 1, and the CDI. Check for 12V at each connector.
Also make sure the points are in good shape and properly set.
Make sure the batteries have about 12v when you try to start it. The voltage regulator and alternator only come into play after the engine is running. You should then have @ 13.8v.

[silver356]

Thanks, guys!. I think I understand all of that, and we will get to that ASAP. I will get back to you as soon as I have some results. It was a heavily used car in its day, and I wouldn't be surprised if the ignition switch had just gone bad as well, but Im starting at the rear panel first. At least I know now there's a direct line from the ignition to the CDI.
Thanks again,
David

[silver356]

Well, we went straight from the battery to the middle connection of the CDI, and we are getting voltage at the coil, but no spark in the distributor. Im a bit stumped, but we are thinking that even though the points look great, and they are gapped correctly, that they may just be bad, so we are either going to replace them or perhaps put in Pertronix to eliminate the issue. Thoughts?
Also noted, as we are checking the voltage on connection "B" of the CDI, it starts out at 12, but slowly decreases to 9. not sure whats going on here, its coming from the battery. Any ideas?
Thanks, of course. This is quite frustrating...

[timmy2]

Check your grounds at the distributor, the coil mount and the negative wire from the coil to the coil mount.
Not sure why your voltage is decaying like that. Is the battery fully charged? Is it happening while you are cranking the motor?
Post some pictures of the rotor, points and distributor with the cap off.
Test continuity of the wires from the A terminal to the coil, the C terminal to the points and the grounds at the CDI to the above mentioned locations. Also check for a good case to body ground on the CDI unit.
See this thread too...
Help With No Electric at the coil