No Spark - CDI Testing

[jjrowe]

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



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



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



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.



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



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

[T77911S]

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

[kodioneill]

what happens when you manually open and close the points?

[jjrowe]

you may be able to remove the plug wire from the cap and plug the wire into the coil for a better connection.

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.

what happens when you manually open and close the points?

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

[kodioneill]

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

[jjrowe]

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

[kodioneill]

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

[jjrowe]

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



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

[kodioneill]

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.

[jjrowe]

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

[kodioneill]

Originally Posted by jjrowe

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.

[jjrowe]

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

[jjrowe]

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

[kodioneill]

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

[equality72521]

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

[jjrowe]

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

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.

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

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

[T77911S]

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.

[jjrowe]

are you triggering the CD with the test light?

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

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.

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

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.

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

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.

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

[rick-l]

Originally Posted by jjrowe

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 History of Bosch CDI ... toubleshooting info, parts list changes, and schematics ... 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.

[rick-l]

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