911T no spark, bad CDI?

[yelcab1]

I need some advice. I have not worked on anything this old in a long time. A 71 911T with Zenith carbs. It was running but no more. No spark at the spark plugs.

The CDI box makes a high pitch noise with key on. It does get 15V at the red wire of the 3 pin connector.

The coil resistance checks out at 0.6 ohm and 670 ohms, primary and secondary.

The distributor cap is new, the rotor is new, and the point is new, which is also gapped at 0.016"

The green-yellow wire at the coil small terminal does go back to the CDI box. The various ground wires seem to be in place.

No spark, nothing else to check. I have to declare a bad CDI box.

Is that a reasonable conclusion?

[dicklague]

how about the points? Are you getting the proper signal from them? check gap and try a dwell meter if you can get one.

It is a bit tricky, because a scope can help measure and check the signal. A square wave generator can substitute for the points and help you test the system.

I wished you lived near me....I could help out.

[yelcab1]

The point is new (from a previous mechanic) and it seems properly gapped. I measured with a feeler gauge. It was running, then just stopped after it ran out of gas. Now no spark. I do have a dwell meter and an oscilloscope so I can try to measure something tomorrow.

I get zero signal at the coil primary winding. Ground, zero, nada. It seems to indicate the CDI box.

[yelcab1]

I wanted to know for sure so I opened up the CDI box. Here is the black mess inside. At least 2 components burned to a crisp. Now I know it is bad for sure.

Nobody knows what those components are?

[Jonny H]

That box has very different looking internals to ones I have seen. I've never seen resistors bridging capacitors and the red cap in the foreground looks newer than the rest of it. The circuit board is an unusual colour too. What is the part number stamped on the case?

I think this goes to show that boxes that have been repaired are still on 'borrowed time', just waiting for the other components to give up the ghost. At least a quarter of the boxes we have in for our CDI+ 'redux' show signs of previous repair.

[T77911S]

15v at the CD is not good.
you need to have the alt and reg tested but it sounds like the reg might be bad.

[yelcab1]

Originally Posted by T77911S

15v at the CD is not good.
you need to have the alt and reg tested but it sounds like the reg might be bad.

Sorry, I mistyped. 12V at the connector. The engine was not running when I did the test, there is no way to get 15V out of the 12V battery. Although I will for sure check it when I get the engine running again.

[yelcab1]

Ingo set me up with a rebuilt unit. It is working great. The battery voltage is at 14.25V if engine is revved at high RPM. A little high but not too excessive. Is there a need to change the regulator?

[dicklague]

good that you got it fixed