gtgrad Posted October 14, 2020 Share Posted October 14, 2020 Hello, I recently had my car run into a no-start issue after changing some dash lights. Whilst changing the dash lights, I created a power-ground short at the speedo. I'm worried this somehow or another created a short back to the CDI via the TD pin from the Tach. Not sure what is possible other than I know the car was working fine before the dash light mishap and is not after. I pulled the old CDI and found that D1 on the internal circuit was a short circuit, not behaving as a diode should. Additionally checked the spark and it was very weak. The CDI had a HV whine, so I don't think it's dead. Just not operating how it should based on the blown diode. The diode also was the incorrect size (must be from a rebuilt) and since it bridges from ground to power, I figure my dash incident could have caused this. Long story short, I bought a CDI+ to replace my old CDI. I'm a bit nervous to plug it in to the car without knowing that the rest of the electronics are fine. Is there some pin test I can do to verify that the plug is seeing the expected operating conditions before plugging the new toy in? As far as I can tell, I fixed the short in the dash and don't seem to have any issues else where. Everything lights up, fuel pump operates, and can't identify any other issues from my limited experience. Appreciate your help and looking forward to the new product! 1 Quote Link to comment Share on other sites More sharing options...
gtgrad Posted October 14, 2020 Author Share Posted October 14, 2020 Should have added. This is a 1978 3.0 motor in a 1976 911S chassis. 6-pin CDI with part number 0.227.300.004. Quote Link to comment Share on other sites More sharing options...
Jonny Retrofit Posted October 15, 2020 Share Posted October 15, 2020 @gtgrad CDI 6 pin connections are as follows looking at the PLUG** on the harness with it's two keyway 'ears' at the top A top left: COIL 'A' 15 top middle: 12V 7 top right: Distributor Signal + 31/1 bottom left: COIL '1' (-) TD bottom middle: Tacho Drive 31d bottom right: Distributor Signal - **Note: The sticker on the side of the CDI case shows the CDI SOCKET connections on the unit as you look into the connector. Basic checks CDI box disconnected. At CDI plug on loom with ignition OFF: Check resistance between pins 7 and 31d. Should be around 600 Ohms. This is the resistance of the pickup winding in the distributor (must be plugged in to distributor of course!). If you can plug the meter in whilst wiggling the green wire, it should stay locked at this reading. If it goes open or short circuit, then the green wire is bad. Check that TD is not shorted to GND. CDI box disconnected. At CDI plug on loom with ignition ON: Check 12V between pins 15 and 31/1. Quote Link to comment Share on other sites More sharing options...
gtgrad Posted October 15, 2020 Author Share Posted October 15, 2020 @Jonny Retrofit Checked both items mentioned and got ~580 Ohms between pins 7 and 31d. Wiggled the green wire and didn't see any changes. TD is not shorted to ground. With the ignition on got battery voltage ~12V across pins 15 and A. Installed the CDI+ unit and gave it a crank. Still not firing. The tacho does a sweep from 0 to 7k RPM to 0 on key on. When I turn the key it goes to 1-2 kRPM sorta of erratically. Turn it off, turn it back on and basically same behavior. I'm assuming (hoping) that the CDI+ should be fine. But I'm not sure what else to chase down here. Quote Link to comment Share on other sites More sharing options...
Jonny Retrofit Posted October 15, 2020 Share Posted October 15, 2020 Tacho sweep on ignition is good - a feature. What coil are you running? And what is the resistance across the two terminals (ignition off, box disconnected)? Quote Link to comment Share on other sites More sharing options...
Jonny Retrofit Posted October 15, 2020 Share Posted October 15, 2020 Next test is remove coil king lead, insert spare spark plug directly into coil top. Ground plug thread with a clip or a jump lead to ground. Crank. Spark? Quote Link to comment Share on other sites More sharing options...
Jonny Retrofit Posted October 15, 2020 Share Posted October 15, 2020 Green LED on top of CDI+ unit should: 1) light up bright for 1 second when ignition turned on 2) flash when cranking. Quote Link to comment Share on other sites More sharing options...
gtgrad Posted October 15, 2020 Author Share Posted October 15, 2020 Black Bosch Coil: 0.221.121.001 Resistance between terminal A and 1 with CDI+ disconnected: 0.6 Ohms Resistance between terminal A and center: 690 Ohms Resistance between terminal 1 and center: 690 Ohms I can't test the cranking visual behavior without my assistant being home. I did check when I turn the key on, the green light turns on for about a second. As soon as she gets home, I'll do a spark test and check for flashing on the CDI+ box. I guess the good news if both of those come back as acting as normal, then I have a functioning ignition system. One step closer to a functioning car I guess.. 🙂 Quote Link to comment Share on other sites More sharing options...
gtgrad Posted October 16, 2020 Author Share Posted October 16, 2020 @Jonny Retrofit Checked status during cranking and got a flashing green LED. Attached a plug to the center lead from the distributor and got a pretty healthy spark on crank. It was for sure brighter than the spark I got with my older CDI, so I think the ignition system to the coil is working. I'm going to check the individual cylinders with a timing light tomorrow to verify each of them is getting a firing signal from the distributor. From there I'm thinking it could be a fueling issue. Quote Link to comment Share on other sites More sharing options...
gtgrad Posted October 16, 2020 Author Share Posted October 16, 2020 @Jonny Retrofit Worked on the fuel system sensors today and found a loose connection in the CSV circuit. It happened to start getting a bit cooler here around the same time I started all this work, so I think this may have been a coincidence. Fixed it and it fired back up! Back to a running vehicle with a much improved ignition system. I also have your fuse kit on the way, should have a functionally modern classic car soon enough. Appreciate all of your suggestions. 1 Quote Link to comment Share on other sites More sharing options...
Jonny Retrofit Posted October 17, 2020 Share Posted October 17, 2020 @gtgrad Good job! 1 Quote Link to comment Share on other sites More sharing options...
Deano Posted November 19, 2020 Share Posted November 19, 2020 Hi Jonny, Ive checked ohms at socket 7 and 31d and got nothing and then unplugged the green wire and wigged it and had around 595 moved it again and now I can’t get anything. It’s a new URO green lead. Could it be faulty or could it be a distributor fault? regards Dean. Quote Link to comment Share on other sites More sharing options...
Jonny Retrofit Posted November 20, 2020 Share Posted November 20, 2020 10 hours ago, Deano said: Hi Jonny, Ive checked ohms at socket 7 and 31d and got nothing and then unplugged the green wire and wigged it and had around 595 moved it again and now I can’t get anything. It’s a new URO green lead. Could it be faulty or could it be a distributor fault? regards Dean. Repeat the test with your meter at the green wire connector. Then, at the connector on the side of the distributor. You should get the same ohm reading as you a measuring across the sensor in the distributor. Narrow it down. My money would still be on the wiring ‘upstream’. The wiring goes through the 14 way plug on the back of the relay panel. Check and clean contacts. You can splay the pins out a bit for better contact. Quote Link to comment Share on other sites More sharing options...
Deano Posted November 21, 2020 Share Posted November 21, 2020 All Sorted Jon. That new green lead had failed. Checked when dizzy was off and still no reading. New wire installed and around 590 ohms. Bingo. Reinstalled dizzy and first turn of key and the glorious flat six burst into life. 1 Quote Link to comment Share on other sites More sharing options...
Jonny Retrofit Posted November 22, 2020 Share Posted November 22, 2020 15 hours ago, Deano said: All Sorted Jon. That new green lead had failed. Checked when dizzy was off and still no reading. New wire installed and around 590 ohms. Bingo. Reinstalled dizzy and first turn of key and the glorious flat six burst into life. Good to know, thanks for reporting back. Quote Link to comment Share on other sites More sharing options...
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