CDI temp sensitive, no spark below 16C

mustseeflop · 07-16-2005, 01:00 PM

Hi ,
I am new to the 911 world. Has been a BMW E24 fan since high school. Owned 630 to M6 even a 3.0 CSI once.
I just got a 75 911S. The car is in good overall condition.
It has a Perma Tune CDI unit (silver one). I have a no spark condition when the ambient temp is below 16C. This morning it is 13C in Calgary and I crank it and no start as expected. I took the CDI unit off and place it on the stove to slowly heat it up to 40C and re-attach it to the 3 pin plug and it start right away. (the whole procedure took 15 min and the ambient hasn't change by more than 1C)
Anyone has similar situation with Perma Tune or other CDI units. (ie warm=easy start; cold=absolute no start)
Any comment as to what may be the cause of this situation.
Thanks
Ed

Early_S_Man · 07-16-2005, 02:20 PM

Intermittent connections, such as bad solder joint, or fractured wire -- with no actual defective components present in the box ... can be either 'fail cold' or 'fail hot' in nature ...

Here is one Permatune failure scenario that at least two Pelicans have experienced and have been able to repair easily and inexpensively:

2.4 ignition problem

mustseeflop · 07-16-2005, 03:43 PM

Hi Warren,
Thanks for the reply. Your suggestion of possible problem is indeed very enlightening. I am glad it is a "fail cold" condition -- it is better than have the car stall while driving (a fail hot condition).
I think it is the expansion & contraction of the bakerlite that cause an open circuit somewhere within the CDI unit. Since the bakerlite prevent any service inside the unit, can you please describe how to add an external 100 ohm 5 W pullup resistor to the CDI unit (which wire/pin to connect to).
I am thankful to have person like youself on this forum.
The car fires, idle & run perfectly when it is above 16C.
Ed

Early_S_Man · 07-17-2005, 09:36 AM

ED,

The easiest, non-intrusive way to add the resistor would be to find the Black/Violet wire to the back of the tach, and get a couple of piggy-back 0.25" Faston connectors [one female, two male tabs] ... solder the resistor leads to lower male tabs of the adapters with appropriate lead lengths to go between +12 Volt terminal and trigger terminal [Black/Violet wire] on the back of the tach. Remove power and trigger wires, install piggyback adapters, then reinstall power and trigger wires.

You can also install the resistor on the Relay/Regulator/CDI panel in the engine compartment ... but that would necessarily involve cleaning the panel and 14-pin connectors!

mustseeflop · 07-19-2005, 08:46 AM

Hi Warren,
I am a civil engineer -- really bad in elec. engr. stuff. Can you please draw a little diagram? My specific confusion is "where each end of the resistor are connected to..." I will be very thankful.
Ed