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Aircooled Breakfast
Join Date: Dec 2007
Location: Wyoming
Posts: 787
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Did I Fry My CDI?
I may have reversed the 3-pin connector on CDI plug. The unit still "whines" when power is applied but there is no spark. Would reversing the input and output kill it?
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04-11-2019, 02:20 PM
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ROW '78 911 Targa
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Input from the coil and output to the points is what you did?
CDI wouldn’t have fired.
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Dennis Euro 1978 SC Targa, SSI's, Dansk 2/1, PMO ITBs, Electric A/C Need a New Wiring Harness? PM or e-mail me. Search for "harnesses" in the classifieds. |
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04-11-2019, 03:36 PM
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Aircooled Breakfast
Join Date: Dec 2007
Location: Wyoming
Posts: 787
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Thanks, Yes, that's what happened due to the connector on the CDI unit being damaged. Good to know that the CDI is OK but I'm even more puzzled over no spark. Would that have damaged anything else? I checked the coil resistance and that looks good.
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04-11-2019, 05:37 PM
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Registered
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3-pin are really easy to troubleshoot. Connect a spark plug directly to the coil HV output and ground it to the engine. Then close the points and open them again and it should release a spark. If not the CDI box needs repair.
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1974 Targa 3.6, 2001 C4 (sold), 2019 GT3RS, 2000 ML430 I repair/rebuild Bosch CDI Boxes and Porsche Motronic DMEs Porsche "Hammer" or Porsche PST2, PIWIS III - I can help!! How about a NoBadDays DualChip for 964 or '95 993 |
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04-12-2019, 10:37 AM
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Aircooled Breakfast
Join Date: Dec 2007
Location: Wyoming
Posts: 787
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@ischmitz - Thank you. I have already tried a version of this test using an inductive clamp timing light on the HT lead to the distributor but I went ahead and did it as you described and - no spark which was the result of the timing light test too.
The other factor I'm dealing with is the mishmash of wiring that has been altered/repaired in this area. I have verified that there is 12VDC to the center terminal of the CDI and that I do have continuity between the "A" terminal on the coil and the distributor lead to the respective pins on the CDI harness plug. The CDI does emit a whine with power supplied. Anything else to look at? John Quote:
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04-12-2019, 01:12 PM
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Registered
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The 12V to the center and GND to the case make it whine. So far so good. Ohm from coil contact in the 3-pin harness connector to chassis GND and make sure you see the coil primary resistance of around 1 Ohm.
You can measure the points and check for +12V and it going to 0V when you close the points. That’s about it. After that it might be a bad CDI box.
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1974 Targa 3.6, 2001 C4 (sold), 2019 GT3RS, 2000 ML430 I repair/rebuild Bosch CDI Boxes and Porsche Motronic DMEs Porsche "Hammer" or Porsche PST2, PIWIS III - I can help!! How about a NoBadDays DualChip for 964 or '95 993 |
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04-12-2019, 02:02 PM
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Registered
Join Date: Jul 2013
Location: South East England
Posts: 1,695
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Some boxes emit a single spark when power is applied. If you had the connector reversed and the points were shut then the 400V coil pulse would be into a short circuit. This is a known failure mode of the units.
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www.classicretrofit.com |
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04-12-2019, 10:48 PM
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Aircooled Breakfast
Join Date: Dec 2007
Location: Wyoming
Posts: 787
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It passed both of these tests so unless there's anything else to check, it looks like the CDI may be culprit. Assume the fix requires more than basic electronic skills?
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04-13-2019, 05:33 PM
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Heinz
1974 Porsche 911 Targa 3.6
