No Spark - CDI Testing
 

After putting my motor back together (with a rebuilt wire harness) I have no spark. The CD box is humming. So I have done the following tests:

1. Check Voltage at CD Box, red center wire in plug under CD box with key on - 12.16V - good

http://forums.pelicanparts.com/uploa...1236622310.jpg

2. Check resistance between CD Box and intake manifold to check ground (.4 ohm) - good

http://forums.pelicanparts.com/uploa...1236622514.jpg

3. Telegraph test - I didn't have a way to hook the plug up to the lead wire from the coil so I used a jumper cable. Anyone see an issue with this? I then ground the base of the plug and then unconnect the signal wire from the distributor. I then turn on the key and tap the signal wire to a ground point. - No Spark

http://forums.pelicanparts.com/uploa...1236622757.jpg

4. So either wiring, coil or CD box is bad. I test the resistance on the coil. Between the two connectors the resistance is .6 ohm.

http://forums.pelicanparts.com/uploa...1236622922.jpg

Resistance from lead to cap and one of the connectors on the coil: 4100 ohms.

http://forums.pelicanparts.com/uploa...1236622991.jpg

I think I need to get a test coil to eliminate that. Also my harness is new, what can I do to check the wiring? Any good tests I am missing?

thanks,
Jesse

1976 911S

just something quick
is the dizzy set up with #1 firing at TDC compression?
you may be able to remove the plug wire from the cap and plug the wire into the coil for a better connection.
it did run before rebuild?
check points gap

what happens when you manually open and close the points?

That is a good idea, I should be able to just plug a spark plug wire end onto my test spark plug and plug the distributor end other end directly on the coil. I will do it that way to make sure I had a good connection.

Well, the purpose of the telegraph test is that it isolates the CD box and coil and takes the distributor and points out of the equation. Since I go no spark with that test I have not moved on to the points or distributor.

Thanks!

Jesse

it appears that you have the jumper cable hooked up to #1 ignition wire not the coil wire in your photo of telegraph test.

Nice catch. Thank you!

I redid the test and still have no spark. All the parts worked before so i suspect the wiring. I will try to find a wire diagram. Switching the two white wires coming out of the CD box would be an easy mistake to make.

Jesse
76 911S

would some pics of that area (under the box molded connector) from my 74 help you out?

I found a diagram from one of early_s_man's (RIP) posts.

http://forums.pelicanparts.com/uploa...1236628413.jpg

I traced the wires at the coil. On my coil point #15 says "A". That traces to "A" on my cd box. I do have ground at the other coil connection. I also trace the "C" wire from CD box to the signal wire on the dizzy. I don't know what else I can do at this point. Unless I messed up the telegraph test it has to be the coil or CD Box. I have to get a spare coil now I guess.

Thanks for all your help.

Jesse
76 911S

keep thinking about and double check what you did. as a mechanic i have almost always found that the problem started when something was taken apart. i doubt that your coil died in the period of time that you replaced the harness.

I agree, I can't help but think I am missing something. After checking the picture it says that my secondary resistance should be 650-790 and mine is 4200. Definitely something is up there.

Jesse
76 911S

that is way out of spec.

I did the test at the end of the wire that feeds the dizzy. I guess that is not right. I pulled the wire off and did the test again but couldn't get a good reading. It is very dirty inside the coil so I need to clean that with a wire brush or something. Can you take a resistance measurement on your coil wire itself? Mine seems high (~900 ohms or .9 Kohm).

Thanks,
Jesse

Okay, update. Still no spark

- I took out the CD Box and cleaned connections.
- I removed the coil and ran the resistance test, still seeing ~4000 ohms between center of coil (4) and gound point (1). Spec on this is 650-790 so it is way off. Inside the coil was dirty and I cleaned it the best I could. Still seeing same numbers. I was told that the secondary resistance is not as important as the primary resistance test which I pass.
- I used a spark plug wire to do the telegraph test instead of my original setup, still no spark!

I am at a total loss. Everything points to the coil but it doesn't make sense that it went bad while the car was apart but worked before.

Jesse
76 911S

the cd box is squealing? does it make any noise when you ground the points wire?

You didn't reverse the wires to the coil did you?

The CD box is squealing. there is no noise when I ground the points. I used a test light hooked to 12V and the points wire, then turned the engine over. Points are opening and closing.

Even when I connect a spark plug directly to the coil and ground it, then ground the points wire, I get no spark.

I thought that maybe the wires in my new harness were reversed but I traced them and they appear to be right. Looking at the CD Box the white wire on the left goes to the connection marked "A" on my coil. The right white wire is the signal wire to the points. I believe that is right.

Is it possible that there is a short to ground somewhere in my harness? My next test was going to be to disconnect the ground and hook my voltmeter in between, then crank the motor and see if my spark is shorting to ground.

Jesse
76 911S

are you triggering the CD with the test light?
thats strange that the CD gets quiet when you ground the points, i thought that is when the oscilator is doing it thing so the cap can recharge. i will have to look into that.

lets back up a second. use the diagram above above and recheck some stuff. there is not much needed to make this work. 12v, a switch (the points) and a voltage multiplier (the coil).

disconnect the CD. with the points closed, veryify ground at pin C and an open when the points open.
verify a short from pin A back to the CD unit,some resistance through coil, but still basically a short. these are done on the wire harness, not the CD itself.
if you need to break it down, pin A to pin 15 on coil, pin 1 back to CD unit.
verify again 12v on pin B with key on. from the pic above, i dont see how you are checking this, the connector should be in the left side of the engine bay, it looks like you are around the coil.

dumb question here, did you remember to put the rotor back on? my mind was wandering one day when i pulled the dizzy and i forgot the rotor.

No - I was testing that the points were opening and closing with the test lamp

Sorry, I miscommunicated that. There is no ADDITIONAL noise when grounding points, CD Box continues to whine the same.

I verified all of this, but I will recheck again. I have chased all the wires and they are going to the right spot.

Rotor is on but I am still "Telegraph" testing so that should take the dizzy out of the equation.

My next test is to check the bare shield wire that connects to the front of the CD Box has not shorted to the signal wire somehow (this is a new harness so a mistake could have been made). This would explain my signal from the points being lost and never initiating a spark.

Thanks,
Jesse
1976 911S

Does you snap on meter have a current setting? If you look at the schematic http://forums.pelicanparts.com/porsche-911-technical-forum/226517-history-bosch-cdi-toubleshooting-info-parts-list-changes-schematics.html?highlight=cdi+schematic the current through the signal wire through the points to ground should be (Vbatt - 0.7) / 1000 ~ 12 milli amps.

Ooops messed up on that, missed a resistor. should be 12 / 33 or 0.36 an amp.

For that matter between the wire that goes to the points and ground you should see Vbatt.

Yes, I can measure current so I will check that by grounding that wire through the meter. Your post reminded me that there is no signal from the points per se, its just supplies a ground when the points close. My first test when I get home will be whether the wire to the points is grounded when disconnected, indicating a short to ground.

Jesse
76 911S

I think I would proceed like this

1. points open 12 volts on the wire
2. points closed 360 mA through the wire

this is at the distributor
Someone will correct me if I'm wrong

Fixed it! There was a short between the signal wire from the distributor and ground. I first checked the resistance and there was none

http://forums.pelicanparts.com/uploa...1236746741.jpg

The wire shield had cracked and the metal shielding was touching the internal wire. I had to peel back the shielding to see it. Once I fixed that the car started on the first crank. I forgot how much fun that car was to drive!

http://forums.pelicanparts.com/uploa...1236746769.jpg

Thanks for all your help

Jesse

I had the same issue as I wasn’t getting the car to start. I jumepered power from the rear window heat fuse to the red wire on the box and i was able to start. Now i know the problem is upstream, or is that downstream, on the other side of the rear firewall :)http://forums.pelicanparts.com/uploa...1634244342.jpg

Good work tracking that down. Short of a lightning strike right on the car, I really don't see that shielding is necessary to prevent spurious triggering of the CDI. I wonder what test scenario or real life circumstance made them do it, or whether it was arbitrary since it's a signal wire? Most CDIs don't need shielding on the points signal wire and I don't think the Bosch 3pin does either. Fred

No spark on my 82' Coupe. So, I swapped the coil on my running 79' Targa and still no spark, however, the White wire to the coil was connected to terminal "1" instead of terminal "A" (as per Bentley's and how it was wired up on my Targa). The coil off of the Coupe started the Targa so it's not the coil and my CDI on the Coupe is whining away. I haven't check the points yet.

You have points on an '82 car? My earlier car (1978SC) doesn't have points; the distributor generates a pulse that passes down the green wire to the CDI. This wire can short. Result no spark. Suggest you check the green wire by putting a meter across the two wires after you have detached it at one end. If you have continuity then there is a short.

What about the wires being effectively crossed between the 2 coils? That is, on the 79' the White wire was connected to the "A" terminal on the 79' but connected to the "1" terminal on the 82'? They both ran that way but I didn't think they were interchangeable. The White to terminal "A" is as per the Bentley manual but the 82' has it wired up backwards.

I haven't pulled the distributor cap and just assumed points.j

When you say: "put the meter across the 2 wires", which 2? The green and...?

The green wire is known to fail. It's actually a shielded cable; if you disconnect it (on my car I can do this somewhere near the oil feed for the chain tensioner) you will find two pins; these two pins should show infinite resistance or no continuity. My meter has a buzzer for checking continuity. If you can't find this connector then disconnect at the CDI and check for continuity between pin 7 and 31d.